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Sound moves
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01/02/2007
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The sound a vehicle makes is one of the key features of the impression it makes, and exhaust systems are a major factor in this respect. Ian Adcock looks at how engineers are tuning their systems to generate a distinctive noise
“Silence,” some would have you believe, “is golden.” In an art gallery studying a masterpiece, at the theatre or opera listening to a great performance, maybe. But a silent car? I’m not so sure.
Last summer I drove an example of what many see as the future of personal transport – a fuel cell powered family hatchback. In this case it was a Ford Focus, but the marque and model are irrelevant. It drove very well. Thanks to the instant torque from its electric motor its acceleration was sprightly and it held its own in the cut and thrust of a commute round the Parisian périphérique.
But what the Focus or any other electric powered car do not do is stir the soul. The reason? Their silence.
The Focus hummed and whirred along with all the emotion of a desk fan. Press the throttle and off we went, no sound of rising revs, no feedback from the motor except a whir. While motorcyclists screamed by, and Porsche and Ferrari drivers barked their engines in frustration at the traffic’s slow pace (or was it just to tell other drivers “I’ve got a sports car”?), I motored on in virtual silence. And it was boring.
Ironically, if fuel cells are the future then there’s a fortune to be made from making them noticeable, otherwise pedestrians literally won’t know what’s hit them.
But before then there’s plenty of time to enjoy what petrol and, increasingly, diesel engines do best: sound good.
Currently European legislation limits sound levels from vehicle exhausts to between 74 to 80db(A) depending on their classification, although most OEMs engineer cars to 71db(A). In fact the challenge facing OEMs today is not pass-by noise requirements, but unwanted noise from the tyre-road interaction.
Manufacturers of high performance cars have come up with ingenious solutions that allow their cars to meet both the legislative requirements and the emotional demands of their customers.
Adaptive silencing uses innovative exhaust flaps that intelligently adapt the tailpipe noise and exhaust back pressure to the driving situation.
In city traffic, particularly during deceleration, low-frequency tailpipe noises dominate. The exhaust flap thus closes one of two tailpipes at low speeds to cut low-frequency exhaust noises by around a third.
The perceived pitch of the exhaust noise rises during faster travel. Damping lower frequencies would now be ineffective, especially since tyre and driving noise is higher than exhaust noise at high speeds. The exhaust back pressure now has to be reduced, so the flap opens. The gas flows through both tailpipes: the exhaust gas back pressure drops and the engine can develop its full performance.
The exhaust flap is switched by a distributor vacuum dashpot mounted on the outside of the tailpipe. A hose provides the necessary suction pressure. Regulation by the engine controller depends on the speed, load and temperature and is via an electric reversing valve. Anyone who has heard Aston Martin’s V12 Vanquish under full bore acceleration will testify to the effectiveness of such technology.
But a thrilling exhaust note is not exclusive to high-end performance cars. Owners of more humble machinery also want their aural highlights. Here the trend of downsizing engines then turbocharging them to recoup the lost power has created its own challenges to acoustic engineers as it robs them of many of the sounds naturally created by the engine.
Engineers from Mann+Hummel working with Ford and DaimlerChrysler have developed a symposer that is used on both the Ford Focus ST and the Mercedes-Benz SLK.
With turbocharged and supercharged engines, the original acoustic behaviour is dampened by the compressor in the air intake system, and the engine noise is, to a large extent, no longer dependent on charge and engine speed. In addition, undesirable sounds are suppressed by improved acoustic insulation of the passenger compartment. For that reason, turbocharged engines require additional sound systems that generate the expected sound and provide acoustic feedback that corresponds to the driving dynamics.
The purpose of the symposer is to transfer engine-generated pulsations from the air intake into the passenger compartment and thus specifically create the engine noise. While high-frequency sounds predominate in front of and directly behind the turbocharger, the dynamic pressure signal downstream of the intercooler nearly equals the characteristics of a naturally aspirated engine.
Connecting the symposer directly in front of the throttle body is therefore ideal to produce the characteristic charge-dependent sound.
The symposer, 60 x 50 x 40mm in size, is made up of four chambers. A movable flap separates the two chambers on the engine side from the two chambers on the outlet side. The pulsations generated by the engine meet the pivoted flap, producing oscillation of the spring-mass system, which is then transferred to the outlet chambers. The pulsations are transmitted by means of a sound pipe through the bulkhead into the passenger compartment, thus creating the desired charge-dependent feedback.
With the application of sound design, a five-cylinder turbocharged engine which would otherwise have conventional acoustics with little charge-dependent sound now produces a sporty, powerful sound with distinctive charge feedback.
Jaguar engineers wanted to achieve a similar result in its new XK sports car. They developed a semi-active exhaust system that varies the gas flow through the main large silencer box, depending on the pressure in the system, with acoustically tuned tailpipes that eliminate low speed boom. This is combined with an underfloor resonator with a chamber for each cylinder bank that balances the sound from both banks.
By tuning both the air induction and exhaust system Jaguar managed to achieve both a low frequency pitch with a technically ‘sophisticated’ higher frequency to portray a feeling of power and performance.
Ensuring the driver and passenger get the full sound effects positive feedback is transmitted from behind the heater pack into the cockpit with a tuned cone and membrane enhancing the frequency and balancing the intake and exhaust sounds.
Porsche’s latest 911 range employs a complex series of baffles and resonators, including a Helmholtz resonator, with carefully designed perforations to harmonise the inlet noise.
Using WAVE matrix for calculating sound absorption, Porsche engineers created a computational model of the exhaust system to accurately assess its damping characteristics. The inlet manifold has been carefully profiled to allow sound into the car’s interior while preventing warm air returning to the engine. High frequency sounds are allowed to pass through a filter on top of the intake where they are absorbed.
Much of this work was done with GEI in Berlin using a new acoustic camera that uses 32 microphones to take an aural and visual picture of the sounds produced while the engine is running. This allows Porsche engineers to track individual sources of noise, eliminating those they do not want and enhancing others.
As manufacturers seek to give their products more personality it will be those that find successful ways of appealing to the buyer’s ear as much as to their eyes that will have the edge.
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Author
Ian Adcock
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