BACKGROUND
1. Technical Field
[0001] This invention relates to loudspeaker systems and, more specifically, to a novel
low frequency loudspeaker system for vehicles.
2. Related Art
[0002] When a loudspeaker driver unit is mounted into a small sealed enclosure, the diaphragm
is primarily influenced by the stiffness of the air as it is compressed into the finite
volume of the sealed enclosure. One effect of this is to raise the coupled resonance
of the driver-cabinet system which may undesirably curtail the low frequency (LF)
extension. Additionally, in vehicles, mounting the loudspeaker into the body in white
often means presenting a complicated impedance condition to the drive unit and may
generate turbulent noise, pressure imbalance and acoustic short circuit. Furthermore
it is often an unknown quantity, and forces the driver performance outside its carefully
predicted bounds. Mounting the driver unit into a small enclosure (e.g., cabinet,
box) requires that it should be mechanically very stiff and heavy to minimise the
influence of the enclosed air stiffness - this in turn will make the driver unit expensive.
[0003] The cavity into which the drive unit is placed, usually some available space in the
vehicle bodywork and doors, typically includes a number of interconnected pathways
often resulting in a non-uniform pressure distribution presented to the rear of the
diaphragm. This loading has been observed to drive asymmetrical mode-shapes in the
driver's more compliant suspension components, in particular, resulting in so-called
"rocking" modes. These modes are not controlled by the dominant motor damping element
in the driver unit and are therefore frequently of a magnitude sufficient to cause
fouling of the software parts against the motor system metalwork. In view of the above,
there is a need for improvements in designing loudspeakers into small and complex
cavities.
SUMMARY
[0004] A loudspeaker system is presented which comprises an enclosure with two separate
air volumes which are acoustically linked by a port enclosing a plug of air; and an
electro-acoustical transducer that converts electrical power into acoustical power
and that includes a diaphragm having a front side and a rear side; the front side
of the diaphragm radiates sound into a listening environment and the rear side is
directly acoustically coupled to one of the volumes of the enclosure. The air volume
which is directly coupled to the diaphragm and the plug of air in the port are tuned
to form a resonant system driven by the transducer; and the volume that is not directly
coupled to the diaphragm, is coupled to the plug of air in the port.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] These and other objects, features, and advantages of the present invention will become
apparent in light of the drawings and detailed description of the present invention
provided below. The components in the figures are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the invention. Moreover,
in the figures, like reference numerals designate corresponding parts throughout the
different views.
FIG. 1 is a cross-sectional view of a novel loudspeaker system having two air volumes
coupled by a short port;
FIG. 2 is an exploded perspective view of the system shown in FIG. 1;
FIG. 3 is a cross-sectional view of another example of a loudspeaker system having
two air volumes coupled by a long port;
FIG. 4 is block diagram illustrating a mechanical model of a novel loudspeaker system
having two air volumes coupled by a port;
FIG. 5 is a diagram depicting the sound pressure levels of a free air loudspeaker
and a loudspeaker placed in a conventional enclosure;
FIG. 6 is a diagram depicting the sound pressure levels of a free air loudspeaker
and a novel loudspeaker system having two air volumes coupled through a port;
FIG. 7 is a diagram depicting the sound pressure level of a loudspeaker placed in
a sealed enclosure in comparison to the sound pressure level of a loudspeaker of a
novel loudspeaker system having two air volumes coupled by a port;
FIG. 8 is a diagram depicting the diaphragm displacement spectra of an Adapter for
Passive Diaphragm Control (APDC);
FIG. 9 is a diagram depicting the calculated Sound Pressure Level (SPL) based on the
diaphragm displacement of an Adapter for Passive Diaphragm Control (APDC);
FIG. 10 illustrates the pressure distribution within an exemplary enclosure for the
case where a driver alone is placed into the enclosure; and
FIG. 11 illustrates the pressure distribution within an exemplary enclosure for the
case where an APDC is used.
DETAILED DESCRIPTION
[0006] FIGS. 1 and 2 illustrate a novel loudspeaker system comprising a loudspeaker drive
unit 1 that includes a sound-producing conical diaphragm 2 having front and rear faces,
and a two parts enclosure located on the rear of said loudspeaker drive unit 1 creating
an acoustical path from the rear face of said diaphragm 2 into the rear enclosure.
The enclosure forms a rear diaphragm control system and includes two air volumes,
an air volume 3 enclosed in an enclosure piece 4 and an air volume 5 enclosed in an
enclosure piece 6, which are acoustically coupled to each other through a port 7 in
a partition wall of enclosure piece 6. Enclosure piece 6 including port 7 will be
also referred to as Adapter for Passive Diaphragm Control (APDC) in the following
description. Volume 5 is directly coupled to the diaphragm 2. Volume 3 is coupled
to a plug of air in the intervening port 7 and, accordingly, not directly coupled
to the diaphragm 2.
