[0001] The present invention relates to sonic emitter arrangements, and it relates more
particularly, though not exclusively, to such arrangements including miniature loudspeakers,
such as microspeakers, and to their incorporation into portable electronic devices
such as mobile cellular phones, digital cameras, portable games consoles and hand-held
computers, or into miniature loudspeaker enclosures, such as earphones. The invention
also encompasses devices, such as portable electronic devices, and miniature loudspeaker
enclosures incorporating such emitter arrangements.
[0002] Portable electronic devices, such as those mentioned above, are becoming increasingly
popular. For example, it is now commonplace for mobile phones and digital cameras
to incorporate music players using the MP3 format, and some of the individual technologies
are converging to create hybrid devices such as mobile phones combined with digital
cameras or gaming consoles.
[0003] Portable electronic devices in general are well-suited for use with personal earphones
or headphones, because they are often used in public places. Furthermore, it has not
been possible (or worthwhile) hitherto for the housings of such devices to incorporate
small loudspeakers which provide anything more than an extremely basic listening experience.
There are several reasons for this.
[0004] Firstly, loudspeakers tend to add significantly to the physical size and cost of
the end product. Secondly, the acoustical output quality of any loudspeaker is critically
dependent on the way in which it is built and mounted. Furthermore, because of the
restricted space available in a hand-held portable electronic device, for example,
the loudspeakers must be placed relatively close together; typically less than 10
cm apart, or even less than 5 cm in a mobile cellular phone. Thus, when stereophonic
audio material is played, the stereo effect is lost because the left and right channels
are being reproduced from virtually the same point in space, whereas stereo is intended
for playback on more widely spaced loudspeakers (typically about 2 metres apart) in
order to create a spatial "sound image".
[0005] In order to address this emerging market for portable electronic devices with more
sophisticated audio capabilities, loudspeaker manufacturers have recently developed
extremely compact loudspeakers, known as "microspeakers", whose dimensions are similar
to those of the driver units used in earphones. Such microspeakers are typically less
than 20 mm in diameter and less than 5 mm in thickness. Despite their small dimensions,
however, such microspeakers have significant power output capability, often with power
ratings of several hundreds of milliwatts.
[0006] Microspeakers have been built into certain types of mobile phones by several manufacturers,
but they are usually attached simply to the inner face of the phone's housing with
their front surfaces exposed via a mesh or grille, or by several small holes in the
housing. Although this is adequate for transmitting simple audio, such as ring-tones,
to the listener, it is not adequate for delivering more sophisticated audio performance,
such as 3D-positional audio for games, or stereo expansion for ring-tone and music
playback.
[0007] The physical requirements for rendering 3D-positional audio via microspeakers are
more demanding than they are for rendering stereo. Firstly, 3D-audio based on HRTF
(Head-Related Transfer Function) processing relies on precise spectral filtering for
its effect, and anything which introduces peaks or troughs into the system frequency
response will degrade the perceived 3D-audio effects.
[0008] Secondly, an intrinsic and critical element of all loudspeaker-based 3D-audio is
transaural crosstalk cancellation, as is described fully in
GB 2,340,005. This relates to the natural acoustic crosstalk which occurs around the head when
an individual is listening to a pair of stereo loudspeakers. When the left-channel
signal is emitted by the left-hand loudspeaker, it travels not only to the left ear,
but also, a little later in time, crosses to the right ear (and vice versa). The brain
recognises the high degree of correlation between the two signals, the primary signal
and the crosstalk signal, and then correctly attributes their source to the left-hand
loudspeaker, hence the sound is perceived to emanate from the left-side loudspeaker.
[0009] Without this crosstalk signal, the left-channel signal would have been delivered
only to the left ear of the listener, just as it would have been using headphones.
This feature, of delivering each sound channel only to its respective ear, is necessary
for the correct operation of HRTF-based 3D-audio.
[0010] The presence of the transaural crosstalk signal inhibits 3D-audio effects, and so
it must be cancelled by generating a signal which is equal in magnitude, and opposite
in polarity, from the opposite loudspeaker, as is described in
GB 2,340,005.
[0011] In order to achieve adequate (say, 90%) crosstalk cancellation, the cancellation
signal must match the crosstalk signal in magnitude and phase within fairly precise
limits, about 3 dB of amplitude and ±20° of phase. This means that the relative time-of-arrival
of the signals at the listener's ears must be synchronised very carefully. Anything
which interferes with the integrity of the left and right-channel signals will degrade
the crosstalk cancellation, and hence degrade the effectiveness of the perceived 3D-audio.
During playback on portable devices, where the loudspeakers might only be 40 mm or
so apart, the timing must synchronise to within a few microseconds for optimum effect.
These constraints, and other considerations, create the following requirements for
the successful rendering of 3D-audio using microspeakers.
- 1. The frequency response of the speakers should be relatively flat and smooth, without
significant notches or peaks.
- 2. The high-frequency (HF) response of the speakers should not be substantially impaired
or degraded in any way, especially for the satisfactory reproduction of music and
MIDI-related sources. Ideally, the frequency response should extend, without significant
reduction in amplitude, up to 10 kHz and preferably beyond.
- 3. The sound for each channel must be emitted from a single point-source, or as close
to this as possible. If the sound source is relatively large, then the wavefronts
will be distributed over the emitting area, and the sound wave will effectively be
"dispersed" in time when it arrives at the listener's ears. This makes transaural
crosstalk-cancellation extremely difficult or even impossible.
- 4. There must be minimal secondary emission, that is, sound emission which occurs
from locations other than the primary microspeaker acoustic output port. For example,
some sources of secondary emission in cell-phones include: (1) the casing and faceplate
if the microspeaker is not well isolated from them; (2) the rear of each speaker,
sound from which actually propagates through the alternate channel speaker, and (3)
various holes in the casing if the rears of the speakers are not properly enclosed.
There should be no additional emission ports other than the primary one.
- 5. The left- and right-channel sources must be placed as far apart as is practical
in order to maximise the time-of-arrival difference between the left and right ears.
- 6. The left- and right-channels should be well matched in all respects, including
loudspeaker matching with respect to both phase and amplitude.
[0012] There are two additional, physical requirements for deploying microspeakers in a
slim casing, such as may be used (for example) for a cell-phone, to render 3D-audio.
Firstly, it is required that the form-factor be suitable for the available space,
and especially that the total thickness profile of the component package is adequately
thin, typically less than 8 mm, in order to fit into the available depth of housing.
