Patent Description:
A vehicle with a fuel-burning engine typically comprises an exhaust system which channels exhaust gases away from the engine so that they can be output from the vehicle. Such an exhaust system normally comprises one or more exhaust components that act on the flow of the exhaust gases, such as a turbo charger which accelerates the flow of gases entering the engine and/or a catalytic converter which converts exhaust gases into less-toxic gases.

As well as channelling exhaust gases, the exhaust system of a vehicle also channels noises from the engine to an output location on the vehicle. Thus, in addition to affecting the flow of gases through the exhaust system, the one or more exhaust components also alter the noises emitted by the engine. In order to reduce muffling and/or alteration of the engine noise by the exhaust components, it is known to provide a sound bypass device within the exhaust system. The sound bypass device is configured to transmit engine-generated sound pulses whilst preventing flow of exhaust gases, thereby providing desirable engine sound noises to the exterior of the vehicle.

A problem associated with known sound bypass devices is that they have associated transmission losses. The transmission loss of a device that transfers energy waves is defined as the ratio between transmitted and incident transmission waves. A high transmission loss value provides a muffling effect on waves, such as sound waves, transferred along a sound bypass device, thereby decreasing the quality of the sound output from the device. This is undesirable for vehicles which comprise sound bypass devices, such as sports cars, because the sound that is generated by such vehicles forms a large part of the impression of the performance of the vehicle. Thus, it is important that the transmission loss is minimised during operative vehicle conditions so that the perceived performance of the vehicle is not adversely affected.

<CIT> discloses an exhaust system for a vehicle with an engine, comprising an exhaust pipe arrangement defining a flow path for exhaust gas from the engine, and a sound transmission device having at least one membrane configured to vibrate so as to mechanically transmit sound waves incident upon it. The sound transmission device defines a sound transmission path receiving sound waves from the source of exhaust, the sound transmission path being distinct from the exhaust flow path.

<CIT> discloses a front muffler for an engine comprising an effective pipe length which changes according to the engine speed.

<CIT> discloses a sound damping device having a vibratory membrane, which stays in acoustic connection with the gas-guiding pipe at a coupling-out point. Another vibratory membrane stays in acoustic connection with a pressure compensation pipe at an excitation point. The acoustic distance between the coupling-out point and the coupling point is equal to the acoustic distance between the excitation point and the coupling point.

<CIT> discloses a vehicle comprising: an internal combustion engine having at least one cylinder, the internal combustion engine comprising an exhaust manifold for collecting exhaust gases expelled from the at least one cylinder; an exhaust system configured to channel exhaust gases along a flow path from the exhaust manifold to at least one exhaust outlet, the exhaust system comprising at least one exhaust component configured to act on exhaust gases flowing though the exhaust component and causing an alteration to engine-generated sound pulses passing through the exhaust component; and a sound bypass device comprising a sound inlet port at a first location on the exhaust system before a first exhaust component along the flow path and a sound outlet port at a second location on the exhaust system after the first exhaust component along the flow path, the sound bypass device being configured to transmit engine-generated sound pulses from the sound inlet port to the sound outlet port whilst preventing flow of exhaust gases from the sound inlet port to the sound outlet port.

According to a first aspect of the present invention there is provided a sound bypass device configured to transmit engine-generated sound pulses from an engine to a sound outlet whilst preventing flow of gases to the sound outlet, the sound bypass device comprising: an input tube configured to conduct the engine-generated sound pulses from the engine; and a sound transmission device connected to the input tube at a first end and to the sound outlet at a second end, the sound transmission device comprising: a first volume connected to the first end, a second volume connected to the second end, and a flexible diaphragm separating the first volume from the second volume and configured to transfer variations in pressure in the first volume to the second volume; wherein the first volume has a cross-sectional area that is greater at the diaphragm than at the first end and the second volume has a cross-sectional area that is greater at the diaphragm than at the second end.

The first and second volumes may be conical in shape, such that the cross-sectional area of the first and second volumes are defined by respective first and second diameters.

The first and second diameters may increase linearly from the first end to the diaphragm and the second end to the diaphragm respectively.

The first volume may be symmetrical to the second volume about a plane that comprises the flexible diaphragm.

The sound bypass device may further comprise an output tube configured to conduct sound pulses to the sound outlet, wherein the output tube has a cross-sectional area that is greater at the sound outlet than at the second end.

The output tube may be conical in shape, such that the cross-sectional area of the output tube is defined by a third diameter.

The third diameter may increase linearly from the second end to the sound outlet.

The first volume may have a length running from the first end to the diaphragm, the second volume may have a length running from the diaphragm to the second end and the output tube may have a length running from the second end to the sound outlet, the length of the output tube being greater than the length of each of the first and second volumes.

A ratio of a minimum to a maximum diameter of the first volume may be between <NUM>:<NUM> and <NUM>:<NUM>. The ratio of the minimum to the maximum diameter of the first volume may be <NUM>:<NUM>.

A ratio of a minimum to a maximum diameter of the second volume may be between <NUM>:<NUM> and <NUM>:<NUM>. The ratio of the minimum to the maximum diameter of the second volume may be <NUM>:<NUM>.

A ratio of a minimum diameter of the first volume to a length from the first end to the diaphragm may be between <NUM>:<NUM> and <NUM>:<NUM>. The ratio of the minimum diameter of the first volume to the length from the first end to the diaphragm may be <NUM>:<NUM>.

A ratio of a minimum diameter of the second volume to a length from the second end to the diaphragm may be between <NUM>:<NUM> and <NUM>:<NUM>. The ratio of the minimum diameter of the second volume to the length from the second end to the diaphragm may be <NUM>:<NUM>.

A ratio of a minimum to a maximum diameter of the output tube may be between <NUM>:<NUM> and <NUM>:<NUM>, and a ratio of a minimum diameter of the output tube to a length from the second end to the sound outlet may be between <NUM>:<NUM> and <NUM>:<NUM>. The ratio of the minimum to the maximum diameter of the output tube may be <NUM>:<NUM>, and the ratio of the minimum diameter of the output tube to the length from the second end to the sound outlet may be <NUM>:<NUM>.

The first volume may be aligned with the second volume along a common axis.

The output tube may be at least partially aligned the second volume along the common axis.

The output tube may be fully aligned with the second volume along the common axis.

The first volume, the second volume and the output tube may be made from steel or titanium.

The diaphragm may be connected across the sound transmission device to prevent flow of gases from the first volume to the second volume.

The flexible diaphragm may comprise a single flexible membrane.

The flexible diaphragm may comprise: a rigid barrier separating the first volume from the second volume; a first flexible membrane located within the first volume; a second flexible membrane located within the second volume; and a connecting member extending though the rigid barrier and connecting the first flexible membrane to the second flexible membrane, the connecting member being configured to transfer sound vibrations from the first flexible membrane to the second flexible membrane.

