FIELD OF THE INVENTION
[0001] This invention relates to reflex-type loudspeakers, and to liners or inserts for
the vented port(s) of such loudspeakers.
BACKGROUND ART
[0002] A significant proportion of loudspeakers which are currently commercially available
are of the type commonly known as bass reflex loudspeakers. A bass reflex system (also
known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that
uses the sound from the rear side of the diaphragm to increase the efficiency of the
system at low frequencies as compared to a typical closed box loudspeaker or an infinite
baffle mounting. A bass reflex loudspeaker typically has one or more openings in the
loudspeaker enclosure (called reflex ports or vents), each of which usually consists
of a rigid pipe, duct or tube (typically circular or rectangular in cross-section)
mounted in the front or rear face of the loudspeaker enclosure, leading from the air
volume behind the driver to the external air. The air in this opening behaves as an
acoustic mass whereas the air contained within the enclosure behaves as an acoustic
compliance; together these form an acoustic resonator known as a Helmholtz resonator.
The frequency at which this acoustic resonance occurs is determined by the length
and cross sectional area of air in the opening, the volume of air within the enclosure,
and the speed of sound. Acoustic radiation from the rear of the loudspeaker passes
through this acoustic resonator, the acoustic resonator providing in a band-pass response.
At the frequency of this acoustic resonance the high pressure in the enclosure reduces
the cone motion and acoustic radiation from the front of the loudspeaker diaphragm.
The combined output from the opening and front of the diaphragm is in the form of
a 4
th order high-pass filter. With suitable choice of driver parameters, enclosure volume
and port dimensions a desired response can be achieved. For example a maximally flat
4
th order butterworth high-pass response is the most obvious choice but, depending on
the design constraints, there are numerous other choices.
[0003] Reflex systems are widely used since they provide a better combination of efficiency
and low frequency extension compared to closed box systems. They also have the benefit
of reducing the diaphragm excursion at frequencies around the enclosure tuning frequency
where the duct provides the main acoustic output.
[0004] In use, the low frequency response of a loudspeaker is strongly dependant on room
dimensions, construction materials and the relative positions of listener and loudspeaker.
Furthermore, what is an acoustically desirable performance is often a subjective choice,
with different listeners preferring enhancement or attenuation of different sound
frequencies; however, any single size and configuration of reflex port will have a
predetermined and largely fixed effect on the overall acoustic performance of a loudspeaker.
One practical method of adjusting the low frequency response of a loudspeaker system
is to vary the Helmholtz resonance by altering the properties of the port.
[0005] For example, to vary the acoustic performance of a reflex-type loudspeaker, cylindrical
plugs (bungs) of foamed polyurethane or felt have been inserted into the opening of
the reflex port, and these adjust the resonance in the port by partially blocking
it; such an approach is rather crude, such "bungs" introduce turbulence and resistive
losses with only a minor change in tuning frequency. The resistive losses introduced
by air flowing through the porous bung reduce the bass output of the port with no
improvement to low frequency extension. The small decrease in tuning frequency does
produce a small improvement in low frequency extension. The turbulence occurs where
high air velocity gradients occur in the air flow due to small port area, abrupt changes
in port area or discontinuities such as those introduced by the bung. The effect of
turbulence is to introduce spurious noise and distortion, and also results in a loss
of low frequency energy; consequently turbulence may limit undistorted bass output.
[0006] A better approach to adjusting the Helmholtz frequency is to provide the user with
some means to adjust the port length and/or area. To avoid turbulence the ports should
preferably be flared at both ends avoiding discontinuities of the wall surface along
the length of the port.
[0007] For example interchangeable ports of different dimensions have been suggested in
GB2352924. In such arrangements, a range of ducts, each of different length and/or cross-sectional
area, is provided, so that a listener can change the acoustic performance simply by
removing and replacing one duct with another of different dimensions. Such arrangements
necessitate the storage of a number of alternative ducts which, because reflex ports
are usually substantially rigid, takes up an undesirable amount of space. In addition,
such replaceable ports need to be firmly fixed in position relative to the loudspeaker
enclosure, so a user needs to have tools available to be able to undo a duct for removal
and to secure a duct in place. Accordingly such systems are not attractive to users.
