Noise attenuator for an induction system or an exhaust system

The present invention provides, with reference to the figure, a noise attenuator for an induction system or an exhaust system comprising a housing (10) having a gas inlet (13), a gas outlet (11) and a first gas flow passage (12) inside the housing connecting the gas inlet (13) to the gas outlet (11). A quarter wave resonator tube (14; 16; 23; 29) is provided inside the housing (10) which opens on to the first gas flow passage. The noise attenuator is an integer which can be installed as a single integrated unit in the induction system or the exhaust system. A Helmholtz resonator (21; 27) is provided inside the housing (10) which opens on to the first gas flow passage (12).

The present invention relates to a noise attenuator for an air induction 
system or an exhaust system. 
The present invention will be described with reference to its use in an air 
induction system or an exhaust system of an internal combustion engine in 
an automobile. However, the noise attenuator of the invention should not 
be considered limited to such a use and it should be appreciated that the 
invention could be used to attenuate noise in many gas flow systems, (e.g. 
air conditioning systems, car heater systems, fan systems or domestic 
appliances) many air induction systems or in many exhaust systems. 
It is at present the generally accepted practice in attenuation of noise in 
air induction systems of internal combustion engines in automobiles to 
attach to the air induction conduit at separate points along the conduit 
Helmholtz resonators and quarter wave tube resonators, each resonator 
being a separate integer and a number of different integers being 
connected to the air inlet conduit along the length thereof. A summation 
of the volumes of the separate resonators typically gives a total volume 
of 12 liters. The different resonators are typically distributed about the 
engine bay. 
In U.S. Pat. No. 5,014,816 there is described a silencer for an air 
induction system or an exhaust system of an internal combustion engine 
which comprises a number of quarter wave resonator tubes provided by 
multiple channels arranged in a single housing. The system is advantageous 
over certain prior art systems because it is more compact in nature than 
the previous prior art systems. However, the arrangement of U.S. Pat. No. 
5,014,816 has a disadvantage in that very long quarter wave tubes must be 
used to attenuate low frequencies. Thus the designer must either design a 
quite large housing to incorporate a long quarter wave tube or 
alternatively the designer must accept that the induction system will not 
attenuate the lower frequencies. 
The present invention provides a noise attenuator for an air induction 
system or an exhaust system comprising a housing having: 
a gas inlet, 
an gas outlet, 
a first gas flow passage inside the housing connecting the gas inlet to the 
gas outlet, and 
a quarter wave resonator tube inside the housing which opens on to the 
first gas flow passage, 
characterised in that there is additionally provided inside the housing a 
Helmholtz resonator which opens on to the first passage, whereby the 
Helmholtz resonator and the quarter wave resonator are together integrated 
in a single unit and the single unit is connectable in and deconnectable 
from the induction system or the exhaust system. 
The housing of the noise attenuator has a Helmholtz resonator which can 
attenuate low frequency noise. The present invention thus has the 
advantage of providing in one housing all of the elements required for 
attenuation of noise of the air induction system or the exhaust system in 
a compact manner. Thus, the housing will not need to have a very long 
quarter wave tube to attenuate low frequency noise. 
A Helmholtz resonator has significant advantages over a quarter wave tube 
resonator in attenuating low frequency noise. Whilst the volume of a 
Helmholtz resonator for attenuating for example 100 Hz frequency noise 
will be 2.4 liters and the volume of a quarter wave tube for attenuating 
the same frequency noise will be less, the quarter wave tube will need to 
be at least 1 meter long and thus will be harder to package than the 
Helmholtz resonator. Furthermore, the Helmholtz resonator will provide a 
better defined frequency band width of noise cancellation than a quarter 
wave resonator tube. 