Secondly, particularly for casings of the "clam-shell" or "ear flip member" type which
can be opened and closed, it is necessary for the phone to reproduce 3D-audio successfully
in both the open and closed positions.
[0013] One feature of all mobile electronic devices is the very limited space which is available
for the user interface, primarily the graphics display, keypad and other controller
devices; the graphics display in particular taking priority when housing space is
allocated during design. Accordingly, there is little or no space for loudspeakers,
however small, to be mounted on the front panel, facing the listener.
[0014] The various alternative options for incorporating microspeakers into a mobile phone
include (a) mounting microspeakers to either side of the device, facing outwards to
the listener's left and right sides, respectively; and (b) mounting the microspeakers
internally of the device body, and delivering the audio output via a conduit, or pair
of conduits, to respective output vents.
[0015] Of the alternative options, side mounting is the easier to implement, for example
by mounting the microspeakers in sealed pods on opposite sides of the device. However,
although the pods might be small, the overall additional bulk makes the phone body
somewhat unsightly and the pods also detract from the smoothness of the body, making
it less easy to move the phone into and out of a pocket, or a holster.
[0016] Internal mounting is thus a favoured option although, as stated previously, the mounting
arrangements for the microspeakers are critical to the resultant performance, and
a number of operational and design-related criteria must be complied with, depending
on the audio performance required. Also, as mentioned above and as discussed in more
detail hereinafter, particular considerations arise in relation to cell-phones which
are designed in the form of clam-shell structures, comprising a pair of hinged body
units (effectively, a body with a corresponding lid) which are closed together when
not in use, and which are unfolded for use when required.
[0017] In any event, if internally-mounted microspeakers are to be used, it is axiomatic
that a conduit of some sort must be used to convey the audio energy to the outside
world. For microspeakers mounted on an internal chassis or frame within an outer housing,
suitable conduits can in principle be formed within the housing to which the front
(sound-emitting) surfaces of the microspeakers are exposed, and emission apertures
can be cut into the housing, at some small distance from the microspeakers, to act
as sound outlet ports, linked via the conduit to the microspeaker.
[0018] Problems arise with such arrangements however, since the conduits and any "dead-space"
volumes adjacent the microspeaker form resonant acoustic cavities. Such cavities generally
exhibit Helmholtz-resonator characteristics (described in Appendix 1), and thus create
undesirable peaks and troughs in the emitted sound spectrum. Moreover, the housing
itself is exposed directly to a considerable amount of sound energy present in the
cavities, to which it is partly transparent, and hence the cell-phone housing itself
becomes an undesirable secondary emission source for both microspeakers, further reducing
the sound quality, and significantly impairing the effectiveness of 3D-positional
audio.
[0019] One method of reducing the peaks and troughs caused by the resonant conduit would
be to simply fill the cavities with sound damping material. Such an approach, however,
inevitably results in the absorption of a considerable amount of the sound energy
indiscriminately across the spectrum, and substantially reduces the emitted volume.
[0020] Various prior-art proposals have sought to address the acoustic limitations which
result when microspeakers and other transducers, such as piezoelectric monomorphs,
are mounted into a housing adjacent to one or more emission apertures; such proposals
mostly involving attempts to insert peaks and troughs, and/or to re-locate unwanted
peaks and troughs, into selected areas of the sound spectrum, in an attempt to compensate
for the peaks and troughs associated with the aforementioned Helmholtz-type cavities
and/or to lift or depress an amplitude versus frequency characteristic which falls
below or exceeds, respectively, a desired sound level. Such approaches are acceptable
where the principal interest lies in a limited part of the spectrum (e.g. for the
reproduction of ring tones and the like) or where the overall amplitude-frequency
characteristic does not need to be controlled to the extent required for accurate
3D-positional performance.
[0021] Such proposals are disclosed, for example, in
WO 83/023304;
US 5,953,414;
US 6,324,052 and
WO 02/340030, none of which successfully addresses the problem of controlling the overall amplitude-frequency
characteristic to an extent sufficient to render music and/or 3D-positional audio
performance of acceptable quality.
[0022] WO 2004/030402, which was published later than the priority date claimed herein, and is thus referred
to hereinafter as "the intermediate document" describes a twin-resonant acoustic structure
for incorporating a microspeaker into a cell-phone housing with the intention of providing
good alerting performance and extended frequency response in the voice frequency range
(between 300 Hz and 3400 Hz). This proposal recognises the problem which arises when
a microspeaker is coupled via a conduit to an opening in the housing, in that the
frequency response becomes dominated by a resonant peak, such that the mid-frequency
and high-frequency responses are very strongly attenuated, and discloses the use of
a first "forward tuning volume", which comprises, in effect, a Helmholtz cavity, adjacent
the face of a loudspeaker. This volume is coupled via a passage both to an opening
in the housing, and to a second forward tuning volume, in fluid communication with
the passageway, lying between the first forward tuning volume and the opening in the
housing.
[0023] Without the second forward tuning volume, the amplitude-frequency characteristic
attributable to this arrangement, however, contains a large resonant peak at about
3.5 kHz, followed by a significant trough at about 5.2 kHz, and exhibits progressive
high-frequency (HF) attenuation. These large spectral perturbations are not acceptable
for 3D-positional audio, nor for music reproduction, both of which require a fairly
flat spectral response.
[0024] When the second forward tuning volume is added to the acoustic structure, another
resonant peak is added to the response in the region of 6 kHz to 7 kHz (which is its
purpose), but the spectral trough at 5 kHz still remains and the HF performance decreases
even further. Thus, although this expedient can be used to increase the apparent volume
of an alerting tone in the 6 kHz to 7 kHz region, these additional spectral fluctuations
and the further HF reduction would further degrade the reproduction characteristics
needed for 3D-positional audio and music.
[0025] It is an object of this invention to provide sonic emitter arrangements, comprising
one or more miniature transducers (such as microspeakers) disposed within a housing,
together with associated control means capable of so controlling the amplitude versus
frequency characteristics of emitted sound as to maintain amplitude excursions attributable
to conduits and/other means for conveying the sound from the transducers out of the
housing to an extent that renders such arrangements capable of emitting sound of acceptable
quality for the reproduction of music and/or for 3-D positional audio imaging.
[0026] From one aspect there is provided a sonic emitter arrangement comprising at least
one sonic transducer encased within a housing dimensioned to be portable or wearable;
said transducer having a sonic emission surface, and the arrangement further comprising
at least one conduit linking said emission surface to an outlet through which sound
produced by said transducer can be emitted from said housing; wherein at least one
dimension of part at least of the length of said conduit is flared so as to increase
toward said outlet, thereby to influence the amplitude versus frequency characteristic
of sound emitted from said outlet.