[0008] Another performance limitation of ports is the response peak due to longitudinal
acoustic resonance occurring within the duct. This resonance is highly undesirable
since it can cause audible colouration. A method of reducing such resonances is described
in
GB2488758. Providing the user with interchangeable ports constructed in this way would be highly
beneficial, but extremely costly.
SUMMARY OF THE INVENTION
[0009] The present invention provides an acoustic insert for lining an outer tube fitted
to a loudspeaker enclosure to form a loudspeaker reflex port, the insert comprising
an elongate hollow tube formed of a flexible material, having an outer wall, sized
and configured to seat within the outer tube, and an inner wall extending in the elongate
direction, the inner wall having a circumference (in the plane transverse to the elongate
direction) which varies along the length of the insert.
[0010] Such inserts are inexpensive to manufacture, easy to use and can be configured to
allow a large range of tuning frequency adjustment since both area and length may
readily be varied. For example, one form of tuning will give a flat response with
more upper bass (such as is suitable for rooms with excessive low frequency absorption)
whereas a different (lower) tuning will give a sloping bass response with more very
low bass and less upper bass. Because the tubes are easily insertable and removable
(partly as a result of their being made of flexible material), it is a simple matter
to remove an insert of one size/configuration/material from the outer tube and replace
it with a different one, thus changing the acoustic effect provided by the reflex
port, enabling a user easily to change the frequency response of the loudspeaker as
desired. Accordingly, the present invention has the potential for loudspeakers to
be made with acoustic performance which can be varied so as to be equally as acceptable
in the home as in applications requiring high accuracy and neutrality. In addition,
the inserts can be used in different lengths on different models in a range of products.
Forming the insert of low density foam material is important to provide the ability
to absorb unwanted sound frequencies, and enables the insert to be flexible. In particular
sound is absorbed at the frequency of longitudinal acoustic resonances in the port,
reducing output above the band-pass region. The foam is preferably closed cell rather
than open or reticulated, so as to form an airtight seal with the outer supporting
tube and so avoid the air leaks and resistive losses which are known to be highly
detrimental to reflex enclosures and result in decreased bass and distortion.
[0011] In addition the inserts can be used with the reflex ports of existing reflex loudspeakers,
provided the existing reflex port (equating to the "outer tube") and the inserts are
of matching size and configuration. The circumference (in the plane transverse to
the elongate direction) of the inner wall of the insert may be substantially symmetric
at any point along the length of the insert. This makes for ease of use (in that a
user does not have to worry about the orientation of the insert when it is inserted
into the reflex port), ease of calculation of acoustic effect, and ease of manufacture.
For essentially the same reasons, the inner wall of the tube may be substantially
symmetric in the elongate direction. The insert (as tubes generally do) has two open
ends, and the inner wall of the insert may be narrower at at least one point between
the ends than it is towards the ends of the tube; such an arrangement means that the
insert acts as a kind of "throttle", the calculations to determine the acoustic effect
of which are reasonably easy to calculate. Similarly, one or both ends of the insert
may be flared outwardly, so as to blend in smoothly with the inner wall of the outer
tube and avoid turbulence.
[0012] Reflex ports are commonly substantially circular, square, elliptical, racetrack or
rectangular shaped along their inner wall; the outer wall of the insert may be similarly
shaped, and the inner wall of the insert may be any one of these shapes (though for
ease of manufacture the shapes of the inner and outer walls of the tube will typically
match, but this need not necessarily be the case). The outer wall of the insert may
be the same size (radially from the elongate axis) as the inner surface of the outer
tube, so as to fit snugly therein, or it may be slightly oversized so as to have a
slight interference fit. This is advantageous, as it helps ensure that the insert
in use remains in the correct longitudinal position within the outer tube, and does
not become dislodged except when a user wishes to remove it. Also, the foam insert
does not rattle or vibrate within the outer tube which could detract from the audible
performance of the loudspeaker. Other ways of retaining the insert in the correct
longitudinal position within the outer tube (apart from when a user wishes to remove
it) will be apparent to the skilled person, such as by providing a releasable adhesive,
or forming the outer wall of the insert with lugs, ribs or recesses which mate with
matching formation on the inner wall of the outer tube. Where cost allows it is also
possible to use the resilience of a flange on the foam insert to provide the retaining
force for a bayonet style fixing. It is also possible to mount the port in the terminal
panel or other convenient part of the loudspeaker.