The present invention provides a noise attenuator as one completely 
integrated unit which has a number of advantages. First, there is a lower 
pressure loss across the integrated attenuator than there would be across 
a prior art system providing similar attenuation but with separate 
resonators distributed throughout the air induction system. This leads to 
an increase in the efficiency of the internal combustion engine 
downstream. Secondly, the applicant has found that a prior art system with 
a total of 12 liters of resonator volume made up of separate resonators 
distributed throughout the air intake system can be replaced with a noise 
attenuator according to the present invention which has a volume in the 
range of 6 to 10 liters, and preferably about 7 liters, whilst in fact the 
attenuation characteristics are improved, with a decrease from 74 dB to 71 
dB in driveby noise (a noise measured by a standard test imposed by 
legislation, comprising measurement of noise at 7.5 meters from a 
vehicle). To achieve the required 3 dB reduction in driveby noise, a 
reduction of 8 dB in intake contribution to that noise is required. Since 
the dB measurement is a measurement on a logarithmic scale, the 3 dB 
decrease represents roughly a halving of noise. The reduced total volume 
of the noise attenuation system has further benefits in reduced weight of 
the system and reduced cost of the system. The attenuator of the present 
invention can achieve the same (and usually better) noise attenuation than 
the prior art distributed system with a reduced total volume; this is due 
to a synergistic effect on noise cancellation of including together in one 
housing a Helmholtz resonator along with quarter wave tube resonators. 
The provision of a complete noise attenuator system as a single integer 
allows design of the noise attenuator to best suit the packaging 
constraints of a particular application. For instance, the noise 
attenuator could be designed with a dual purpose in mind, the unit 
functioning for instance as both a noise attenuator and a wheel arch 
liner, both a noise attenuator and a bonnet liner or both a noise 
attenuator and part of an automobile bumper. 
The provision of a complete noise attenuation system in one integer further 
enables reduction in noise by facilitating connection of the integer to 
the remainder of a vehicle by isolators, for instance rubber isolators. In 
the past each of the separate components of the noise attenuator system 
would be able to rattle and it was very difficult and costly to connect 
each separate component to the remainder of, for instance, an automobile 
to prevent noise generation. The plurality of walls in the noise 
attenuator of the present invention also allows it to be made stiff, which 
helps keep vibrational noises low. 
The positioning of the plurality of distributed resonators of the prior art 
systems whilst restricted by packaging requirements, was chosen so that 
the positioning of a quarter wave tube or a Helmholtz resonator in the air 
induction system optimised cancellation of a particular frequency by the 
resonator. However, it has been found against accepted practice that the 
disadvantage of locating all of the resonators together at one point in 
the air intake system is not significant and is outweighed by the 
advantages of the present invention. 
It has been found that provision of a Helmholtz resonator with an inlet 
passage of a non-circular (and preferably rectangular) axial 
cross-section, in particular in conjunction with resonator tubes of 
non-circular (and preferably rectangular) axial cross-section is 
particularly advantageous. When circular cross-sections are used the noise 
attenuation characteristics are good but a standing wave tends to be 
established in the gas flow tube through the noise attenuator. The 
applicants have discovered advantageously that the waveform of the 
standing wave can be varied by using non-circular axial cross-sections. 
The present invention in one aspect has two or more gas flow passages 
through the housing which can be beneficial since different aspect ratios 
(i.e. the ratios between the cross-sectional areas of the gas flow 
passages and the cross-sectional areas of the resonators) can be chosen 
for each gas flow passage, which allows better tuning of the noise 
attenuator. 
Sound deadening material can be incorporated in the housing walls of the 
resonators to enhance noise cancellation. 
Injection moulding of parts of the resonator is preferred if accurate 
tolerances are required, since injection moulding is a precise moulding 
method (more precise than blow moulding for instance). Polypropylene could 
be used in the moulding process. 
The number of resonators in a housing would vary upwardly from a minimum of 
one Helmholtz resonator and one quarter wave tube resonator to any number 
of either resonator depending on the application and the quality of noise 
cancellation required. The layout of the resonators would also vary 
depending on packaging requirements and noise optimisation.

In FIG. 1 it can be seen that the noise attenuator of the invention is a 
single integer which comprises a housing 10 which is a moulded plastic 
housing. The housing 10 has a maximum depth of 100 mm. The housing 10 can 
be seen to have an inlet orifice 13. This orifice 13 could be an inlet for 
air in an air induction system of an internal combustion engine. 
Alternatively the orifice could be an inlet for exhaust gases when the 
noise attenuator is connected in an exhaust system of an automobile, in 
which case the housing 10 would be made of metal or some other heat 
resistant material. 