[0027] According to the invention, said housing encases first and second sonic transducers;
said transducers each having a respective sonic emission surface, and wherein the
arrangement further comprises a respective conduit linking each said sonic emission
surface to a respective emission outlet through which sound produced by said transducer
can be emitted from said housing; and wherein each said conduit is flared so as to
increase toward its respective outlet, thereby to influence the amplitude versus frequency
characteristic of sound emitted from said outlets.
[0028] Another example, utilising a single sonic transducer, provides first and second conduits
linking the transducer's sonic emission surface to respective emission outlets through
which sound produced by said transducer can be emitted from said housing; and wherein
each said conduit is flared so as to increase toward its respective outlet, thereby
to influence the amplitude versus frequency characteristic of sound emitted from said
outlets.
[0029] In the arrangements described in both of the two immediately preceding paragraphs,
it is preferred that substantially identical flaring is applied to each of said conduits.
[0030] The flaring may be applied over part only of the length of a conduit. In such circumstances,
it is preferred (though not essential) that such flaring occurs adjacent the outlet.
[0031] The flaring is preferably smooth and may conform to a substantially linear profile
or follow an exponential or other curvilinear form, though it may alternatively, or
in addition, incorporate one or more discrete steps.
[0032] Arrangements of the kind described in the foregoing paragraphs are efficient, can
be implemented at relatively low cost, and can be readily adapted to a wide range
of different transducer types and sizes, though it is preferred that a miniature loudspeaker,
such as a microspeaker, is used.
[0033] In further preferred forms of the invention, the arrangement comprises one or more
acoustic resonant absorbers linked to said emission surface and/or to at least one
conduit, in order to further influence said characteristic. By this means, fine-tuning
and/or additional compensation for unwanted characteristics can be achieved.
[0034] In one embodiment, at least one of the acoustic resonant absorbers comprises a Helmholtz
resonator, such resonators being relatively simple to construct and exhibiting reliable
performance over a wide range of operating conditions.
[0035] In another embodiment, at least one of the acoustic resonant absorbers comprises
a quarter-wavelength tube, channel or groove device, or plurality thereof; such resonant
devices being relatively simple either to mould in plastic materials or to cut in
metallic materials, and therefore well-suited to mass-production means.
[0036] Where quarter-wavelength absorber tubes are utilised, it is preferred to provide
a distributed array of quarter-wave channels conforming substantially to a concentric
elliptical array pattern.
[0037] Any arrangement in accordance with the invention may conveniently be fabricated,
at least in part, from superposed laminar components; such components being of metal,
plastics or any other material which can be readily machined, moulded or otherwise
formed to the required tolerances and which, when assembled, exhibits suitable acoustic
performance.
[0038] Where such laminar components are used, edge-emission conduits can conveniently be
formed by opening an otherwise enclosed aperture, such as an aperture located substantially
centrally within a plate, through to an edge of the plate by removal of the plate
material, bearing in mind that other laminae will lie above and below the plate in
question, thereby defining part at least of the conduit.
[0039] If desired, any one or more of the resonant absorbers may be fabricated of, or may
contain or have associated therewith, acoustic damping material of any convenient
kind, such as cotton fibre wool or tissue paper.
[0040] Arrangements according to any embodiment of the invention may be incorporated into
electronic devices such as mobile telephones, digital cameras, mobile games consoles
or portable sound and/or multimedia equipment.
[0041] In such circumstances, the said housing usually serves as the overall housing for
the invention. Typically, such a device incorporates two or more such arrangements,
preferably matched in performance, permitting the device to exhibit sophisticated
audio performance characteristics, such as 3D-positional audio or enhanced stereophonic
sound.
[0042] One or more arrangements according to any example of the invention may be incorporated
into portable and/or wearable loudspeaker enclosures such as earphones, intended to
be worn by a listener.
[0043] It will be appreciated from the foregoing statements of the invention that the invention
calls for at least one dimension of part at least of a conduit acoustically coupling
a microspeaker to an emission outlet to be flared so as to increase toward said outlet,
thereby to influence the amplitude versus frequency characteristic of sound emitted.
[0044] The principle of deploying a flared conduit, as used in the present invention, should
not be confused with other audio applications in which flared devices are known. For
example, a flared trumpet device was used in the earliest gramophone machines as a
mechanical transformer to transmit efficiently the mechanical vibrations from a small
needle into a large-area, resonant and reflective surface, thus increasing its loudness.
Also, flared horn-type arrangements are widely used to increase the efficiency of
loudspeakers used in public-address systems, by acting as an acoustic transformer
to match the impedance of the piston-like diaphragm to that of the air.
[0045] In contrast to these known applications of flared acoustic devices for impedance
matching and the like, the present invention employs a flared topography for a very
different reason, namely to create a minimally resonant emission conduit linking the
emission surface of the microspeaker with the emission aperture, in order to provide
a desired, flat frequency response emission characteristic.
Minimal Resonance Principle
[0046] Embodiments of the present invention were devised by designing and constructing the
relevant elements of a Helmholtz-type conduit (as defined in Appendix 1 hereinafter)
so as to minimise its resonant properties, according to the following two principles
devised by the inventor.
[0047] Firstly, it was decided to reduce the magnitude of the resonant peak by minimising
the Q factor of the resonance, within practical constraints, in order to minimise
the "insertion response"; a term which is defined hereinafter. Secondly, it was decided
to locate the spectral peak associated with the residual resonance at as high a frequency
as would be practically possible, in order to avoid the HF attenuation related to
the prior-art Helmholtz-type cavities.
[0048] By inspection of equations (1) and (2) in Appendix 1, it can be deduced that both
of these objectives can be achieved by the following principles:
- 1. Minimising the internal volume of the conduit, V;
- 2. Minimising the length factor, L; and
- 3. Maximising the sonic emission area, S.
[0049] Conformance to these principles is not straightforward, however, because these parameters
are mutually interdependent. For example, in a conduit of constant rectangular section,
if it were required to increase the emission area, S, then this could be achieved
by increasing the thickness of the conduit. An increase in the thickness of the conduit
would proportionately increase the emission area, S. However, it would also proportionately
increase the volume, V, and this would create a conflict in satisfying the above principles,
in which S must be maximised, and V must be minimised. This conflict also occurs if
the width of the conduit were to be increased.
