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Nowadays there are sophisticated systems that not only reduce but also modulate the noise from car exhausts. But the applications don’t end there...

Nicodemo Angì

THERE ARE THOSE who believe that the sound from their car engine is heavenly music and share it on Facebook. There are those who modify the exhaust on their darlings so that they have an even louder voice and, obviously, there are also those who want their car to be very quiet because the mechanics and the exhaust are simply “noise”. How can these two needs be reconciled? You would need an exhaust whose voice can be varied in some way and, in effect, there are already solutions of this kind. They use, for example, a kind of valve that changes the exhaust geometry when it opens – not unlike the tapping of the musician’s fingers on the holes of a flute – and perhaps with the help of special “boxes” (resonators) that operate in certain frequencies.
But before talking about the recent developments in these sound modulating systems, we must say something about sound itself and about the characteristics that allow it to be effectively reduced and shaped.
Sound is propagated through air in the form of waves; each wave has a peak (the highest point of oscillation) and a trough, the lowest point. This type of phenomenon has quite special properties regarding, for example, the concept of “summing”. Adding one wave to another does not always signify an increase: because they oscillate, adding one to another could lead to the almost total disappearance of them both. This can happen when the peak of one corresponds to the trough of the other; in other words, when they are 180° out of sync they could cancel each other out because the oscillation of one is equal and opposite to that of the other.
This characteristic has been known about for centuries but it is only recently that, thanks to advances in electronics, it has begun to be used with a certain amount of ease in cancelling noise, for example. The disturbance must be added to another sound in such a way that its peaks fall into the troughs of the disturbance and vice-versa. The practical implementation of this principle normally includes the use of a microphone that instant-by-instant captures ambient noise in the case of a circuit for filtering external noise so that music and words can be heard better. The signal is processed, amplified and sent to a loudspeaker – or earpiece – in the form of an “anti-sound” signal that will cancel the disturbance but not the music. Thanks to increasingly powerful and ever smaller processors, this technology is now extremely widespread because it used in cell phones, speakerphones, headphones and even in cars! Vehicles are in fact fairly efficient generators of noise because of the typical rumble of the exhaust and various other noises (from gears, wheels, aspiration and so on). The stratagem of adding a suitably calculated sound to the noise itself will “modulate” the noise as well as reducing it.

Nicer noises, smaller exhausts
A specific example of the application of this technique is the ActiveSound and ActiveSilence systems recently developed by Eberspächer to “treat” noise. They change the noise made by the exhaust by means of a loudspeaker that selectively attenuates or amplifies certain frequencies. It is regulated by a control unit developed by the Eberspächer Electronics division, which processes the signals from the engine power unit and a microphone (we already know what it is used for) situated in the exhaust system.
The data are calculated more than 100 times a second, a speed that allows the correction signal to be sent to the loudspeaker even before the noise has had time to leave the exhaust pipe. We all know about downsizing, the reduction in the number of engine cylinders, with turbochargers to give the same power as a bigger engine but which consumes and pollutes less; however, this has affected exhaust noise. The ActiveSound applications include the new Audi A6 Avant, with its top-of-the-range TDI engine (313 HP) which “sounds” like a naturally aspirated, fuel injection V8, without the typical diesel clattter thanks to the loudspeaker in the exhaust system that attenuates certain frequencies and accentuates others, while staying within homologation standards. With electronic control systems of this type the exhaust systems can also be simplified because most of the noise attenuation is done by active cancellation. This means that a smaller silencer could be used with the elimination of resonators, intermediate chambers and valves. Components that are not only bulky and heavy, but also create damaging – in terms of performance and consumption – back pressure: with the reduction in weight and improved engine breathing, consumption can be reduced by up to 3%, which is also thanks to an exhaust systems that can be as much as 30% lighter. If active sound modulation allows high performance cars to have a sporty sound even with a simpler engine, ActiveSilence gives more comfort by attenuating the correct frequencies with the advantages of ActiveSound: reduction in back pressure, up to 60% saving in size and 40% in weight, fewer CO2 emissions and up to 10% less noise.
The importance of these systems is also destined to grow in the electric and hybrid vehicle sector, an area that does not involve engines in the strict sense. The heat produced by electric engines – they are over 85% more efficient so there is less heat dissipation – is not sufficient to heat the vehicle interior which means that battery power or small burners are used, which must be silenced because the noise emissions from an electric engine are very much lower than those from other engines. By optimizing combustion and using a silencer for exhaust gases, Eberspächer has reduced burner noise to below 50 dB, but the application of ActiveSilence technology would bring it to below 45 dB. The Airtronic (air) and Hydronic (water) units run on bioethanol with hardly any combustion residue or emissions, which are lower than the regulation 90%; and because they do not use the batteries, vehicle range is not affected.
Obviously there are systems that use the vehicle’s loudspeakers to reduce noise from the road, the engine and so on: the principle is the same with the advantage that the stereo system’s loudspeakers can be used to transmit the ant-disturbance sound.

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