In FIG. 2 the cross-sectional view of the housing shows that the housing 
has a first gas flow passage 12 passing through the housing 10 from the 
inlet orifice 13 to an outlet orifice 11. In use the housing 10 can be 
connected such that the inlet orifice 13 is connected to an air filter and 
the outlet orifice 11 is connected to an induction manifold for an 
internal combustion engine, for instance in an automobile. Alternatively 
in use the housing 10 can be connected in an exhaust system of an 
automobile such that the inlet orifice 13 is connected to a pipe leading 
to the exhaust manifold of the internal combustion engine and the outlet 
orifice 11 is connected to a pipe which exhausts combusted gases to 
atmosphere. 
The housing 10 will be formed in two parts 10A and 10B (see FIG. 1). The 
parts are each formed by simple injection moulding operations. Injection 
moulding has a benefit of producing parts of finer tolerances than are 
achievable in some other moulding techniques (e.g. blow moulding). The 
parts 10A and 10B could be moulded from polypropylene or from a 
nylon-based material, which (whilst more expensive) would lead to a 
stiffer structure less prone to vibration. 
The part 10A is formed with a number of partitions, so that when the two 
parts 10A and 10B of the housing 10 are joined together the two parts 
together define tubes and cavities, as will now be described. The greatest 
dimension of the housing 10 is 540 mm. 
In FIG. 2 it can be seen that the housing 10 comprises a first quarter wave 
resonator tube 14 which is the longest quarter wave resonator tube in the 
housing 10. The quarter wave resonator tube 14 is open at its end 15 to 
the first gas flow passage 12. The quarter wave resonator tube 14 is 
L-shaped and extends along two sides of the housing 10. 
A shorter quarter wave resonator tube 16 is also provided in the housing 10 
and this tube has an end 17 which is open to the first passage 12. As air 
or exhaust gas passes through the first passage 12 from the inlet orifice 
13 to the outlet orifice 11, the gas sequentially passes first past the 
opening 15 of the quarter wave tube 14 and then past the end 17 of the 
quarter wave tube 16. 
The gas which has passed the end 17 of the quarter wave resonator tube 16 
next passes an end 22 of a Helmholtz resonator 18. The Helmholtz resonator 
18 comprises an inlet passage 20 which extends into a cavity 21. Both the 
inlet passage 20 and the cavity 21 are defined by the shape of the two 
parts 10A and 10B of the housing 10 when the two parts 10A and 10B are 
brought together. 
After the gas passes the open end 22 of the tube 20, the gas passes an open 
end 24 of a quarter wave resonator tube 23. The quarter wave tube 23 is 
L-shaped as viewed in FIG. 2 and extends first at right angles to the 
first passage 12 and then curves through 90.degree. to lie parallel to the 
end portion of the quarter wave resonator tube 14. 
After the gas passes the open end 24 of the quarter wave resonator tube 23, 
the gas next passes an open end 28 of a Helmholtz resonator 25. The 
Helmholtz resonator 25 comprises an inlet passage 26 which opens into a 
cavity 27, the inlet passage 26 and the cavity 27 both being defined by 
the shape of the two parts of the housing 10. 
The gas passing along the first passage 12 after passing the open end 28 of 
the Helmholtz resonator 25 next passes the open end of the shortest 
quarter wave resonator tube 29. The quarter wave resonator tube 29 is 
defined when the two parts 10A and 10B of the housing 10 are brought 
together. 
The gas passing along the first passage 12 before it reaches the outlet 11 
lastly passes an open end 32 of a quarter wave resonator tube 31. Whilst 
the quarter wave resonator tubes 14, 16, 23 and 29 lie on one side of the 
gas flow passage 12, the quarter wave resonator tube 31 lies on the 
opposite side of the gas flow passage 12 but in the same plane. 
Also shown in FIG. 2 is a removable panel 33 defined in the housing 10. The 
housing 10 is designed to be positioned on the top of an internal 
combustion engine when in use and the removable panel 33 can be removed to 
allow access to an oil filler cap lying below the housing 10. 
It will be appreciated that the noise attenuator of the invention can be 
made economically, because only two different moulded parts need be made, 
these then being joined together to form the housing with the quarter wave 
resonator tubes and the Helmholtz resonators defined in the housing by a 
series of partitions formed during the moulding process of one part 10A of 
the housing 10, which co-operate with the other part 10B of the housing 10 
to form the resonators. The parts 10A and 10B are not equal in size and it 
can be seen in FIG. 1 that part 10A occupies four fifths of the total 
depth of the housing 10 and part 10B the other fifth. 