[0007] The drive unit 1 is mounted in the enclosure piece 6 that serves as a ported coupling
device and interfaces the drive unit 1 with the environment into which it is placed,
e.g., in the present case enclosure piece 4 having a tube-like shape with an end wall.
FIGS. 1 and 2 illustrate a system in which the volume 5 is predetermined and volume
3 does not need to be predetermined. Volume 3 may be replaced with any alternative
sealed or even leaky volume representing a less deterministic part of the enclosure
and may therefore be a more hostile acoustic environment, e.g., a cavity within a
vehicle bodywork.
[0008] In the system of FIGS. 1 and 2, the enclosure piece 6 is a tube-like ring with a
cone-shaped end piece (partition wall) carrying port 7 and extending into enclosure
4. However, enclosure piece 6 may have any other shape that encloses the same volume.
As the first volume 3 is larger than the second volume 5, the distance from the diaphragm
2 to the rear of the enclosure piece 6 is relatively short and the intended operating
frequency sufficiently low, such that standing waves in that direction within enclosure
piece 6 are not a problem. If desired, sound absorbent material (not shown) can be
provided within enclosure piece 6, enclosure piece 4 or both to provide an additional
damping element should this prove to be necessary as an optimisation step.
[0009] The port 7 may be, e.g., a simple aperture or a connector tube that may run at virtually
any angle to the loudspeaker drive unit's axis. The aperture (or apertures) may be
in the form of a circular hole in the partition wall as shown in FIGS. 1 and 2. As
shown in FIG. 3, the aperture may be a connector tube 14 arranged in the centre of
the partition wall extending outwardly from the front and (or) the rear of the partition
wall of the enclosure piece 6. The tube 14 has a specific diameter and a specific
length defining an acoustic mass in the tube 14 referred to as plug of air. Many other
arrangements of apertures and tubes are applicable such as multiple coupling ports,
apertures/tubes placed out of the central regions of the partition wall, or apertures/tubes
having a cross-section other than circular. Tubes with dimensions which change with
distance from the loudspeaker drive unit are applicable as well. For specific applications,
a second diaphragm 15 suspended in the aperture/tube interconnecting the two volumes
of the enclosure, e.g., in or at the port, may be used to increase the port mass (plug
of air) and reduce turbulence and losses at high power.
[0010] Beside the diaphragm 2, the drive unit 1 includes an apertured chassis 8, a magnet
assembly 9, a voice coil 10, a surround 11, a spider 12, and a dust cap 13 in the
centre of the diaphragm 4. When the loudspeaker drive unit 1 is in operation, sound
waves are able to pass rearwardly from the rear of the diaphragm 2 into the sound
path established by the two enclosure pieces 4, 6 and the port 7. From the front of
the diaphragm 2 sound waves travel through the apertured chassis 3 to the listening
environment. The loudspeaker drive unit 1 is of the modern reduced physical depth
type, also known as inverse motor loudspeaker, where the magnet assembly 9 is arranged
at the front side of the diaphragm 2, i.e., in the sound path leading to the listening
environment. Therefore, beside the minimum depth of the loudspeaker system, this loudspeaker
design provides also very good heat dissipation for the motor system, i.e., the magnet
assembly 9 and the voice coil 10.
[0011] FIG. 4 shows a mechanical abstraction of the loudspeaker system of FIGS. 1-3 using
standard dynamic theory for modelling purposes. In the model, an air mass Mv enclosed
in the port 7 is tied to ground G by a spring (spring constant K1) provided by the
first volume 3 and to the diaphragm 2 (mass Mmd) by another spring (spring constant
K2) provided by volume 5. A force F is applied to the diaphragm 2 moving its mass
Mmd which is tied to ground G by a spring (spring constant K3) and a damper (damping
constant C) provided by the suspension 11 and the spider 12.
[0012] The system is similar in many ways to the traditional bass reflex design, however
the air mass Mv in the port 7, being enclosed, does not form part of the acoustic
radiation and is thus tied to ground G via the spring having spring constant K1 in
the mechanical model. This second degree of freedom serves only to provide the necessary
rear loading to control the diaphragm in the tuned system. The force F applied to
the driver and that experienced by the air mass Mv in the port 7 are expressed mathematically
and the resulting equations solved to provide the diaphragm displacement. The sound
pressure level (SPL) at 1m, assuming the system radiates from an infinite baffle can
be calculated from the diaphragm displacement. Solving the equations below for x
1 will provide a prediction of the piston displacement x
1 from which diaphragm displacement and, accordingly, SPL predictions can be made.