[0013] The insert may be the same length as the outer tube, or it may be longer or shorter.
The insert may be provided with a lip of greater size than the inner circumference
of the outer tube at one end of the tube, so that it is easy for a user to place the
insert inside the outer tube at the correct longitudinal position, and not insert
it "too far"; also, such a lip makes it easier for a user to extract the insert when
desired.
[0014] The insert may be formed of or comprise any acoustic absorptive material, such as
foam, felt,; we have made inserts out of closed cell foamed polymer material, as this
is flexible yet robust and relatively inexpensive and easy to manufacture. The inner
surface of the insert may be slightly rough, so as to introduce a small amount of
turbulence in the adjacent boundary layer of air.
[0015] The invention extends to a kit comprising a number of such inserts, each of different
dimensions, configuration and/or material, so that a user can easily tailor the frequency
response as required. Such a range of insert lengths may be used with a set of interchangeable
outer tubes of different lengths so as to provide further scope for tuning the loudspeaker
performance. A loudspeaker comprising one or more such inserts is also within the
ambit of this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention will now be described by way of example, and with reference to the
accompanying drawings, in which;
Figure 1 is a schematic cut away view of an acoustic insert in accordance with the
invention in position inside an outer tube forming a reflex port.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0017] Figure 1 is an isometric, cross-sectional schematic of an arrangement 2 comprising
an acoustic insert 6 seated within an outer tube 4 which is fitted to the enclosure
of a loudspeaker (not shown). The acoustic insert 6 and the outer tube 4 together
form an open ended, cylindrical tube, with a first end 8 opening at the wall of the
loudspeaker enclosure and a second end 10 inside the enclosure. The insert flares
outwardly at the first and second ends, with a central portion 14 of smaller radius
(around the X axis shown) than at the two ends. The first end of the outer tube 4
has a lip 16 which engages with the wall of the loudspeaker enclosure as is known
in the art. The insert, which is made of low density foam material, is also formed
with a matching lip 18, which seats on the lip 16 when the insert is fully inserted
within the outer tube 4; this prevents the insert from being pushed too far inside
the outer tube, and instead ensures that the insert 6 is in the correct longitudinal
position (along the X axis) relative to the outer tube 4. Also, the lip 18, being
outside the loudspeaker enclosure, is easy to grasp if the insert 6 is to be removed.
In use, the insert tunes the resonance of the arrangement to attenuate certain frequency
ranges (but not others), according to the configuration and dimensions of the inner
wall of the insert 6), so as to alter the acoustic performance of the loudspeaker.
The resilience of the material from which the insert is formed may be such that in
use it deforms slightly and/or the inner wall of the insert deflects and so absorbs
sound.
[0018] It will of course be understood that many variations may be made to the above-described
embodiment without departing from the scope of the present invention. For example,
there may be additional means to hold the insert 6 in place within the outer tube
4, as described above; a suitable place for the application of a releasable adhesive
is at the interface between the lips 16, 18; if a peelable adhesive is applied to
one or other lip, this holds the insert securely in place until, when the insert is
to be removed, the insert lip 18 is peeled away from the port lip 16 and the insert
can then be pulled out of the outer tube. Other means could be used to releasably
connect the lips 16, 18, such as a bayonet fixing, and/or the lip 16 could be separate
from the outer tube 4: this would be advantageous, as such a separate lip could then
be configured for mounting the arrangement to a loudspeaker enclosure, with different
configurations being available for use with different enclosures. Where the foam insert
is self-supporting, part of it may protrude beyond the end of the outer tube. The
insert 6 is shown with only a single constriction (generally at 14), however there
could be two or more constrictions spaced along the X axis, and the longitudinal shape
could be like a wave (e.g. sinusoidal). The inner surface of the tube 6 is shown as
being longitudinally symmetric (i.e. along the X axis) and concentric, but there may
be benefits in some applications for it to be asymmetric (e.g. the "peaks" and "troughs"
in a wave-shaped inner surface may not be equally spaced). As stated above, the inner
circumference of the insert is symmetrical at any point along the X axis, however
it may be any shape, including asymmetric; in general however, the acoustic calculations
are simplest for where the insert is cylindrical and/or symmetric/concentric. Those
skilled in the art will understand that, for a generally cylindrical insert (as illustrated),
the internal diameter may be varied, as well as its length, to give a wider range
of tuning frequencies for less variation in depth, and that similar considerations
apply to non-symmetric and/or non-concentric inserts. The inner surface of the insert
6, being made of foam, has a certain roughness, however this surface could in some
configurations be coated so as to have a smooth surface; alternatively the insert
6 could be formed with a smooth inner surface. For example by moulding the insert
in a foamed material the surface of the moulding forms a solid skin which is sufficiently
smooth. The insert 6 could be coloured or bear suitable markings to indicate its effect
on tuning the frequency response of the loudspeaker (for example, the inserts could
vary in shade, with darker hues indicating a "lower" tune and lighter hues a "higher"
tune, or they could be numbered according to a list of different degrees of tuning).