The figures do not fully illustrate the fact that the depth of the 
Helmholtz resonators is greater than the depth of the quarter wave 
resonator tubes. The opposed surfaces of the two parts 10A and 10B of the 
housing 10 will each have a complex three dimensional shape, designed so 
that the quarter wave resonator tubes and the Helmholtz resonators have 
the required three dimensional shapes when the two parts 10A and 10B of 
the housing 10 are brought together and joined to one another. The bottom 
of each of the Helmholtz resonators 18 and 25 (as seen in FIG. 2) will be 
flat. 
The quarter wave resonator tubes in the preferred embodiment each have a 
roughly rectangular axial cross-section, the corners of the rectangular 
axial cross-section being rounded. Also, the inlet passages 20 and 29 of 
the Helmholtz resonators 21 and 27 have roughly rectangular axial 
cross-sections, the corners of the axial cross-section being rounded. 
It has been found that it is surprisingly important to have non-circular 
axial cross-sections. Whilst circular axial cross-sections do provide 
reasonable noise attenuation, a standing wave can form in the gas flow 
passage 11 which can contribute significantly to noise levels. The 
waveform of the standing wave in the gas flow passage 11 can be changed by 
choosing non-circular (and preferably rectangular) axial cross-sections 
with a resulting decrease in noise. The axial cross-sections could be 
oval, hexagonal or any other non-circular shape, but it is preferred that 
the smallest dimension of the cross-section is parallel to the axis of the 
gas flow passage 11. 
The exact dimensions of the quarter wave resonator tubes and the Helmholtz 
resonators and the layout of the resonators will be chosen for a 
particular application, having in mind the acoustic frequency spectrum of 
the gas flow which is to be attenuated. 
For a quarter wave tube used for an air induction system the basic equation 
f=C/4L (a very simplified equation which ignores, for example, temperature 
and end effects) can be used to calculate chosen lengths (although more 
complicated mathematical models are preferred), where f is the tuned 
frequency, C is the approximate speed of sound in air at 20.degree. C. and 
L is one length of the centre line of the channel. For example with a 
centre length of 0.6 meters then f=340/2.4 and f=141 hertz. If the centre 
length were 0.45 meters then f=340/1.8, in other words f=189 hertz. 
For a Helmholtz resonator the dimensions of the tube and the cavity forming 
the Helmholtz resonator are tuned to attenuate specific frequencies. This 
is done for an air induction system using the basic equation 
f=C/2.pi..sqroot. (A/LV) (a very simplified equation which ignores, for 
instance, temperature and end effects), where f is the tuned frequency, C 
is the speed of sound in air, A is the cross-sectional area of the tube 
leading to the cavity, L is the length of the tube leading to the cavity 
and V is the volume of the cavity. In practice a more complicated 
mathematical model would be preferred. 
For example the tube 20 of Helmholtz resonator 18 could be chosen to have a 
length of 100 mm and a cross-sectional area of 1256 mm.sup.2. The cavity 
21 could be chosen to have a volume of 1.47 liters. In this case the tuned 
frequency would be 141 hertz. The tube 26 of the Helmholtz resonator 25 
could be chosen to have a length of 45 mm and a cross-sectional area of 
1256 mm.sup.2. The volume of the cavity 27 of the Helmholtz resonator 25 
could be chosen to be 1.40 liters. In this case the tuned frequency would 
be 191 hertz. 
If the two equations for calculation of frequency f are considered 
carefully it can be seen that whilst a long length is needed for a quarter 
wave resonator to damp low frequencies, the length of the tube of the 
equivalent Helmholtz resonator can be made quite short, because the 
frequency is very dependent on the area and length of the tube and the 
volume of the Helmholtz resonator. A tube with a small area and a cavity 
with a large volume can be chosen to attenuate low frequencies, without 
the problem of having to package a very long quarter wave resonator tube 
in the housing of the noise attenuator. 
The equations given above are only the basic equations for the resonators 
which are given merely to demonstrate the different characteristics of 
Helmholtz and quarter wave resonators. The exact tuning frequencies are 
dependent on many factors such as the dimensions of the openings of the 
resonators. 