[0050] Embodiments of the present invention conceived to satisfy the minimal resonance principles,
above, utilise a "flared emitter" structure. By minimising the cross-sectional area
of the emission conduit adjacent the microspeaker, and maximising its area at the
point of emission, then the internal volume, V, is minimised whilst maximising the
sonic emission area, S.
[0051] One implementation of these principles is to minimise the frontal volume adjacent
the microspeaker, with a small, constant cross-sectional area in this region, from
which is provided a flared conduit to the emission aperture. Another approach is to
provide a progressive increase in cross-sectional area throughout the length of the
conduit between the microspeaker and the emission aperture. By this means, the emission
conduit less resembles a Helmholtz resonator, but rather becomes more like an open
tube structure, and is minimally resonant.
[0052] In practice, for an orthogonal emission arrangement, it will be appreciated that
the length factor, L, cannot be less than the radius of the microspeaker, or thereabouts.
[0053] In order that the invention may be clearly understood and readily carried into effect,
certain embodiments thereof will now be described, by way of example only, with reference
to the accompanying drawings, of which:
Figures 1(a) and 1(b) show an idealised representation of the relationship between
a microspeaker and an outlet slot, or port, for the acoustic emissions therefrom;
Figures 2(a) and 2(b) show a flip-top, or "clam-shell", type of mobile phone in closed
and open conditions respectively;
Figures 3(a) and 3(b) are similar to Figures 2(a) and 2(b) respectively, but illustrate
a convenient disposition and configuration for outlet ports conducive to use with
embodiments of the present invention;
Figure 4 shows, in exploded diagrammatic perspective, part of an arrangement, in accordance
with one example of the invention, utilising a plate formed with a flared emission
slot;
Figure 5 shows an amplitude-frequency response characteristic measured in relation
to an arrangement of the kind shown in Figure 4, and a characteristic, similarly measured,
for a microspeaker alone for comparison;
Figure 6 shows an idealised representation of the relationship between a microspeaker
and an outlet slot, or port, to illustrate measurements of insertion loss in acoustic
emissions;
Figure 7 shows an insertion loss characteristic measured in relation to an arrangement
of the kind shown in Figure 4;
Figures 8(a) and 8(b) show a flared emitter plates incorporating respectively a Helmholtz
resonator and quarter-wave stub resonators, each intended individually to replace
the plate shown in Figure 4, in arrangements in accordance with second and third embodiments
respectively of the invention;
Figure 9 shows an amplitude-frequency response characteristic measured in relation
to an arrangement including a plate of the kind shown in Figure 8(a), and a characteristic,
similarly measured, for a microspeaker alone for comparison;
Figure 10 shows an insertion loss characteristic measured in relation to an arrangement
including a plate of the kind shown in Figure 8(a);
Figure 11 shows an amplitude-frequency response characteristic measured in relation
to an arrangement including a plate of the kind shown in Figure 8(b), and a characteristic,
similarly measured, for a microspeaker alone for comparison;
Figure 12 shows an insertion loss characteristic measured in relation to an arrangement
including a plate of the kind shown in Figure 8(b);
Figure 13 shows a dually flared emitter plate, incorporating an array of quarter-wave
stub resonators, intended to replace the plate shown in Figure 4, in arrangements
in accordance with a fourth embodiment of the invention and utilising a single microspeaker;
and
Figure 14 defines certain parameters used herein in describing Helmholtz resonator
characteristics.
[0054] The invention aims to provide an efficient sonic emitter structure with physical
and acoustic properties that are especially well-suited (inter alia) to application
in cell-phones and other portable devices for the reproduction of stereophonic music
and, especially, 3D-positional audio.
[0055] In terms of physical properties, the invention permits construction of the components
and their assembly into a thin, substantially planar configuration, and provides sonic
emission from an aperture, or port, 10 formed in one edge of its structure and orthogonally
oriented with respect to the emission plane of an internal microspeaker 20 as shown
in Figure 1, in which L-L' represents the propagation (sound emission) axis, and C-C'
represents the central emission axis of the microspeaker. This is especially beneficial
for use in clam-shell type cell-phones where the flat form factor enables deployment
in a thin structure, in pairs (Figure 1(b)), and where the edge emission property,
being front-back symmetrical, lends itself to operation in both open and closed positions.
[0056] This latter feature is an important attribute of certain embodiments of the invention.
The clam-shell phone structure (see Figures 2 and 3) comprises a pair of hinged body
units (a body 100 with a corresponding lid 200) which are closed together when not
in use, and which are unfolded for use when required. If a pair of microspeakers were
to be mounted inside the upper surface of the lid unit 200 directly behind respective
sound emission apertures 300 and 400, the apertures would be exposed towards the listener
when held in the hand, in the closed mode, as shown in Figure 2(a). However, with
the lid 200 opened in order to use the cell-phone, then the emission apertures 300
and 400 would face away from the listener (Figure 2(b)). Furthermore, the emission
apertures would be occluded by the lid 200 itself, thus reducing the perceived volume
and high-frequency content. This represents a major problem for the use of video games
on a clam-shell type phone, in which the user will wish to use the phone in open mode,
in order to see the visual display, but also will require 3D audio playback from the
microspeakers. This problem is not solved by incorporating the speakers into the lower,
body unit, because the user's hand is likely to occlude any emission apertures which
would be present therein.
[0057] However, by engineering the embodiment of the invention of Figure 1(b) into a cell-phone
body so as to provide edge emission from a pair of rectangular slit apertures such
as 500 formed in opposing sides of the lid structure 200, as depicted in Figure 3,
then the emission is symmetrical in both the open and closed positions, as shown in
Figures 3(a) and 3(b) respectively, which is ideal for the reproduction of 3D-sound
in both circumstances.
[0058] A further valuable property of the invention relates to the minimal acoustic dispersion
of the propagated signal. Acoustic dispersion occurs when there is not a single, well-defined
path from the source to the listener, as is the case when the sound emitting device
is not a point source. If it were, then the acoustic path length to the listener's
ear would be well-defined. In practice, however, microspeaker sound is emitted from
a finite area. If this area were divided into a matrix of elemental areas, then each
element would have a slightly different path length to the listener's ear. This variation
in path lengths represents the acoustic dispersion range. For 3D-audio, as described
previously, it is important to ensure that the wavefronts arrive separately and synchronously
at the listener, and hence with minimal dispersion.