The rigid barrier may further comprise a channel through which the connecting member is able to extend, and the flexible diaphragm may further comprise a seal positioned within the channel and configured to hold the connecting member in place within the channel.

The diaphragm may further comprise one or more first balance orifices which are located in the first flexible membrane.

The diaphragm may further comprise one or more second balance orifices which are located in the walls of the second volume.

According to a second aspect of the present invention there is provided a vehicle comprising: an internal combustion engine having at least one cylinder, the internal combustion engine comprising an exhaust manifold for collecting gases expelled from the at least one cylinder; an air intake system for providing a supply of air to the internal combustion engine; an exhaust system configured to channel gases from the internal combustion engine along a flow path from the exhaust manifold to at least one exhaust outlet, the exhaust system comprising at least one exhaust component configured to act on gases flowing though the exhaust component and causing an alteration to engine-generated sound pulses passing through the exhaust component; and a sound bypass device as claimed in any preceding claim, wherein the inlet tube is connected to a first location on either the air intake system or the exhaust system, and the sound outlet is located at a second location on the exterior or within the cabin of the vehicle.

The following description is presented to enable any person skilled in the art to make and use the invention and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art.

<FIG> illustrates an example of a vehicle <NUM>. In this example, the vehicle comprises an arrangement of components that is symmetrical about a line X. The line X traverses the length of the vehicle <NUM> and bisects the width of the vehicle. Thus, the number and arrangement of components on a first side of this line X are the same as the number and arrangement of components on the second side of the line. Each type of component comprised within the vehicle <NUM> has only been labelled once in <FIG>. It would be understood by the skilled person that, as the vehicle is symmetrical about the line X, a component labelled on a first side of the line X corresponds to the same component located on the second side of the line X. The vehicle arrangement illustrated in <FIG> is for exemplary purposes only and it would be understood that this is not limiting to the exact arrangement of components that may be comprised within a vehicle according to the present invention.

The vehicle <NUM> comprises an internal combustion engine <NUM>, which may be coupled to a drive system for the transference of an engine torque to other components of the vehicle. More specifically the torque generated by the internal combustion engine <NUM> may be transferred from the engine <NUM> to moveable elements <NUM> of the vehicle <NUM>. Alternatively, internal combustion engine <NUM> may be coupled to the drive system for the transference of an engine torque to one or more first electrical machines for the generation of drive power. The one or more first electrical machines may be coupled to one or more second electrical machines to receive the drive power and generate motor torques to moveable elements <NUM> of the vehicle <NUM>. These electrical machines together with the internal combustion engine <NUM> may together form a powertrain of the vehicle <NUM>.

The internal combustion engine <NUM> of vehicle <NUM> could be a straight, flat or V-engine having any number of cylinders. The internal combustion engine <NUM> may be part of a hybrid drive system for the vehicle. For example, the internal combustion engine may be part of a parallel hybrid drive system whereby one or more electrical machines and the internal combustion engine each generate torques that can be used separately and/or in combination to drive the vehicle. In an alternative example, the internal combustion engine may be part of a series hybrid drive system whereby the internal combustion engine is coupled to one or more first electrical machines which generate power from the engine torque generated by the internal combustion engine. The power generated by the one or more first electrical machines may be transferred to one or more second electrical machines to generate motor torques for driving the vehicle.

The vehicle <NUM> may comprise a plurality of movable elements <NUM>, <NUM> for supporting the vehicle <NUM> on a surface. In the example illustrated in <FIG>, the moveable elements are wheels. However, it would be appreciated that the moveable elements may be any alternative components that are capable of supporting the vehicle on a surface and transferring engine torque into a driving force for the vehicle, such as tracks. The moveable elements will from this point forward be referred to as wheels. Some of those wheels may be drive wheels and some of those wheels may be non-drive wheels, such as wheels <NUM>. It will be appreciated that any configuration of drive <NUM> and non-drive <NUM> wheels may be used depending on the particular drive characteristics required by the vehicle <NUM>.

The vehicle <NUM> may comprise an air intake system <NUM> for providing a supply of air to the internal combustion engine <NUM>. The intake system <NUM> may comprise an intake manifold <NUM> that is fed an air mixture by at least one intake port. In the example shown in <FIG>, the vehicle comprises two intake manifolds <NUM> that are fed an air mixture by air inlet pipes <NUM>. Air flows into the intake system from one or more intake inlets <NUM> via the air inlet pipes <NUM>. Generally, these are located on the exterior of the vehicle to permit air to flow into the inlets. The flow of air into the intake system may be assisted by one or more induction devices. The induction devices may be one or more turbochargers and/or superchargers. In the example shown in <FIG>, a turbocharger <NUM> is provided for each intake manifold. Each turbocharger <NUM> is connected between the intake inlet <NUM> and its respective intake manifold <NUM>.

The flow of air mixture, via the at least one intake port <NUM>, into the intake manifold <NUM> may be regulated by at least one throttle. The intake manifold <NUM> permits the flow of the air mixture from the intake ports <NUM> to the one or more cylinders of the engine <NUM>. The one or more cylinders each house a piston which is caused to move by the ignition of fuel present in the respective cylinder. The pistons are each coupled to a drive an axel of the engine <NUM> to enable generation of the engine torque by means of the movement of the pistons. The entry and exit of gases into and out of the cylinders are regulated by a plurality of valves for each cylinder. The plurality of valves comprises intake and exhaust valves. Generally, the intake valves regulate the flow of combustion gases into a cylinder and the exhaust valves regulate the flow of exhaust gases out of a cylinder.

The internal combustion engine <NUM> may comprise one or more exhaust manifolds <NUM> which collect the exhaust gases expelled from the cylinders of the engine <NUM>. The exhaust gases are expelled from the cylinders via the plurality of exhaust valves. In the example shown in <FIG>, the engine <NUM> comprises two exhaust manifolds <NUM>. Each exhaust manifold collects exhaust gases expelled from a separate set of cylinders of the engine <NUM>.

The vehicle <NUM> further comprises an exhaust system <NUM> which channels the exhaust gases from the exhaust manifold to at least one exhaust outlet <NUM>. If there is only one exhaust manifold present in the vehicle then the exhaust system <NUM> may channel the exhaust gases from that exhaust manifold to at least one exhaust outlet <NUM>. In some vehicles there may be more than one exhaust outlet <NUM> to which the exhaust gases are channels from the one exhaust manifold. In the example shown in <FIG>, the engine <NUM> comprises two exhaust manifolds and the exhaust system <NUM> channels exhaust gases from a first exhaust manifold to at least one first exhaust outlets and from a second exhaust manifold to at least one second exhaust outlets. The exhaust system may combine the flows of exhaust gases from multiple exhaust manifolds along the path from the exhaust manifolds to at least one exhaust outlet <NUM>.