In FIG. 2 the quarter wave resonator tubes are each shown with a closed 
end. In fact, in practice each quarter wave resonator tube may have a 
small hole in its end, in order to allow drainage of moisture from the 
quarter wave resonator tube. Although, air induction systems are ideally 
watertight some moisture does enter and there must be a means for escape. 
The hole will be chosen to be small enough to have a minimal effect on the 
acoustic properties of the quarter wave tube. Similarly, the Helmholtz 
resonators may each have a small hole in order to allow drainage of 
moisture from within the housing 10. Again, the holes in the Helmholtz 
resonators will be chosen to be small enough to have a minimal effect on 
acoustic properties of the Helmholtz resonators. 
Whilst above it is mentioned that the housing is made of plastic material 
by injection moulding, the housing could also be manufactured by stamping 
two metal sections and the joining the two metal sections together. Indeed 
the housing could be made by many different manufacturing techniques, e.g. 
rotary moulding or in many different materials, e.g. fibre glass or any 
fibrous material. The two parts of the housing could be moulded together 
or secured together using mechanical fastening or in any other suitable 
way. Alternatively the housing could be made as a unitary member. 
Whilst in the embodiment mentioned above there are five quarter wave 
resonator tubes and two Helmholtz resonators, this is not critical and the 
number of quarter wave resonator tubes and Helmholtz resonators can be 
varied for different applications. What is important in each application 
is to analyse the frequency spectrum of the acoustic noise to be 
attenuated and then to choose the best combination of quarter wave 
resonator tubes and Helmholtz resonators to attenuate the acoustic noise. 
Generally, the Helmholtz resonators are chosen to attenuate low frequency 
portions of the frequency spectrum and the quarter wave resonator tubes 
are designed to attenuate high frequency components of the acoustic 
frequency noise spectrum, although there will be a cross-over for 
mid-range frequencies. The quarter wave resonator tubes and the Helmholtz 
resonators can be made in many different shapes according to packaging 
requirements and the quarter wave resonator tubes can for instance be 
straight tubes or can be curved. Indeed, some quarter wave resonator tubes 
can be turned through any angle (e.g. 90.degree.). Also the quarter wave 
resonator tubes could be made with a three-dimensionally varying shape, 
e.g. one could be formed as a helix. It is preferred that the 
cross-sectional area of each quarter wave resonator tube is substantially 
uniform over its entire length. 
It will be appreciated that the housing 10 has a thickness dimension (110 
mm) which is much smaller than the other dimensions of the housing. This 
permits the housing to be located for instance, above an engine, between 
the engine and a bonnet, where space is limited. The housing can in fact 
be located easily anywhere in the engine bay, for instance attached to a 
side wall of the engine compartment. Indeed, the noise attenuator can be 
provided anywhere in a vehicle, not necessarily in the engine bay. Also 
the housing could serve another purpose in the vehicle (e.g. the housing 
could be part of a bumper of the vehicle). 
Whilst above the housing 10 is formed of two separate parts 10A and 10B, it 
is envisaged that the housing could equally well be formed of any number 
of different parts and indeed the housing could be formed as one structure 
as a single part. 
The housing 10 can be fabricated by moulding resin or from a fibrous 
material. For instance lightweight polymeric materials such as 
thermoplastic or thermosetting resins can be used. Also composite 
materials can be used. 
The noise attenuator described above has been described for use in 
attenuating noise in an air induction system or an exhaust system of an 
internal combustion engine, but the noise attenuator could equally well be 
used with a compressor, a turbine or a pump. Indeed the noise attenuator 
could be used in any system (e.g. an air conditioning system) which has a 
plurality of ducting components and a component which generates noise or 
in any system where gas has to flow through a variety of chambers of 
different dimensions. 
In the embodiment described above one of the quarter wave resonator tubes 
lies on a side of the air gas induction passage which is opposite to the 
other quarter wave tubes. This a preferred feature because it improves 
packaging characteristics. 