[0059] However, as described already, and illustrated in Figure 1, a feature of embodiments
of the invention is that the aperture 10, from which sonic emission occurs, can usefully
be formed as a narrow, rectangular slit, typically 2 mm or less in width and 10 mm
or so in length. Consequently, in a clam-shell cell-phone configuration, if the two
apertures such as 500 in Figure 3 are arranged and oriented so as to lie along the
opposing, lateral edges of the lid 200, as described above for front-back symmetry,
they also present the listener with a pair of suitably narrow sound sources, maximally
separated by the width of the clam-shell lid 200. The resultant vertically oriented
emission slits such as 500 are near perfect for 3D-audio reproduction, owing both
to the maximal separation and to the minimal acoustic dispersion associated with the
very narrow emission area.
[0060] Figure 4 shows, in exploded diagrammatic form, the principal components of a sonic
emitter arrangement in accordance with one example of the invention. The arrangement
is, in this example, constructed in laminar form from overlaid elements of aluminium
sheet stock, and incorporates a 16 mm diameter, 2 mm thick microspeaker unit 20 (Foster
type 364870), with an active emission surface area of 9 mm diameter located centrally
thereof, mounted with epoxy sealant into an aluminium flange plate 22 of thickness
4 mm, measuring 28 x 28 mm and formed with a central aperture of diameter 16.2 mm.
[0061] A plate 30 is secured to the plate 22; the plate 30 being formed as shown with a
conduit configured as a flared slot 32 to provide a gradual transition from a 9 mm
wide central aperture, overlying the active transducer area, to a 16 mm wide emitting
aperture open at one edge of the plate. In order to avoid any impedance discontinuities,
a linearly flared profile was adopted for the slot 32 in this example, as shown. Such
a conduit is referred to hereinafter for convenience a "9-16 flared emitter", referring
to the 9 mm central aperture diameter and the 16 mm length of the emitting edge, respectively.
[0062] It is important to note that flaring characteristics other than linear (for example
exponential or other curvilinear characteristics) can be used if preferred. Moreover,
the flaring need not be smooth, and thus steps or other discrete dimensional variations
can be used instead of, or in addition to, smooth flaring profiles. Still further,
and in relation to any flaring characteristic used, the flare need not extend all
of the way from the central aperture to the edge of the plate 30. In some embodiments,
the width of the slot 32 initially is maintained at 9 mm for a predetermined distance
away from the central aperture; the flare then commencing and continuing to the edge
of the plate 30.
[0063] In the orthogonal emission format (as shown in Figure 1), where the conduit is relatively
flat and wide, there is clearly more scope to flare the width dimension of the conduit,
rather than the height dimension. It is not important which of the two dimensions
is flared (or indeed both might be flared); what is important is that the cross-sectional
area of the conduit be increased along at least a part of its path from the microspeaker
surface to the outlet port, and that there is no substantial decrease in cross-sectional
area at any point (because this would create an increase in the Helmholtz-like resonant
properties of the conduit).
[0064] Also secured to the plate 22 is a rear enclosing cavity 40 of volume about 2 ml.
The cavity 40 is coupled to the rear surface of the microspeaker 20, and comprises
an 18 mm diameter tube 42 which, in this embodiment, is 7.9 mm in length and is provided
with flanges 46, 48 at either end, and a plain sealing plate 44 secured to that flange
(48) of the tube 42 remote from the microspeaker 20. In general, that surface of the
microspeaker 20 which is intended to be the sound-emitting surface is herein designated
the "front" surface; the other (parallel) major surface of the microspeaker correspondingly
being designated as the "rear" surface.
[0065] The upper surface of the plate 30 is covered with a simple flat blanking or capping
plate 34.
[0066] In this example, most of the laminar components are made of aluminium sheet stock
of thickness 2 mm, and the use of such 2 mm thick material for the 9-16 flared emitter
plate 30 provides an emission area of 32 mm
2. For this constant thickness plate, the intrinsic volume is linked to the aperture
area, because they are both dependent upon the thickness of the plate.
[0067] Figure 5 shows both the measured, on-axis amplitude-frequency response characteristic
31 of the arrangement shown in Figure 4, and thus incorporating a conduit comprising
the 9-16 flared emitter 30, against, as a reference, the original on-axis response
21 of the microspeaker 20 without the flared emitter in place. This characteristic
(31) compares favourably with those attributed to the proposals of the aforementioned
intermediate document, particularly in respect of the performance required for stereophonic
music and 3D-positional audio. Compared with the characteristics attributed to the
aforementioned proposals, the main resonant peak has been reduced in magnitude and
distributed over a wider frequency range. Moreover, in an arrangement according to
this example of the present invention, the principal peak now lies at 5 kHz, well
above the voice-band (300 Hz to 3.4 kHz) as opposed to lying in the vicinity of 3.2
kHz. Compared to the aforementioned proposals, the invention also improves, by around
3 dB, the gain in the important region extending from 1 kHz to 1.5 kHz.
[0068] Equally importantly, the high-frequency response associated with use of the flared
emitter is usefully sustained to 10 kHz and beyond. As can be seen from Figure 5,
at 10 kHz, the response is only about 6 dB less than the reference microspeaker itself.
[0069] In terms of acoustic properties, therefore, use of this embodiment of the invention
provides an excellent high frequency response, suppressing the Helmholtz resonance
otherwise present intrinsically in its structure.
[0070] It will be appreciated that a complete arrangement configured for stereophonic sound
reproduction and/or 3D-positional audio will comprise a housing that incorporates
a pair of constructions such as that shown in and described with reference to Figure
4, and that the emission slits for the two conduits will typically be disposed to
either side of the housing, as shown, for example, in Figures 1 and 3. It will also
be appreciated that, in such circumstances and order to provide matched performance
from the two microspeakers, the two conduits will in general be configured to exhibit
substantially identical flarings.
[0071] In another embodiment of the invention, described hereinafter, the arrangement also
provides a relatively flat, smooth frequency response by means of the additional integration
of a resonant absorber device.
[0072] In contrast to prior-art devices and other proposals intended to provide a signal
boost at specific, narrow regions of the spectrum in order to compensate for an otherwise
poor HF response, or to accentuate a ring alert, certain embodiments of the present
invention seek to provide a relatively flat frequency response over an extended part
of the audible frequency spectrum.
[0073] In contrast to prior-art devices and other proposals employing resonant volumes and
chambers to amplify the sonic output at certain frequency ranges by resonance, the
present invention uses a resonance-suppressing structure.
[0074] In contrast to proposals employing Helmholtz-type resonant cavities to amplify the
transmitted signals at certain frequencies, some embodiments of the present invention
use a resonant cavity structure to selectively suppress, by absorption and/or attenuation,
residual device resonance at certain frequencies. Furthermore, the resonant cavity
structure is not restricted to a Helmholtz-type configuration, but one preferred embodiment
of the present invention employs an array of quarter-wave stubs as a resonant absorber
array.