The exhaust system <NUM> may comprise one or more exhaust components that acts on the exhaust gases being channelled through the exhaust system. The one or more exhaust components may comprise a turbocharger <NUM>. An exhaust inlet of the turbocharger <NUM> may be connected to the exhaust manifold <NUM> by an exhaust pipe <NUM>. The exhaust inlet permits exhaust gases to flow into the turbocharger <NUM>. An exhaust outlet <NUM> of the turbocharger <NUM> may permit exhaust gases to flow out of the turbocharger <NUM>. The exhaust outlet <NUM> may be connected to an exhaust pipe <NUM> to channel the exhaust gases towards the one or more exhaust outlets. The turbocharger assists the flow of air into the intake manifold by obtaining power from the flow of the exhaust gases through the turbocharger. The turbocharger may comprise a first impeller which assists the flow of air into the intake manifold. This first impeller can be powered by the flow of exhaust gases flowing over a second impeller connected to the first impeller. The turbocharger comprises the second impeller. The presence of the turbocharger in the flow path of the exhaust gases from the exhaust manifold to the one or more exhaust outlets alters the engine sounds that are transmitted along the exhaust system to the one or more exhaust outlets <NUM>. This may mean that the engine sounds from the engine are muffled or otherwise changed. For instance, a turbocharger can add a whining sound to the engine sound being transmitted along the exhaust system.

The one or more exhaust components may alternatively or additionally comprise an exhaust gas treatment device. The exhaust system may comprise more than one exhaust gas treatment device for each channel of exhaust gases from the exhaust manifold to exhaust outlet(s). Examples of exhaust gas treatment devices are catalytic convertors, and gasoline particulate filters otherwise known as anti-particulate filters. Each of these devices acts on the exhaust gases in some way to change the constituents of the exhaust gases. The exhaust system may comprise a catalyst followed by an anti-particulate filter in series connected together by exhaust pipes <NUM>. The exhaust system may comprise a catalyst followed by an anti-particulate filter in series connected together by exhaust pipes <NUM> for each channel between an exhaust manifold <NUM> and exhaust outlet <NUM>.

Alternatively or additionally to the above, the one or more exhaust components may comprise a silencer <NUM>. The silencer <NUM> acts on the flow engine sounds along the exhaust system to change the sounds and/or reduce the level of sounds that flow along the exhaust system.

The vehicle illustrated in <FIG> comprises a turbocharger <NUM>, a catalytic convertor <NUM> and an anti-particulate filter <NUM> along a first set of exhaust pipes <NUM> that channel exhaust gases from a first exhaust manifold to at least one exhaust outlet. These exhaust components also act on the exhaust gases produced by the engine <NUM> so as to alter the sound of the engine that is transmitted along the exhaust system to the one or more exhaust outlets <NUM>. The changes to the engine sounds that are produced by the exhaust components and transmitted along the exhaust system can be detrimental to the perception of the vehicle in certain circumstances. For instance, if the vehicle is a high-performance sports car then the exhaust components may serve to alter the sounds emanating from the exhaust outlets such that there is a reduction in the perception that the vehicle is high-performance.

To address this issue, the vehicle is provided with at least one sound bypass device <NUM>. The sound bypass device <NUM> allows the engine generated sounds to bypass one or more of the exhaust components while the exhaust gases still flow through the exhaust components. The sound bypass device <NUM> may or may not reconnect to another part of the exhaust system after it has bypassed one or more exhaust components in the system. In an example, the sound bypass device <NUM> may be configured to bypass the entire exhaust system. The sound bypass device <NUM> is configured to transmit engine-generated sounds but not to permit the flow of exhaust gases. In other words, the sound bypass device <NUM> is configured to prevent the flow of exhaust gases through the sound bypass device <NUM>.

The sound bypass device <NUM> comprises a sound inlet <NUM> which is connected to a first location on the exhaust system <NUM> before one of the exhaust components along the flow path of the exhaust gases. That is, the sound inlet <NUM> is located closer to the exhaust manifold <NUM> along the flow of the exhaust gases within the exhaust system than that exhaust component. The sound inlet <NUM> may be connected to the exhaust system <NUM> before all of the exhaust components. That is, the sound inlet <NUM> may be connected to the exhaust system <NUM> between the exhaust manifold and the first exhaust component along the exhaust system in the direction of flow of the exhaust gases. The sound inlet <NUM> may be connected to an exhaust pipe which is connected between the exhaust manifold and the first exhaust component. The first exhaust component may be a turbocharger <NUM> as shown in <FIG>. In this case, the exhaust pipe to which the sound inlet <NUM> is connected may be connected to the exhaust inlet of the turbocharger <NUM>. The sound bypass device <NUM> comprises a sound outlet <NUM> from which sound is output from the vehicle. The sound outlet <NUM> may be located at a number of different positions on the exterior of the vehicle. In one example, the sound outlet <NUM> is located on the roof of the vehicle. In another example, the sound outlet is located on the base of the vehicle. In a further example, the sound outlet <NUM> is located at the side of the vehicle exterior, next to the engine <NUM>. The sound outlet <NUM> may alternatively be located inside the body of the vehicle. For example, the sound outlet <NUM> may be connected to an exhaust outlet <NUM>. The sound outlet <NUM> may alternatively be located within a cabin of the vehicle, within which a driver of the vehicle is seated.

The sound bypass device <NUM> comprises a sound transmission device <NUM> coupled to an input tube <NUM> at a first end <NUM> and an output tube <NUM> at a second end <NUM>. The input tube <NUM> is configured to conduct engine-generated sound pulses from the first location on the exhaust system <NUM> to a sound transmission device <NUM>. The sound transmission device <NUM> is configured to receive the sound pulses from the input tube <NUM> and transmit them to the sound outlet <NUM>. In one example, a path from the second end <NUM> of the sound transmission device to the sound outlet <NUM> is provided by means of an output tube <NUM>. The output tube <NUM> is configured to conduct sound pulses generated by the engine <NUM> from the sound transmission device <NUM> to the sound outlet <NUM>.