[0013] Force F acting upon piston:
![](https://data.epo.org/publication-server/image?imagePath=2009/28/DOC/EPNWA1/EP07024287NWA1/imgb0001)
[0014] Force acting upon air mass in port (losses ignored):
![](https://data.epo.org/publication-server/image?imagePath=2009/28/DOC/EPNWA1/EP07024287NWA1/imgb0002)
wherein K
tot = (K
1 • K
2) / (K
1 + K
2) + K
3, K
3 is the acoustical stiffness element of the loudspeaker, S
d is the area of the diaphragm, S
v is the area of the port, and x
2 is the displacement of the plug of air in the port.
[0015] FIG. 5 depicts the results of a comparison of a loudspeaker drive unit operated in
free air (solid line) and a loudspeaker drive unit placed in an enclosure enclosing
a 5.5 litre (L) air volume (dotted line). As can be seen, the sound pressure level
(SPL) in dB over frequency f in Hz of the free air drive unit exhibits a higher sound
pressure level at low frequencies (e.g., below 100 Hz) than the boxed drive unit exhibiting
a peak at frequencies above 100Hz.
[0016] FIG. 6 depicts the results of a comparison of a loudspeaker drive unit operated in
free air (solid line) and loudspeaker drive unit placed in an enclosure enclosing
a 5.5 litre (L) air volume and having an APDC (dotted line) as shown in FIGS. 1-3
(the sum of the two air volumes are equal to the volume of a conventional enclosure).
In the present example, the port diameter is 20mm, the port length is 20mm, the coupler
volume (of enclosure piece 6) is 0.2L and the rear volume (of enclosure 4) is 5.3L.
The sound pressure level (SPL) in dB over frequency f in Hz of the free air drive
unit exhibits a higher sound pressure level at low frequencies (e.g., below 100 Hz)
than the drive unit that has ripples at frequencies below and above 100Hz.
[0017] However, comparing the loudspeaker drive unit in a sealed enclosure with APDC with
the same drive unit in a sealed enclosure without APDC, the low frequency peak in
the response provides an improvement in the bass extension for woofer and subwoofer
applications and the following dip in the response forms the roll off which may be
augmented with an active or passive filter, e.g., an active filter implemented in
a digital signal processor (DSP). FIG. 7 depicts the results of the comparison of
the loudspeaker drive unit in the sealed enclosure with APDC (dotted line), and the
same drive unit in the sealed enclosure without APDC (solid line), where the loudspeaker
drive unit in the sealed enclosure with APDC is supplied with double input power (+3dB).
The driver used in the modelling was the JM85 Land Rover rear door woofer. The system
is scalable and can be used also with loudspeakers with lower bass extension. It can
be seen from FIG. 7 that while adding a sealed box reduces the low frequency efficiency,
the APDC restores this efficiency to equal the free air case at lower frequencies
facilitating bass reproduction in small boxes.
[0018] FIGS. 8 and 9 show results of the experimentally measured diaphragm displacements
over a range of low frequencies at both 1v and 6v using a Klippel analyser. These
measurement results are compared to the simulation from the derived equations. From
these displacement curves, it is possible to predict the SPL in an infinite baffle.
It can be seen that there is some loss effect at high power levels due to turbulent
effects. Changing (in particular increasing) the port dimensions can optimize this
even at the expense of a little of the desired coupler effect. The results demonstrate
that the diaphragm is indeed influenced in the same manner as predicted.
[0019] The plug of air in the port is separating the tuned air volume coupled directly to
the diaphragm from the untuned volume established by, e.g., a car body cavity. So
the hardly controllable behaviour of the untuned air volume is no longer determining
the acoustic properties of the complete loudspeaker system. As shown in the examples
of FIGS. 1 and 3, an appealing aspect of this is that the second volume 5 directly
coupled to the diaphragm and the port can be formed from a simple funnel-shaped moulding
(e.g., even part of the basket or frame) and therefore would be cheap to add. The
benefits would be reducing or eliminating coil rubs due to rocking modes and knowing
precisely what the in-situ diaphragm velocity is and being able to simulate or tune
the full system more accurately. The APDC may also function as a simple protective
rainwater cowl.
[0020] Referring to FIGS. 10 and 11, a conceptual enclosure 15 was created representing
a vented subwoofer box having an opening 16. The opening 16 is to interface with the
vehicle bodywork and a convenient boundary condition was applied to simulate entry
into a long pipe so it is neither sealed nor vented by design. A driver having a diaphragm
17 was loaded into the enclosure both with and without an APDC coupler 18. A coupled
acoustic analysis (half model) of the driver plus enclosure and driver in APDC plus
enclosure was computed. The results are shown in FIGS. 10 and 11 illustrating the
pressure distribution within the enclosure 15 up to the rear surface of the diaphragm
17 for the case where a driver alone is placed into the enclosure 15 (FIG. 10) and
where an APDC coupler 16 is added to the system (FIG. 11). Only the air is shown as
a volume which extends to the rear surface of the diaphragm 17 and the pressure variation
is described by a change on a grey scale (dark - higher pressure, pale - lower pressure).