Insertion Response
[0075] A simple method of defining clearly the properties of the present invention, and
for quantifying its beneficial characteristics, relates to the concept of the "Insertion
Loss" of a component. The Insertion Loss of a device in a transmission circuit is
defined as the difference between the transmission response without the device in
place, and the transmission response with the device in place. More rigorously, we
can define an "Insertion Response", which will define the change in the transmitted
spectrum owing to the presence of a device (rather than just a simple gain-factor
at a single, specified frequency).
[0076] This concept of an Insertion Response is a powerful method for characterising the
acoustic properties of a device, because the resultant data are independent of the
drive transducer. Figure 6 shows a simple method for measuring the Insertion Response
of a microspeaker emission conduit. First, as shown in Figure 6(a), a microspeaker
is mounted on to a small, fixed sealed rear enclosure having a volume that is typical
of the final application (for cell-phones, this is typically 2 ml). The microspeaker
characteristics are measured by conventional impulse or frequency sweep methods in
an anechoic chamber, using a reference-grade microphone in an on-axis (C-C') position
(Figure 6(a)) directed at an emission outlet, and typically at a distance of about
5 or 10 cm. Next, as shown in Figure 6(b), an emission conduit is attached to the
front surface of the microspeaker enclosure and the measurements are repeated. Here,
L-L' represents the on-axis direction of propagation, now orthogonal to the microspeaker
central axis, C-C'. The Insertion Response is calculated by subtracting the second
response from the first, yielding on-axis results, and can be plotted as a function
of gain (dB) against frequency, as illustrated later. In all of the examples described
later, Insertion Responses have been calculated using the on-axis data for both the
emission conduit measurements and the reference microspeaker measurement, where this
is defined to be aligned along the central emission axis of the transducer or conduit.
[0077] Embodiments of the invention permit simultaneous adherence to the following operational
criteria:
- 1. The Insertion Gain characteristic should be smaller than +10 dB within the voice
spectrum range of 300 Hz to 3400 Hz.
- 2. The Insertion Loss characteristic should be smaller than -10 dB in the frequency
range 500 Hz to 10 kHz (and, ideally, to about 15 kHz).
[0078] From practical experimentation, it has been found that these criteria provide acceptable
quality music reproduction and the effective rendering of 3D-positional audio on a
cell-phone platform; and Figure 7 shows at 33 an insertion response curve for the
arrangement of Figure 4.
[0079] It is a further objective of the invention to produce a still higher-quality sonic
emitter by further reducing or even eliminating the still-present, residual resonant
peak present in the emitted spectrum of the flared emitter. An obvious approach is
to insert damping material into the conduit. However, as already noted, this expedient
significantly reduces the emitted volume level across the spectrum, and the microspeaker
cannot be simply driven harder to compensate for the reduction in sound volume if
it is already operating near its maximum output capability.
[0080] A further embodiment of the invention therefore utilises a compensating resonant
absorber cavity, linked to the flared emitter conduit 30, 32; tuned to the unwanted
residual resonant peak, and having an appropriate Q factor. The compensating resonator
absorbs acoustic energy specifically at the relevant resonant frequency and to the
desired spectral profile, without reducing the sound output across the spectrum, as
would happen if damping material were simply introduced into the cavity. The inventor
has discovered that either a Helmholtz-type resonant absorber or, alternatively, one
or more miniature quarter-wave tube-type resonant structures can be used to reduce
or eliminate residual resonant peaks in the response of the flared emitter.
[0081] Accordingly, an arrangement comprising a compensated structure of this kind using
a Helmholtz-type absorber was fabricated using stacked laminar components as shown
in Figure 4, but with the plate 30 replaced by a plate 50 shown in Figure 8(a), in
order to address the residual spectral peak at about 5 kHz in the characteristic 31
of Figure 5.
[0082] The new "integrated" sonic emitter plate 50 was fabricated by integrating a flared
emission conduit and a suitable Helmholtz absorber into the same plate, and is therefore
readily manufactured. The Helmholtz parameters to absorb the residual 5 kHz resonance
were calculated, and a corresponding circular cavity 54 and a linking channel 56 were
incorporated into the plate 50, which also was formed with a flared 9-16 emitter conduit
52, with the Helmholtz absorber neck coupled to the flared conduit adjacent the microspeaker
surface, opposite the path leading to the emission aperture, as shown in Figure 8(a).
The circular cavity 54 is 7 mm in diameter, and is linked to the flared conduit by
a 2 mm wide channel 56, which is 2 mm deep (the thickness of the plate).
[0083] A cotton damping material (not shown) was used to fill the 7 mm diameter absorbing
cavity 54 in order to match the Q factor of the residual resonance, thereby providing
a damped resonant cavity; a resonant absorber.
[0084] The 9-16 flared emitter 50 with integral Helmholtz absorber, as shown in Figure 8(a),
was inserted into the arrangement of Figure 4 in place of the plate 30 and was characterised
using the same method as for the previous devices, with the results being shown in
Figure 9.
[0085] Figure 9 shows both the measured frequency response 51 of the flared emitter with
integrated Helmholtz absorber, and, as before, the original, on-axis response 21 of
the microspeaker without the flared emitter in place as a reference. Although the
response is not perfectly flat, it more closely resembles the "pure" response of the
reference microspeaker on its own. Figure 10 depicts at 53 the Insertion Response
of the flared emitter with integrated Helmholtz absorber.
[0086] This characteristic has proven excellent for the reproduction of music and for rendering
3D-positional audio; the HF response extending beyond the plot shown, and the overall
response being 800 Hz to 15.8 kHz ±4 dB, and totally free from any sharp peaks or
troughs. At 10 kHz, the response is only about 2 dB less than the reference microspeaker
level. By comparison, a response attributed to the Helmholtz resonator-based arrangement
proposed in the intermediate document was about 30 dB below this reference point at
the same frequency.
[0087] In order to further simplify the mass production of the invention by dispensing with
the need to use cotton damping material (or alternative damping materials), research
was carried out to discover whether one or more quarter-wave tube absorbers could
be used as an alternative to the Helmholtz-type resonant absorber, above.