The first end <NUM> of the sound transmission device <NUM> may otherwise be referred to as a sound inlet. The input tube <NUM> is connected to sound inlet <NUM>. The second end <NUM> of the sound transmission device <NUM> may otherwise be referred to as a sound outlet <NUM>. The output tube <NUM> may be connected to the sound outlet <NUM>. The sound transmission device <NUM> is configured to permit engine-generated sound pulses to be transmitted from the sound inlet <NUM> to the sound outlet <NUM>. The sound transmission device <NUM> is configured to prevent the flow of exhaust gases through the sound transmission device <NUM> from the sound inlet <NUM> to the sound outlet <NUM>. The sound transmission device <NUM> comprises a diaphragm <NUM> housed within a chamber <NUM>. The diaphragm may be formed of one or more membranes. In <FIG>, the diaphragm <NUM> is illustrated as a single membrane. In the case where the diaphragm <NUM> is formed of more than one membrane, the membranes may be spaced apart from each other. The one or more membranes that form the diaphragm <NUM> may be flat (as illustrated in <FIG>) or may be of any alternatively suitable shape such as conical. The shape of the membrane can advantageously be used to couple the required acoustic performance of the sound transmission device with its structural requirements. An alternative example of the arrangement of a diaphragm for use in the sound bypass device illustrated in <FIG> is described below with respect to <FIG>.

The diaphragm <NUM> is connected across the width of the chamber <NUM> such that sound pulses travelling down the input tube <NUM> into the chamber drive the motion of the diaphragm. The diaphragm <NUM> moves in response to changes in pressure generated by the engine <NUM> in the exhaust system <NUM>. Because the diaphragm <NUM> is configured to move in accordance with sound pulses received from the engine <NUM>, the variations in a first volume <NUM> located on a first side of the chamber to which the input tube <NUM> is connected are transferred into a second volume <NUM> located on a second side of the chamber to which an output tube <NUM> is connected. Sound pulses travelling down the input tube may therefore pass through the diaphragm <NUM> and into output tube <NUM>. The diaphragm <NUM> prevents exhaust gases from flowing from the first volume <NUM> to which the input tube <NUM> is connected to the second volume <NUM> to which the output tube <NUM> may be connected.

The sound bypass device <NUM> therefore enables engine-generated sound pulses to be transmitted from one position along the exhaust system <NUM> to the exterior of the vehicle. The device <NUM> therefore bypasses the sound-altering exhaust components to provide a greater range of and/or better sounding sound pulses to the exterior of the vehicle.

The particular configuration of an improved sound bypass device <NUM> to be inserted into the vehicle of <FIG> is illustrated in <FIG>. The sound bypass device <NUM> may, for example, correspond to or replace the device <NUM> in <FIG>.

As mentioned above, the sound bypass device <NUM> is configured to transmit engine-generated sound pulses from an engine to a sound outlet <NUM> whilst preventing flow of exhaust gases to the sound outlet <NUM>. The sound bypass device <NUM> comprises an input tube <NUM> which may correspond to input tube <NUM> of the vehicle <NUM> illustrated in <FIG>. The input tube <NUM> is configured to conduct the engine-generated sound pulses from the engine, which may correspond to engine <NUM> in <FIG>. The sound bypass device <NUM> further comprises an output tube <NUM> which may correspond to the output tube <NUM> of the vehicle <NUM> illustrated in <FIG>. The output tube is configured to conduct sound pulses to the sound outlet <NUM>, which may correspond to outlet <NUM> in <FIG>.

The sound bypass device further comprises a sound transmission device <NUM> which may correspond to the device <NUM> in <FIG>. The sound transmission device <NUM> is connected to the input tube <NUM> at a first end <NUM>. The first end <NUM> may otherwise be referred to as the sound inlet or inlet port, as it provides an inlet for exhaust gases and engine noise into the sound transmission device from the inlet tube <NUM>. The sound transmission device <NUM> is connected to the output tube <NUM> at a second end <NUM>. The second end <NUM> may otherwise be referred to as the sound outlet or outlet port as it provides an outlet for engine noise out of the sound transmission device into the output tube <NUM>. The sound transmission device <NUM> comprises a chamber with a first volume <NUM> connected to the first end <NUM> which may correspond to volume <NUM> illustrated in <FIG>. The chamber of the sound transmission device <NUM> further comprises a second volume <NUM> connected to the second end <NUM> which may correspond to volume <NUM> illustrated in <FIG>. The sound bypass device also comprises a flexible diaphragm <NUM> that separates the first volume <NUM> from the second volume <NUM>. The term "flexible" implies that at least part of the diaphragm is capable of flexing in response to sound vibrations passing through the sound bypass device. In other words, at least one component of the diaphragm is capable of flexing in response to sound vibrations. The flexible diaphragm <NUM> may correspond to the diaphragm <NUM> illustrated in <FIG>.

The flexible diaphragm <NUM> is configured to transfer variations in pressure in the first volume <NUM> to the second volume <NUM>. In one example the diaphragm <NUM> is formed of metal. The diaphragm may alternatively be formed from any other material that is able to withstand the temperatures and pressures exerted by engine exhaust gases whilst also being deformable so as to transfer variations in pressure across the chamber of the sound transmission device <NUM>. The diaphragm <NUM> is connected across sound bypass device <NUM> to prevent flow of exhaust gases from the first volume to the second volume. In other words, the diaphragm <NUM> is connected across the chamber formed by the first and second volumes <NUM>, <NUM> to prevent exhaust gases from flowing from the first side of the chamber to the second side of the chamber.

The first volume <NUM> comprises a first length L<NUM> which extends between the first end, or sound inlet, <NUM> and the diaphragm <NUM>. In an example, the first length L<NUM> is aligned along an axis <NUM>. The first volume further comprises a cross-sectional area A<NUM> that is perpendicular to the first length L<NUM> at any given point along the first length L<NUM>. The second volume <NUM> comprises a second length L<NUM> which extends between the second end, or sound outlet, <NUM> and the diaphragm <NUM>. In an example, the first length L<NUM> is aligned with the second length L<NUM> along the common axis <NUM>. In other words, the first volume <NUM> may be aligned with the second volume <NUM> along the common axis. The second volume further comprises a cross-sectional area A<NUM> that is perpendicular to the second length L<NUM> at any given point along the second length L<NUM>.

The output tube <NUM> comprises a third length L<NUM> which extends between the second end <NUM> and the sound outlet <NUM>. In an example, the third length L<NUM> is at least partially aligned with the second length L<NUM> along the common axis <NUM>. In other words, the output tube <NUM> may be at least partially aligned with the second volume <NUM>. Where the second volume <NUM> is aligned with the first volume <NUM>, the output tube is also at least partially aligned with the first volume <NUM>. In a further example, the third length L<NUM> is fully aligned with the second length L<NUM> along the common axis <NUM>. That is, the output tube <NUM> is fully aligned with the second volume along the common axis <NUM>. The output tube <NUM> further comprises a cross-sectional area A<NUM> that is perpendicular to the third length L<NUM> at any given point along the first length L<NUM>.