[0021] At higher frequencies - in the present case 310Hz, where pressure within the enclosure
varies spatially, there is an uneven acoustic loading at the rear of the loudspeaker
diaphragm. This means that loudspeakers that have structural rocking modes located
in this frequency range are vulnerable to having these activated by the pressure imbalance
at the rear of the diaphragm. The dominant motor damping mechanism is ineffective
for this type of mode as the motion of the coil does not orthogonally cut the flux
lines in the motor system. In the case where an APDC is used, the fluid load acting
directly upon the rear of the diaphragm is more uniform, decoupled from the strong
pressure variation by the structure of the APDC housing. This means that little energy
is transferred into rocking modes and therefore are only weakly excited.
[0022] The novel loudspeaker system provides a deterministic rear load and alleviates rocking
modes. It is tuneable where an increased port diameter restores the original response
and retains uniform loading to the rear of the loudspeaker drive unit, e.g., a woofer
or subwoofer. The use of an Adaptor for Passive Diaphragm Control (APDC) provides
a sharper roll in/roll off. Furthermore, the acoustic radiation is performed by the
diaphragm only and the tuning parameters will be optimised such that no audible port
noise is evident.
[0023] The performance of the novel loudspeaker system is - compared to conventional systems
- predictable and a small enclosure can be added with little or no change in designed
system resonance. Natural sharp roll off after the Helmholtz resonance of the enclosure
reduces interference with other drivers complementing electronic filtering. Having
an engineered multi degree of freedom acoustical system controlling the diaphragm,
a low frequency extension is possible without adding acoustically absorbent wadding
within an enclosure which is often impractical for car audio applications.
[0024] Due to a lower efficiency, novel loudspeaker systems may be used in connection with
D-Class amplifiers in order to have more power available. In this case, when using
an inverse motor loudspeaker drive unit as outlined above, the potentially hot motor
is on the outside of the enclosure providing very good heat dissipation to the atmosphere
considering the higher power supplied. When subwoofers are used in conjunction with
small enclosures, they are often designed to have very stiff suspension and high moving
mass to minimise the effect of the added stiffness of the sealed box. Such a driver
must be driven at high power to achieve reasonable acoustical power output and efficiency
is generally quite low. In the novel loudspeaker system, lower cost drive units having
a lower mass and/or lower stiffness and/or magnetic flux density may be used.
[0025] Although examples of the present invention have been described herein above in detail,
it is desired, to emphasis that this has been for the purpose of illustrating the
present invention and should not be considered as necessarily limitative of the invention,
it being understood that many modifications and variations can be made by those skilled
in the art while still practising the invention claimed herein.
1. A loudspeaker system comprising:
an enclosure that comprises two separate air volumes which are acoustically linked
by a port; the port enclosing a plug of air; and
an electro-acoustical transducer that converts electrical power into acoustical power
and that comprises a diaphragm having a front side and a rear side; the front side
of the diaphragm radiates sound into a listening environment and the rear side is
directly acoustically coupled to one of the volumes of the enclosure; where
the air volume which is directly coupled to the diaphragm and the plug of air in the
port are tuned to form a resonant system driven by the transducer; and
the volume that is not directly coupled to the diaphragm, is coupled to the plug of
air in the port.
2. The loudspeaker system of claim 1 where the port has a tube-like shape.
3. The loudspeaker system of claim 1 or 2 where the port and the volume directly coupled
to the diaphragm, are tuned using standard dynamic theory.
4. The loudspeaker system of one of claims 1-3 where the volume not directly coupled
to the diaphragm is larger than the volume directly coupled to the diaphragm.
5. The loudspeaker system of one of claims 1-4 where the plug of air comprises a volume
that is smaller than the two volumes.
6. The loudspeaker system of one of claims 1-5 where the diaphragm of the electro-acoustical
transducer has a cone-like shape.
7. The loudspeaker system of one of claims 1-6 where the volume coupled directly to the
diaphragm has a cone-like shape.
8. The loudspeaker system of one of claims 1-7 where the enclosure comprises rigid walls.
9. The loudspeaker system of one of claims 1-8 where the electro-acoustical transducer
comprises a motor system that is arranged on the side of the diaphragm being opposite
to the enclosure.
10. The loudspeaker system of one of claims 1-9 where the volume that is not directly
coupled to the diaphragm, is formed by body parts of a vehicle.
11. The loudspeaker system of one of claims 1-10 further comprising another diaphragm
located at or in the port.
12. The loudspeaker system of one of claims 1-11 where the electro-acoustical transducer
is a woofer or subwoofer.
13. The loudspeaker system of one of claims 1-12 where the transducer comprises a frame
and the enclosure comprises two parts; one part of which being also part of the transducer's
frame.