[0088] This was done by milling narrow, rectangular channels of rectangular profile into
the upper surface of a 9-16 flared emitter plate, such that when the capping plate
34 was added to the surface, the channels formed closed tubes ("stubs"), with open
ends exposed to the flared conduit, adjacent the microspeaker surface, and opposite
to the path leading to the emission aperture. The resulting plate 60, as shown in
Figure 8(b), comprises a 9-16 flared emitter slot 62, as before, and in addition a
plurality of stub absorbers 64a to 64e. The stub absorbers could, of course alternatively
be formed in the underside of the capping plate 32. As a further alternative, some
stub absorbers could be formed in the plate 60 and some in the plate 32, or part of
some or each of the stub absorbers could be formed in juxtaposed, facing surfaces
of each plate.
[0089] Use of this principle was found to be successful, with resonant absorption occurring
at predictable frequencies. It was also found that, if the channels were more than
several square millimetres in cross sectional area, the absorption was greater than
required and not well matched to the Q factor of the residual resonance. Also, these
relatively large section area channels often introduced artefacts in the response
in the form of side-lobe accentuation adjacent the absorption frequency. By reducing
the cross-sectional area of the quarter-wave channels to 1 mm
2 or less, it was found that the resistive losses caused by fluid interaction with
the sidewalls became relatively greater, thus reducing the Q factor and the artefacts.
However, the frequency band over which absorption took place was also reduced to several
hundred Hertz, whereas it was required to effect absorption over about 1 kHz, bandwidth,
as indicated by the peak in Figure 9.
[0090] Accordingly, a distributed array of five quarter-wave channels 64 (a) to 64(e) was
milled onto the surface of a 9-16 flared emitter 60, 62, as shown in Figure 8(b).
Each stub channel 64 was 1 mm in width, and 0.7 mm in depth. In order to expose all
of the channels 64 correctly to the emission conduit adjacent the microspeaker, and
to pack them in the restricted space, a concentric elliptical array pattern was used.
Initially, the lengths of the channels were calculated so as to provide absorption
at 500 Hz intervals from 4.1 kHz to 6.1 kHz. This was successful, but a slightly improved
performance was obtained by adjusting the length of one of the channels (empirically)
to a higher frequency, owing to the interaction between them. The resulting five-stub
array was made according to the following frequency and length parameters, where the
stub reference letters are shown in Figure 8(b).
Stub Reference |
Frequency (Hz) |
Length (mm) |
64(d) |
6100 |
14.1 |
64(b) |
6989 |
12.3 |
64(a) |
5100 |
16.8 |
64(c) |
4600 |
18.6 |
64(e) |
4100 |
20.9 |
[0091] This proved to be very successful, as the results shown in Figure 11 demonstrate.
[0092] Figure 11 shows both the measured frequency response 61 of the flared emitter 60
with integrated quarter-wave array absorber 64a to 64e, and as before, as a reference,
the original, on-axis response 21 of the reference microspeaker without the integrated
flared emitter in place.
[0093] The response 61 follows closely the "pure" response 21 of the microspeaker on its
own. It is excellent for the reproduction of music and for rendering 3D-positional
audio. The HF response extends beyond the plot shown, being 800 Hz to 15.30 kHz ±3
dB, and is totally free from any sharp peaks or troughs. Figure 12 shows at 63 the
insertion loss response of the flared emitter with integrated quarter-wave array absorber.
[0094] It will of course be appreciated in relation to embodiments of the invention utilising
modified emitter plates such as 50 (Figure 8(a)) and 60 (Figure 8(b)) that, as mentioned
above in relation to the basic construction of Figure 4, a complete arrangement configured
for stereophonic sound reproduction and/or 3D-positional audio will comprise a housing
that incorporates a pair of microspeakers, each associated with a respective flared
and compensated conduit, and that the emission slits for the two conduits will typically
be disposed to either side of the housing, as shown, for example, in Figures 1 and
3. It will also be appreciated that, in such circumstances and order to provide matched
performance from the two microspeakers, the two conduits will in general be configured
to exhibit substantially identical flaring and resonant absorption characteristics.
[0095] In certain circumstances, it may be desirable for each plate, such as 50 or 60, to
incorporate more than one resonant absorber. In such circumstances, the resonant absorbers
may or may not be of the same kind; i.e. a Helmholtz-type absorber and one or more
quarter-wavelength channels may be coupled to the same flared conduit.
[0096] In general, it is noted in relation to the invention that:
- 1. The parameters of the sonic emitter can be selected and adapted so as to enable
its integration into a wide range of differing device body sizes and shapes.
- 2. Although aluminium was used for device fabrication in the above examples, it is
equally effective to use other materials, such as plastics materials, although it
is much preferred to use a rigid plastic material rather than a soft plastic (such
as polystyrene) in order to minimise secondary emission.
[0097] It will be appreciated that the invention may be implemented in other forms and utilised
in other applications than those specifically described herein, and that accordingly
the examples used herein are not intended to limit the scope of the claims hereof.
For example, embodiments of the present invention discussed hitherto in this specification
have been principally focussed upon the use of arrangements utilising a pair of microspeakers,
each producing sound intended to emerge from a respective output aperture so as to
provide stereophonic and 3D sound capabilities of acceptable quality. In some other
embodiments of the invention, however, aimed at meeting different criteria, useful
results have been obtained by coupling sound from a single microspeaker via respective
conduits to two or more emission apertures. In such embodiments, it is usual that
both (or all) apertures are coupled to the front surface of the microspeaker. Such
embodiments of the invention find application, for example, to mobile telephones intended
to be operable "hands-free" and which thus may be placed on a stand or other support
so that a user can (for example) conduct a call or listen to music, using the telephone,
whilst doing something else.
[0098] Figure 13 shows a plate member 70 for use in such an embodiment of the invention;
the plate member 70 being generally similar to plate member 60 of Figure 8(b), but
formed with dual flared conduits 72 and 74, each linking the output surface of a single
sonic emitter, such as a microspeaker (not shown), to a respective emission slot region
76, 78. The flared conduits 72 and 74 are merged in a centrally located region 73
which overlies the emission surface (not shown) of the microspeaker. It will be appreciated
that, apart from the fact that two conduits (72, 74) are used to link a single sonic
transducer to a pair of emission slots, the construction of arrangements using plates
of the kind shown at 70 in Figure 13, and their incorporation into electronic devices
utilising them, is substantially the same as for other embodiments of the invention
disclosed herein.
[0099] Referring again to Figure 13, it will be observed that the plate 70 also includes
an array 80 of quarter-wave stub absorbers; the array in this case comprising five
stubs, 80(a) to 80(e), of different lengths, split for convenience into groups of
two and three disposed respectively to either side of the aforementioned central region
73 at which the dual conduits merge.