The insert may advantageously also comprise means for damping longitudinal resonance
such as the impermeable, frequency-dependent acoustic leakage path arrangements described
in
GB2488758 (which provides a frequency-dependent acoustic leakage path in the reflex port tube,
such as by having a port conduit acoustically coupling the interior of the enclosure
to a region external thereto, the port conduit comprising an acoustic leakage path
through a motile part thereof in a direction transverse to a longitudinal axis of
the port conduit, the acoustic leakage path having a relatively high acoustic impedance
at a first frequency value, and a relatively low acoustic impedance at a second, lower,
frequency value; this can be achieved with a plurality of holes in the port tube and
an impermeable membrane lining the conduit). The arrangement could be provided with
a front flare, or annular facing element which has the dual purpose of clamping the
front flange or lip 18 of the insert 6 to the lip 16 and of improving the appearance
of the arrangement 2 when assembled. Furthermore, where different variations or alternative
arrangements are described above, it should be understood that embodiments of the
invention may incorporate such variations and/or alternatives in any suitable combination.
1. An acoustic insert for lining an outer tube fitted to a loudspeaker enclosure to form
a loudspeaker reflex port, the insert comprising an elongate hollow tube formed of
a flexible material, having an outer wall sized and configured to seat within the
outer tube and an inner wall extending in the elongate direction, the inner wall having
a circumference which varies along the length of the insert.
2. An acoustic insert according to Claim 1 wherein at any point along the length of the
insert, the circumference of the inner wall of the insert is substantially symmetric.
3. An acoustic insert according to Claim 1 or 2 wherein the inner wall of the insert
is substantially symmetric in the elongate direction.
4. An acoustic insert according to Claim 1, 2 or 3 wherein the insert has two open ends
and the inner wall of the insert is narrower at at least one point between the ends
than it is towards the ends of the insert.
5. An acoustic insert according to any preceding claim wherein the insert has two open
ends and the inner wall of the insert is flared outwardly at at least one of the ends.
6. An acoustic insert according to any preceding claim wherein the inner and/or outer
wall of the insert, when viewed along the elongate direction, is substantially circular,
elliptical, square or racetrack shaped.
7. An acoustic insert according to any preceding claim wherein the outer wall of the
insert is substantially the same size as an inner surface of the outer tube.
8. An acoustic insert according to any preceding claim wherein the outer wall of the
insert is sized so as to be an interference fit inside the outer tube.
9. An acoustic insert according to any preceding claim wherein the insert is the same
length as the outer tube.
10. An acoustic insert according to any preceding claim wherein the insert is provided
with a lip of greater size than the inner circumference of the outer tube at one end
of the insert.
11. An acoustic insert according to any preceding claim wherein the insert comprises an
acoustic absorptive material.
12. An acoustic insert according to any preceding claim wherein the inner wall of the
insert deflects so as to absorb sound.
13. An acoustic insert according to any preceding claim wherein the length of the tube
is substantially the same as the length of the outer tube.
14. An acoustic insert according to any of Claims 1 to 12 wherein the length of the tube
is not the same as the length of the outer tube.
15. A kit comprising a plurality of acoustics inserts according to any preceding claim,
of differing dimensions, configuration and/or material.
16. A loudspeaker of the reflex port type comprising one or more acoustic inserts according
to any of Claims 1 to 13.