In the embodiment of the invention described above the gas flow through the 
first passage 12 in the housing after passing the open end of one 
Helmholtz resonator must then pass the open end of a quarter wave tube 
before passing the open end of the second Helmholtz resonator. When 
designing the layout of the noise attenuator the designer will have in 
mind the fact that the main flow path (i.e. the gas flow path 11) will 
itself resonate at a particular frequency and therefore will include in 
the attenuator a quarter wave resonator tube or a Helmholtz resonator 
designed to attenuate noise created by the resonance of the main flow 
path. The positioning of this quarter wave or Helmholtz resonator will be 
chosen to maximise the benefit of the noise attenuator. When this position 
is fixed then the relationship of the other resonators to one another will 
preferably be chosen such that resonators which open consecutively (in the 
direction of gas flow) on to the main flow path have resonant frequencies 
distant from each other in order that maximum benefit is obtained from the 
noise attenuation provided by each. In other words, it is beneficial to 
separate resonators which have similar resonant frequencies. However, the 
resonators do not have to be positioned in this way and could be packaged 
in any way which gives a good compromise between packaging and acoustic 
performance. 
While the divider walls described above which divide the resonators are 
solid walls, they could equally well be cavity walls, with two skins 
separated by an air gap. 
Separate spaced divider walls could be provided for each resonator, the 
externally facing surfaces of the divider walls being separated from each 
other for instance by an air gap. This could be done to strengthen the 
housing since the divider walls could form reinforcing corrugations for 
the housing. 
Whilst the housing described above is shaped like a rectangular box and 
this is advantageous for manufacturing practicalities and for packaging 
considerates, the housing could have any form, e.g. it could be 
cylindrical or spherical in nature (although both of these forms take up 
more space in situ than a rectangular box of a similar volume). 
When the attenuator is used in an air induction system it can be located on 
the "dirty" or the "clean" side of the air filter (i.e. either before or 
after the air filter in the direction of gas flow). It may be preferred to 
enhance the noise attenuating performance of the noise attenuator by 
coating the inwardly facing surfaces of the resonators with a secondary 
noise deadening (e.g. fibrous) material. In this case the noise attenuator 
would be located on the "dirty" side of the air filter so particles coming 
loose from the sound deadening material will not enter the engine. 
While in the embodiment described above the inwardly facing surfaces of the 
gas flow path is a smooth plastic surface, this surface could be 
deliberately given a roughness to improve attenuation characteristics and 
could be provided with a series of inclined reflecting surfaces as in an 
anechoic chamber. 
A second embodiment of the present invention will be now described with 
reference to FIG. 3 in which there is shown a resonator comprising a 
housing 40. The housing has an inlet 41 which in use is connected to an 
air filter of an internal combustion engine. The air flows through a gas 
flow passage 42 in the housing 40 from the air inlet 41 to an air outlet 
42, which in use will be connected to the inlet manifold of an engine. As 
the air flows from the air inlet 41 to the air outlet 43 via the air flow 
passage 42 it will flow sequentially past: 
an L-shaped quarter wave resonator tube 43, 
a Helmholtz resonator 44 which has an L-shaped inlet passage 45 opening 
onto the gas flow passage 42; 
a quarter wave resonator 46; 
a quarter wave resonator 47; and 
a quarter wave resonator 48. 
Thus it will be seen that the noise attenuator of FIG. 3 comprises four 
quarter wave resonators and one Helmholtz resonator. Also shown in the 
Figure are three rubber isolators 49A, 49B and 49C which allow connection 
of the housing 40 to a vehicle body. The isolators 49A, 49B and 49C 
attenuate transmission of vibration from the housing 40 to the vehicle 
body and thus lower the noise experienced by the driver. 
A third embodiment of the present invention is shown in FIG. 4 where the 
noise attenuator has a housing 50 which has an air inlet 51 which in use 
will be connected to an air filter of an internal combustion engine. The 
housing 50 also have an air outlet 52 which in use will be connected to an 
air inlet manifold of an internal combustion engine. The air inlet 51 is 
connected to the air outlet 52 by a gas flow passage 53 which comprises 
two separate flow paths 53A and 53B through the housing 50. Air flowing 
through the flow path 53 will flow initially through the air inlet 51 and 
then will divide into a first air flow through the path 53A and a second 
air flow through the path 53B. The air flows through the paths 53A and 53B 
will combine again before passing through the air outlet 52. In the 
embodiment shown the cross-sectional area of the air flow path 53A will be 
different to the cross-sectional area of the air flow path 53B. Opening 
onto the air flow path 53A are a quarter wave tube resonator 54, a 
Helmholtz resonator 55 and a quarter wave tube resonator 56. Opening onto 
the air flow path 53B are a Helmholtz resonator 57, which comprises an 
L-shaped inlet passage 58, and an L-shaped quarter wave tube resonator 59. 