[0100] In one embodiment, the merged central region 73 of the conduits 72, 74 is 8 mm in
width, and the width of each conduit then flares linearly and smoothly to about 20
mm at its respective emission aperture region. In that embodiment, and in order to
suppress an intrinsic resonance of the system, occurring at about 4360 Hz, the array
80 of five quarter-wave stubs, split into groups as aforesaid for convenience, was
integrated into the plate. Each stub comprised a channel 1.0 mm wide and 0.8 mm deep,
but with their respective lengths matched to the required absorption frequencies in
accordance with the following table:
Stub Reference |
Frequency (Hz) |
Length (mm) |
80(a) |
3360 |
25.5 |
80(b) |
3860 |
22.2 |
80(c) |
4360 |
19.7 |
80(d) |
4860 |
17.6 |
80(e) |
5360 |
16.0 |
[0101] It may be of assistance to the reader to provide certain information about the properties
of Helmholtz resonators and quarter-wave tubes, and thus the following appendices
are furnished for convenience.
Appendix 1: Helmholtz Resonator Properties
[0103] Diagrams of a theoretical Helmholtz resonator, without and with flanges, are shown
in Figures 14(a) and 14(b) respectively. The resonator comprises a rigid walled cavity
1 of volume V, connected to the ambient by way of a neck 2 having length L, and cross
sectional area S. The structure behaves as a resonant system when the dimensions of
all of these parameters are significantly smaller than the wavelengths λ under consideration.
For audio waves in the relevant range for the present invention (say 500 Hz to 6 kHz),
the wavelengths lie between 680 mm and 57 mm. It is assumed that the neck constriction
is sufficiently short that all fluid particles may be assumed to move in phase when
actuated by a sound pressure wave.
[0104] Referring to the aforementioned publication of Kinsler et al., if λ>>L, then the
mass of air (or other fluid) in the neck moves as a unit, represented by an inertance.
Similarly, if λ>>V
⅓, then the volume of air V represents a compliance element (with associated stiffness),
and if V>>S
½, the opening radiates sound as a simple source, corresponding to a resistance element.
This acoustical system is directly analogous to a mechanical oscillator, in which
its inertance, compliance and resistance correspond to the mass, compliance and resistance
of, for example, a damped, sprung piston, and also to the inductance, capacitance
and resistance of an L-C-R resonant electrical circuit.
[0105] When sound energy is incident on to the entrance to the tube, the mass of air in
the neck moves back and forth against the compliance of the internal fluid; resonance
occurs when the reactance approaches zero. The structure is characterised by its resonant
frequency, ω
0 (radians/sec), [or f
0 (Hz)] and the dimensionless "quality factor" or Q, by the following two equations.
[0106] Here, the factor L' is used for the effective length of the neck, rather than the
physical length L because of its radiation-mass loading.
[0107] According to Kinsler et al., at low frequencies a circular opening of radius a is
loaded with a radiation mass equal to the fluid contained in a cylinder of area πa
2, and length 0.85a, if terminated in a wide flange 3 (Figure 1 (b)), and 0.6a if not
flanged (Figure 1 (a)). From this, it can be shown that the relationship between L'
and L is as follows.
[0108] If the neck opening is a circular opening of radius a then, in the limit when L approaches
0, L' approaches a value of 1.7a (flanged) or 1.5a (not flanged).
[0109] The Q factor of a Helmholtz resonator can be defined as the quotient of the resonant
frequency, ω
0, and the width of the resonant peak at the half-power points (ω
1 and ω
2) lying at the -3 dB points flanking the peak at ω
0.
[0110] Kinsler et al. also note, in the publication referenced above, that the resonator
acts as an amplifier, in effect, with Q representing the ratio of the pressure amplitudes
inside and outside the cavity, and hence gain factor, G (dB), is given by:
[0111] Hence a typical Q value of 96, obtained experimentally, would represent a gain factor
of 40 dB.
[0112] As has been noted in the description of the invention, by consideration of equation
2, the Q factor of the emitting cavity can be reduced either by reducing V, reducing
L, or increasing S, or by any combination of or, preferably, all of these. In addition
to the Q factor, another important feature of the resonant peak of a resonant system
is its overall shape, and this is governed by the presence of any associated non-reactive
impedances, such as resistive components, and their relative values. Their effect
is to dampen the resonance and limit the range of the impedance of the system at resonance,
such that it does not approach an infinitely large value or a zero value. Accordingly,
it is possible to have two resonant systems featuring an identical Q value, but which
possess differing spectral profiles. One might display a resonant curve in the frequency
domain featuring concave-upwards flanks to the resonant peak, for example, whereas
the other might exhibit flanks having a concave-downwards profile. In one aspect of
the present invention, resistive elements are introduced in order to refine and match
the shape of the resonant profile of a compensating cavity to that of an emitting
cavity. The equations described herein can be extended to include such resistive elements
and model resonant system behaviour in even more detail.
Appendix 2: Quarter-Wave Tube Resonator Properties
[0113] The acoustic properties of various closed- and open-tube (or pipe) configurations
are well known to those skilled in the art. The relevant configuration here is the
"single-ended, closed, quarter-wave tube", or "λ/4 tube". For frequency F
0, this is a tube that is one-quarter of a wavelength in length, being closed at one
end and open at the other. When exposed to a sonic sound field at frequency F
0, some of the incident wave energy travels in to, and along, the tube, and then it
and is reflected back from the closed end. After a time period corresponding to the
passage of one half of a wavelength, the incident wave at the tube entrance will have
undergone a 180° phase shift. Consequently, destructive interference occurs when it
interacts with the original, reflected wave emerging from the quarter-wave tube; it
is resonant at frequency F
0. Sound energy primarily in the region of that one specific wavelength interacts with
the tube.
[0114] The amount of energy interaction between the sound field and the tube is dependent
on the relative cross sectional area of the λ/4 tube. The "Q" factor is determined
by the resistive losses relating to frictional interaction between the fluid medium
(air) and the tube walls, and also by thermal energy loss to the walls of the tube.
If the tube diameter is made very small, then the resistive losses by both mechanisms
increase, and the "Q" factor decreases. Accordingly, by making very narrow diameter
tubes (in the form of rectangular section grooves), the inventor has found that it
is possible to manufacture resonant absorbers having characteristics that can be controlled
very accurately, and which can be used advantageously for compensating the residual
resonance of a minimally resonant conduit.
[0115] For example, where L is the length of the tube and C is the speed of sound, under
ambient conditions, the absorption frequency, F
0, is given by:
and hence an absorbing quarter-wave tube for, say, a frequency of 5 kHz, would be
required to have a length of 17.2 mm.