The cross-sectional area A<NUM> of the first volume <NUM> varies across the length L<NUM> of the first volume. More specifically, the cross-sectional area A<NUM> of the first volume <NUM> is greater at the diaphragm <NUM> than it is at the first end <NUM> which connects to the input tube <NUM>. That is, the cross-sectional area A<NUM> of the first volume increases between the first end <NUM> and the diaphragm <NUM>. In an example, as illustrated in <FIG>, the cross-sectional area A<NUM> of the first volume increases continuously between the first end <NUM> and the diaphragm <NUM>. The term "continuously" in this context means that the rate of increase of the cross-sectional area is constant along the length of the volume. In other words, the cross-sectional area A<NUM> of the first volume increases monotonically between the first end <NUM> and the diaphragm <NUM>. In an alternative example, the increase between the first end <NUM> and the diaphragm <NUM> is discontinuous. That is, there may be parts of the first volume <NUM>, along its length L<NUM>, in which there is no change in cross-sectional area. There may alternatively or additionally be parts at which the rate of increase of cross-sectional area changes along the length L<NUM> of the first volume, or at which the cross-sectional area decreases. However, in all examples the cross-sectional area A<NUM> at the end of the first length L<NUM> at which the first volume <NUM> is connected to the diaphragm <NUM> is greater than the cross-sectional area A<NUM> at the end of the first length L<NUM> at which the first volume <NUM> is connected to the input tube <NUM>.

The cross-sectional area A<NUM> of the second volume <NUM> also varies across the length L<NUM> of the second volume. More specifically, the cross-sectional area A<NUM> of the second volume <NUM> is greater at the diaphragm <NUM> than it is at the second end <NUM> which connects to the output tube <NUM>. That is, the cross-sectional area A<NUM> of the second volume decreases between the diaphragm <NUM> and the second end <NUM>. In an example, as illustrated in <FIG>, the cross-sectional area A<NUM> of the second volume decreases continuously between the diaphragm <NUM> and the second end <NUM>. The term "continuously" in this context means that the rate of decrease of the cross-sectional area is constant along the length of the volume. In other words, the cross-sectional area A<NUM> of the second volume decreases monotonically between the diaphragm <NUM> and the second end <NUM>. In an alternative example, the decrease between the diaphragm <NUM> and the second end <NUM> is discontinuous. That is, there may be parts of the second volume <NUM>, along its length L<NUM>, in which there is no change in cross-sectional area. There may alternatively or additionally be parts at which the rate of increase of cross-sectional area of the second volume <NUM> changes, or at which the cross-sectional area decreases. However, in all examples the cross-sectional area A<NUM> at the end of the second length L<NUM> at which the second volume is connected to the diaphragm <NUM> is greater than the cross-sectional area A<NUM> at the end of the second length L<NUM> at which the second volume is connected to the output tube <NUM>.

The cross-sectional area A<NUM> of the output tube also varies across the length L<NUM> of the output tube. The cross-sectional area A<NUM> of the output tube is greater at the sound outlet <NUM> than it is at the second end <NUM> of the sound transmission device <NUM>. That is, the cross-sectional area A<NUM> of the output tube increases between the second end <NUM> of the sound transmission device and the sound outlet <NUM>. In an example, as illustrated in <FIG>, the cross-sectional area A<NUM> of the output tube <NUM> increases continuously between the second end <NUM> and the sound outlet <NUM>. The term "continuously" in this context means that the rate of increase of the cross-sectional area is constant along the length of the output tube. In other words, the cross-sectional area A<NUM> of the output tube increases monotonically between the second end <NUM> and the sound outlet <NUM>. In an alternative example, the increase in cross-sectional area A<NUM> is discontinuous. That is, there may be parts of the output tube <NUM>, along its length L<NUM>, in which there is no change in cross-sectional area. There may alternatively or additionally be parts at which the rate of increase of cross-sectional area changes, or at which the cross-sectional area decreases. However, in all examples the cross-sectional area A<NUM> at the sound outlet <NUM> is greater than the cross-sectional area A<NUM> at the end of the second length L<NUM> at which the output tube <NUM> is connected to the sound transmission device <NUM>.

The first and second volumes, and the output tube, of the sound bypass device may be of any suitable shape. In one example, the first and second volumes are conical in shape. That is, the first and second volumes may resemble the shape of a cone. The first and second volumes may be conical frustums. Thus, the cross-sectional area A<NUM> of the first volume may be circular, and therefore defined by a diameter d<NUM>. The cross-sectional area A<NUM> of the second volume may be circular, and therefore defined by a diameter d<NUM>.

Where the first and second volumes are conical frustums, the first and second diameters d<NUM>, d<NUM> of the first and second volumes respectively vary linearly along their lengths. In a specific example, the first and second volumes may be right conical frustums. That is, the first diameter d<NUM> of the first volume <NUM> may increase linearly between the first end <NUM> and the diaphragm <NUM>. In other words, first diameter d<NUM> may increase in the axial direction of the first volume <NUM>. The diameter of the first volume <NUM> can be measured at any point along this axial length in the radial direction. The second diameter d<NUM> of the second volume <NUM> decreases linearly between the diaphragm <NUM> and the second end <NUM>. Put differently, the second diameter d<NUM> of the second volume <NUM> increases linearly from the second end <NUM> to the diaphragm. In other words, first diameter d<NUM> may increase in the axial direction of the second volume <NUM>. The diameter of the second volume <NUM> can be measured at any point along this axial length in the radial direction.

The geometry of the first volume <NUM> may be symmetrical to the geometry of the second volume <NUM> about a plane <NUM> that comprises the flexible diaphragm <NUM>. That is, the length L<NUM> of the first volume may be the same as the length L<NUM> of the second volume. The cross-sectional area A<NUM> of the first volume may vary at the same rate along its length L<NUM> as the corresponding variation in cross-sectional area A<NUM> of the second volume along its length L<NUM>. Where the first and second volumes are conical in shape, the minimum diameter of the first volume <NUM> may be the same as the minimum diameter of the second volume <NUM>. Similarly, the maximum diameter of the first volume <NUM> may be the same as the maximum diameter of the second volume <NUM>.

The output tube <NUM> may be conical in shape. That is, the output tube <NUM> may resemble the shape of a cone. The output tube <NUM> may be a conical frustum. In a specific example, the output tube <NUM> may be a right conical frustum. Thus, the cross-sectional area A<NUM> of the first volume may be circular, and therefore defined by a diameter d<NUM>. Where the output tube <NUM> is a conical frustum, the diameter d<NUM> of the output tube increases linearly along its length L<NUM>. That is, the diameter d<NUM> of the output tube <NUM> increases linearly between the second end <NUM> and the sound outlet <NUM>. The increase in third diameter d<NUM> may increase in the axial direction of the frustrum. The diameter of the output tube <NUM> can be measured at any point along this axial length of the output tube <NUM> in the radial direction.