By diverting the air through separate flow paths 53A and 53B, the 
illustrated resonator can provide greater opportunity for tuning of the 
resonator to effectively cancel noise. By choosing the cross-sectional 
area of the air flow path 53A to be different to that of the air flow path 
53B, different aspect ratios (i.e. the ratios between the cross-sectional 
areas of the gas flow paths and the cross-sectional areas of the 
resonators) can be made available. 
FIG. 5 shows a fourth embodiment of the invention in which the noise 
attenuator comprises a housing 60 which is shaped to provide a wheel arch 
liner for an automobile. Thus, it will be appreciated that the housing 60 
serves a dual function since it functions both as a housing for the noise 
attenuator and also functions as a structural component of a vehicle, 
namely a wheel arch liner. 
The housing 60 has an air inlet 61 and an air outlet 62, with an air flow 
path 63 connecting the air inlet 61 and the air outlet 62. Air flowing 
through the air flow path 63 (which is a curved path, due to the curved 
nature of the wheel arch liner), passes sequentially past: 
a Helmholtz resonator 64 having an L-shaped inlet passage 65; 
a U-shaped quarter wave tube resonator 66; 
an L-shaped quarter wave tube resonator 67; 
a Helmholtz resonator 68 having an L-shaped inlet passage 69; 
a quarter wave tube resonator 70; 
an L-shaped quarter wave tube resonator 71; and 
a Helmholtz resonator 73 having an L-shaped inlet passage 72. 
The use of the housing 60 to provide a wheel arch liner will have an 
overall cost and weight saving advantage for the automobile which will not 
require separate components of a noise attenuator and a wheel arch liner. 
Furthermore, the use of the housing 60 as a wheel liner is a good use of 
dead space in the vehicle so that the engine bay can be kept uncluttered. 
It will be appreciated that the present invention in all of its embodiments 
has numerous advantages. Whereas a current distributed resonator system in 
an automobile comprises roughly 12 liters of resonator volume, this can be 
cut down to around 7 liters, with a decrease in drive-by noise from 77 dB 
to 74 dB. Furthermore, the integrated unit provided by the present 
invention is of reduced weight in comparison with the distributed 
resonator system and is also of reduced cost. Furthermore, the pressure 
drop across the integrated unit is less than the combination of the 
pressure drops across distributed units and this can lead to a power 
output improvement of the engine. The integrated unit can be used as a 
structural component of the vehicle, for instance a wheel arch liner or a 
bonnet liner. The integrated unit can be made stiffer than the separate 
components that are currently used and also it is easy to connect the 
integrated unit via isolators to a vehicle body; both of these factors 
decrease the vibration transmitted to the vehicle body by the noise 
attenuator. 
It has been found that the interaction of Helmholtz and quarter wave 
resonators within one integrated unit has a beneficial effect in achieving 
greater degrees of noise reduction with reduced volume. Locating the 
quarter wave and Helmholtz resonators together in one integrated unit 
leads to a synergistic effect in noise cancellation. This is because 
before it was assumed that it would be best to locate the separate noise 
attenuators at different parts in the air flow path of an air induction 
system of a vehicle, to take account of the wave form of the pressure 
profile of the air flowing through the air inlet path. 
The Helmholtz resonator in the integrated unit will provide a better 
defined bandwidth of noise cancellation than the bandwidth provided by the 
quarter wave tube resonators. The interaction of the Helmholtz and quarter 
wave tube resonators in the one integrated unit leads to optimisation and 
this means that the total resonator volume of the integrated unit can be 
decreased relative to the volume obtained by summing the resonators were 
they to be connected as separate components. 
The present invention can lead to a cost saving, because the integrated 
units provided by the present invention can be manufactured by moulding 
process in two parts. An injection moulding process using a nylon-based 
material would be particularly beneficial in providing a resonator with 
high tolerances, but a good degree of stiffness.