The output tube <NUM> may have a different length L<NUM> to the lengths L<NUM>, L<NUM> of the first and second volumes. In an example, the length L<NUM> of the output tube is greater than the length of each of the first and second volumes L<NUM>, L<NUM>. Where the first and second volumes <NUM>, <NUM> are symmetrical about a plane <NUM> comprising the diaphragm <NUM>, the first and second lengths L<NUM>, L<NUM> are the same. In alternative examples, the first length L<NUM> may be different from the second length L<NUM>.

Where the first volume <NUM> is conical in shape, the minimum diameter d<NUM> defining the minimum cross-sectional area of the first volume <NUM> may be characterised with respect to the maximum diameter d<NUM> defining the maximum cross-sectional area A<NUM> of the first volume <NUM>. That is, the minimum cross-sectional area A<NUM> of the first volume may differ from the maximum cross-sectional area A<NUM> of the first volume <NUM> by a predefined ratio. In an example, the ratio of the minimum to the maximum diameter of the first volume <NUM> is between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum to the maximum diameter of the first volume <NUM> is <NUM>:<NUM>. Similarly the minimum diameter d<NUM> defining the minimum cross-sectional area A<NUM> of the second volume <NUM> may be characterised with respect to the maximum diameter d<NUM> defining the maximum cross-sectional area A<NUM> of the second volume. In an example, the ratio of the minimum to the maximum diameter of the second volume may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum to the maximum diameter of the second volume may be <NUM>:<NUM>.

The minimum diameter d<NUM> defining the minimum cross-sectional area A<NUM> of the first volume <NUM> may be characterised with respect to the length L<NUM> of the first volume. In an example, the ratio of the minimum diameter d<NUM> to the length L<NUM> of the first volume is between <NUM>:<NUM> and <NUM>:<NUM>. That is, the ratio of the minimum diameter d<NUM> to the length between the first end <NUM> and the diaphragm <NUM> is between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum diameter d<NUM> of the first volume to the length of the first volume is <NUM>:<NUM>. Similarly, The minimum diameter d<NUM> defining the minimum cross-sectional area A<NUM> of the second volume <NUM> may be characterised with respect to the length L<NUM> of the second volume <NUM>. The ratio of the minimum diameter d<NUM> to the length L<NUM> of the second volume may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum diameter d<NUM> to the length L<NUM> of the second volume <NUM> is <NUM>:<NUM>.

The maximum diameter d<NUM> of the first volume <NUM> may also be defined with respect to the length L<NUM> of the first volume. The ratio of the maximum diameter d<NUM> of the first volume <NUM> to the length L<NUM> of the first volume <NUM> may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the maximum diameter d<NUM> of the first volume <NUM> to the length L<NUM> of the first volume <NUM> is <NUM>:<NUM>. The maximum diameter d<NUM> of the second volume <NUM> may also be defined with respect to the length L<NUM> of the second volume <NUM>. The ratio of the maximum diameter d<NUM> to the length L<NUM> of the second volume <NUM> may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the maximum diameter d<NUM> to the length L<NUM> of the second volume <NUM> is <NUM>:<NUM>.

The minimum diameter d<NUM> of the output tube <NUM> may additionally or alternatively be defined with respect to the maximum diameter d<NUM> of the output tube <NUM>. In an example, the ratio of the minimum to the maximum diameter of the output tube <NUM> is between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum to the maximum diameter of the output tube <NUM> is <NUM>:<NUM>. The minimum diameter d<NUM> of the output tube <NUM> may also be defined with respect to the length L<NUM> of the output tube <NUM>. That is, the minimum diameter d<NUM> of the output tube <NUM> may also be defined with respect to the length from the second end <NUM> to the sound outlet <NUM>. The ratio of the minimum diameter d<NUM> to the length L<NUM> of the output tube <NUM> may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the minimum diameter d<NUM> to the length L<NUM> of the output tube <NUM> is <NUM>:<NUM>. The maximum diameter d<NUM> of the output tube <NUM> may also be defined with respect to the length L<NUM> of the output tube <NUM>. The ratio of the maximum diameter d<NUM> of the output tube to the length L<NUM> of the output tube may be between <NUM>:<NUM> and <NUM>:<NUM>. In a more specific example, the ratio of the maximum diameter d<NUM> to the length L<NUM> of the output tube <NUM> is <NUM>:<NUM>. The relative lengths and diameters of the components in the sound bypass device are selected to ensure the optimal transference of pressure through the sound transmission device, and therefore to minimise transmission loss values generated by the device.

The alternative configuration of a diaphragm for use in the improved sound bypass device of <FIG> is illustrated in <FIG>. With the exception of the configuration of the diaphragm, the features of the sound bypass device illustrated in <FIG> correspond broadly to those of the respective device that is illustrated in <FIG>. That is, the bypass device of <FIG> comprises an input tube <NUM> and a sound transmission device <NUM> corresponding to the respective components illustrated in <FIG>. The bypass device may further comprise an output tube <NUM>. The sound transmission device <NUM> comprises a first volume <NUM> and a second volume <NUM> separated by the alternative diaphragm <NUM>.

The diaphragm <NUM> in <FIG> differs from that which is illustrated in <FIG> in that, instead of consisting of a single membrane, it comprises two membranes that are spaced apart. That is, the diaphragm comprises a first flexible membrane <NUM> and a second flexible membrane <NUM>. The first flexible membrane <NUM> is located within the first volume <NUM> of the sound transmission device <NUM>. The first flexible membrane <NUM> is connected across the width of the first volume <NUM>. This enables sound pulses travelling through the first volume from the first end <NUM> of the sound transmission device to drive the motion of the first flexible membrane <NUM>. The second flexible membrane <NUM> is located within the second volume <NUM> of the sound transmission device <NUM>. The second flexible membrane <NUM> is connected across the width of the second volume <NUM>. The first and second membranes <NUM>, <NUM> may be formed of metal. The first and second membranes may alternatively be formed from any other material that is able to withstand the temperatures and pressures exerted by engine exhaust gases whilst also being deformable so as to transfer variations in pressure from the first volume to the second volume of the sound transmission device. The first and second membranes are configured to vibrate in response to pressure variations that are transmitted through the sound transmission device <NUM> whilst preventing flow of exhaust gases from flowing from the first volume to the second volume.

The first and second flexible membranes <NUM>, <NUM> move in response to changes in pressure that are transmitted through the sound transmission device. The first and second flexible membranes <NUM>, <NUM> are connected together by a connecting member <NUM>. The connecting member <NUM> allows sound vibrations to travel between the first flexible membrane <NUM> and the second flexible membrane <NUM>. The first flexible membrane <NUM> is configured to move in accordance with sound pulses received from the inlet of the sound transmission device. Thus, by connecting the first flexible membrane <NUM> to the second flexible membrane <NUM>, the connecting member <NUM> permits variations in pressure experienced by the first flexible membrane <NUM> to pass through the diaphragm to the second flexible membrane <NUM>. In other words, the connecting member <NUM> transfers sound vibrations through the diaphragm <NUM> from the first flexible membrane <NUM> to the second flexible membrane <NUM>. As the first flexible membrane <NUM> is in the first volume <NUM> and the second flexible membrane <NUM> is in the second volume <NUM>, the connecting member <NUM> transfers sound vibrations through the diaphragm <NUM> from the first volume <NUM> to the second volume <NUM>. To optimise the transmission of sound pulses from the first flexible membrane <NUM> to the second flexible membrane <NUM>, the connecting member may be in the form of a rigid shaft.

The diaphragm <NUM> further comprises a rigid barrier <NUM> separating the first volume <NUM> from the second volume <NUM>. The rigid barrier <NUM> is configured to restrict the transmission of heat between the first and second volumes <NUM>, <NUM>. The rigid barrier <NUM> may prevent the transmission of heat between the first and second volumes <NUM>, <NUM>. In other words, the walls of the rigid barrier <NUM> may be impermeable to heat and pressure variations. The rigid barrier acts to thermally insulate the first volume from the second volume, and vice versa. The presence of the rigid barrier <NUM>, in addition to that of the first and second membranes <NUM>, <NUM>, is configured to prevent exhaust gases from flowing from the first volume <NUM> to the second volume <NUM>.

The rigid barrier <NUM> comprises a channel <NUM> through which the connecting member <NUM> is able to extend. The channel <NUM> allows the connecting member <NUM> to pass between the first volume <NUM> and the second volume <NUM> in order to transfer vibrations from the first flexible membrane <NUM> to the second flexible membrane <NUM>. The diaphragm may further comprise a seal <NUM> positioned within the channel <NUM> and configured to hold the connecting member <NUM> in place within the channel <NUM>. The seal <NUM> comprises an outer diameter that is substantially the same as the inner diameter of the channel <NUM>, and an inner diameter that is substantially the same as the outer diameter of the connecting member <NUM>. This ensures a tight fit between both the seal <NUM> and the channel <NUM>, and the seal <NUM> and the connecting member <NUM>. In one example, the seal <NUM> may be press fitted into the channel <NUM>. Additionally, or alternatively, the connecting member <NUM> may be press fitted onto the seal <NUM>. The seal <NUM> also ensures that, whilst the connecting member <NUM> is able to transfer sound vibrations between the first volume <NUM> and the second volume <NUM> through the rigid member <NUM>, the transmission of heat between the first and second volumes <NUM>, <NUM> is minimised. The transmission of exhaust gases between the first and second volumes <NUM>, <NUM> is also minimised by the seal <NUM>. The seal <NUM> may be made of any material that is capable of withstanding the pressure and temperature variations experienced by the sound transmission device. The seal may be constructed from industrial rubber, Polytetrafluoroethylene (PTFE), Fluorosilicone (FVMQ), Polyurethane, or any other suitable material.

The first and second flexible membranes <NUM>, <NUM> are connected to the first and second volumes <NUM>, <NUM> respectively by elastic connectors 314a, 314b. Each flexible membrane may have a single elastic connector extending around the inner circumference of the wall of the first or second volume (as illustrated in <FIG>). That is, the first flexible membrane <NUM> may comprise a first elastic connector 314a connecting it around the circumference of the inner wall of the first volume <NUM>. The second flexible membrane <NUM> may comprise a second elastic connector 314b connecting it around the circumference of the inner wall of the second volume <NUM>. The diaphragm <NUM> may comprise any alternative number of elastic connectors. For example, each of the first and second flexible membranes may comprise two elastic connectors. The purpose of the elastic connectors is to allow the first and second flexible membranes to vibrate with respect to the walls of the first and second volumes. The elastic connectors may be formed from any suitable material that allows the membranes to vibrate with respect to the walls of the first and second volumes. In one example, the elastic connectors may be rubber springs.

The configuration of the diaphragm <NUM> illustrated in <FIG> is advantageous because it minimises the pressure gradient experienced by the membranes of the diaphragm due to the variation in temperature across the first and second volumes. During operation of the sound transmission device, the first volume <NUM> is exposed to gases that flow in from the exhaust system of a vehicle. The gases travelling through the exhaust system are hot, and so the first volume <NUM> is exposed to high temperatures. The second volume <NUM> is exposed to gases that flow in from the exterior or within the cabin of a vehicle. These gases are cool, relative to those that travel through the exhaust system, and so the second volume <NUM> is exposed to lower temperatures than the temperatures to which the first volume <NUM> is exposed. Separating the hot first volume <NUM> from the cooler second volume <NUM> using a diaphragm comprising a single membrane exposes that membrane to a steep heat gradient, and therefore to a steep pressure gradient that is experienced across the sound transmission device. This pressure gradient may cause a single membrane to deform, negatively affecting its ability to transfer sound vibrations across the sound transmission device. In contrast, by thermally isolating the first volume <NUM> from the second volume <NUM> using a rigid barrier <NUM>, and by transmitting sound vibrations across the device using two flexible membranes <NUM>, <NUM> and a connecting member <NUM> connecting the first flexible membrane to the second membrane, the pressure differential experienced by the diaphragm is minimised whist the ability of the diaphragm to transmit sound vibrations from the first volume to the second volume is maintained. This diaphragm arrangement can therefore be used to optimise the performance of the sound transmission device.

The diaphragm <NUM> may further comprise one or more balance orifices for equalising the pressure that is built up in the first and second volumes <NUM>, <NUM> respectively. The first volume <NUM> may comprise first orifices 318a, 318b. The second volume <NUM> may comprise second orifices 320a, 320b.

The first orifices 318a, 318b may be located in the first flexible membrane <NUM> of the first volume. The first orifices may pass through the first flexible membrane <NUM>, connecting a first side of the first volume <NUM> located between the first flexible membrane <NUM> and the first end <NUM> of the sound transmission device from a second side of the first volume <NUM> located between the first flexible membrane <NUM> and the rigid barrier <NUM>. The first orifices 318a, 318b may be holes of any suitable shape, such as circular. In <FIG> the first volume comprises two orifices 318a, 318b. In alterative examples the first volume may comprise one orifice, or more than two orifices. The orifices act to prevent pressure from building up in the second side of the first volume <NUM> by providing one or more openings that allow gases from the second side of the first volume to be distributed to the first side of the first volume. The orifices thereby ensure that the pressure gradient across the first volume <NUM> is minimised. Thus, the presence of the orifices in the first flexible membrane <NUM> act to prevent the first flexible membrane from deforming by minimising the pressure gradient across the membrane.

It is advantageous that the orifices in the first volume <NUM> are located in the first flexible membrane <NUM>, instead of being located in the walls of the first volume <NUM>, because this ensures that exhaust gases present in the first volume are unable to escape from the sound transmission device into other parts of the vehicle such as the cabin.

The second orifices 320a, 320b may be located in the walls of the second volume <NUM>. The first orifices may pass through the walls of the second volume <NUM>, connecting the inside of the second volume <NUM> from the exterior of the sound transmission device. The second orifices 320a, 320b may be holes of any suitable shape, such as circular. In <FIG> the second volume comprises two orifices 320a, 320b. In alterative examples the second volume may comprise one orifice, or more than two orifices. The second orifices 320a, 320b act to prevent pressure from building up in the second volume <NUM> by providing one or more openings that allow compressed gases from the second volume to be distributed to the atmosphere. The orifices may be located on a first side of the second volume <NUM> located between the rigid barrier <NUM> and the second flexible membrane <NUM>. The orifices thereby ensure that the uniform pressure gradient on either side of the second flexible membrane <NUM> in minimised by dispersing gases on the first side of the second volume to the atmosphere.

It is advantageous that the orifices in the second volume are located in the walls of the second volume <NUM> and not the second flexible membrane <NUM> because it is cheaper to form these orifices in the walls of the second volume than it is to form them in the second flexible membrane. As there are no exhaust gases in the second volume <NUM>, it is less important if gases from the second volume are expelled out of the sound transmission device than it is if they are expelled from the first volume. Another benefit of the second orifices 320a, 320b being located in the walls of the second volume <NUM> is that this maintains the acoustic performance of the second flexible membrane <NUM>. It is beneficial to maintain the acoustic performance of the second flexible membrane <NUM> because it is this membrane that carries sound vibrations from the vehicle engine out of the sound transmission device.

In an alternative example, the second orifices 320a, 320b may be located in the second membrane <NUM>. In other words, in this second example, the second orifices may pass through the second flexible membrane <NUM>, connecting a first side of the second volume <NUM> located between the first flexible membrane <NUM> and the rigid barrier <NUM> of the sound transmission device from a second side of the second volume <NUM> located between the second flexible membrane <NUM> and the second end <NUM> of the sound transmission device.

In the examples described above, the sound bypass device comprises a sound inlet which is connected to a first location on the exhaust system before one of the exhaust components along the flow path of the exhaust gases. However, in an alternative examples, the inlet of the sound bypass device may be connected to the air intake system <NUM> for the vehicle. Connecting the inlet of the sound bypass device to the air intake system (instead of the exhaust system) has little effect on the sound vibrations that are transferred from the system to the sound transmission device but does expose the first volume <NUM> of the device lower temperatures than those experienced if the inlet is connected to the exhaust system. This reduces the difference in heat experienced across the sound transmission device, which is beneficial in ensuring that suitable transmission loss values are met.

Similarly, the gases that are prevented from passing from the first volume <NUM> to the second volume <NUM> of the sound transmission devices described above are described as "exhaust gases". However, in alternative examples such as an example where the inlet of the sound bypass device is connected to the air intake system, the gases prevented from passing to the second volume <NUM> may be referred to as "intake gases". The gases prevented from passing from the first volume to the second volume may alternatively be a combination of intake and exhaust gases.

<FIG> illustrates the advantages afforded by the sound bypass device of <FIG> as compared to known alternative bypass devices. The graph of <FIG> illustrates the variation in transmission loss relative to the frequency of sound vibrations. Sound vibrations, in the context of the present invention, are generated by the engine of the vehicle. The frequency of such vibrations is measured in hertz (Hz) and is plotted along the x-axis of the graph in <FIG>. Transmission loss is measured in decibels (dB) and is plotted along the y-axis of the graph.

To achieve the desired sound profile for the engine noises of a vehicle comprising a sound bypass device, the transmission loss associated with the bypass device should be below a predetermined threshold value. Transmission loss is defined as the ratio between transmitted and incident pressure waves. Thus, as the transmission loss increases the difference in pressure between the incident and transmitted sound waves increases, so that transmitted sound waves become more damped. In other words, a transmission loss characteristic above the predetermined threshold value for a vehicle will mean that the sound waves output by the engine will be muffled by the sound bypass device. This will degrade the quality of sound output by the vehicle and therefore the perceived performance of the vehicle. An exemplary predetermined threshold value for transmission loss value is illustrated by the line <NUM> in <FIG>.

During operation of a vehicle, the frequency of sound vibrations generated by its engine are within a defined range of values. In <FIG>, this defined range of frequency values is illustrated by reference numeral <NUM>. It would be appreciated that the range of frequency values produced by the engine during its operation varies for different vehicles. The operative conditions of a vehicle may also be characterised by defined pressure and temperature values. For example, the pressure within the vehicle exhaust system during operation may be between <NUM> and 5bar, and the temperature within the exhaust system may be between <NUM> and <NUM>.

The transmission loss characteristic of a sound bypass device is highly dependent on the shape of the components within the device. A known design of a sound transmission device comprises cylindrical first and second volumes with a constant cross-sectional area across their lengths. An indication of the transmission loss characteristic produced by this known design is illustrated by line <NUM> in <FIG>. It can be seen that, within the range of frequencies generated by the engine during operative conditions, the transmission loss of the bypass device is above the threshold value <NUM>. Thus, this known design is not desirable for optimising the perceived performance of the vehicle.

The transmission loss characteristic produced by the design of a sound bypass device as illustrated in <FIG> is depicted by line <NUM> in <FIG>. It can be seen that, within the range of operative frequencies of the vehicle demonstrated in <FIG>, the transmission loss experienced by the bypass device is below the threshold value <NUM>. Thus, the muffling of the noise produced by the engine of the vehicle is reduced to an acceptable level and the perceived performance of the vehicle is optimised.

Claim 1:
A sound bypass device (<NUM>) configured to transmit engine-generated sound pulses from an engine (<NUM>) to a sound outlet (<NUM>) whilst preventing flow of gases to the sound outlet, the sound bypass device comprising:
an input tube (<NUM>) configured to conduct the engine-generated sound pulses from the engine;
a sound transmission device (<NUM>) connected to the input tube at a first end (<NUM>) and to the sound outlet at a second end (<NUM>), the sound transmission device comprising: a first volume (<NUM>) connected to the first end, a second volume (<NUM>) connected to the second end, and a flexible diaphragm (<NUM>) separating the first volume from the second volume and configured to transfer variations in pressure in the first volume to the second volume, wherein the first volume has a cross-sectional area (A<NUM>) that is greater at the diaphragm than at the first end; and
an output tube (<NUM>) configured to conduct sound pulses to the sound outlet;
the sound bypass device being characterized in that
the output tube has a cross-sectional area (A<NUM>) that is greater at the sound outlet than at the second end and the rate of increase of the cross-sectional area of the output tube is constant along the length of the output tube; and
the second volume has a cross-sectional area (A<NUM>) that is greater at the diaphragm than at the second end.