Although there are actually four types of silencers, mainly the reflection- and the absorbtion silencers are installed, either singly or combined. In motorcycles, the first of these is best known, because it's relatively effective, for the amount of space which it needs and for the special demands required of it. In addition it scores points by, because of the constant performance, right up to the time when it has to be replaced. Nevertheless, there are models which have a certain amount of absorbtion through having a double-pipe, internally holed system. One should also be aware of, in contrast to the motor car, the residual heat on the the outside surface.
The inside of the actual silencer itself is made up of chambers, which are connected to each other through pipes with varying diameters. At the in-flow opening, some have a specially sysnchronised air funnel. Disturbing frequencies are thus reduced by the overlapping the flow-waves and the exhaust gas flow is throttled is little as possible, the noise is diminished without cutting the performance.
All this sounds much simpler than it is. The exhaust gas flow is not a constant value. It is then particularly vehement, when, on one of the cylinders, the exhaust valve opens. This does not occur suddenly, when the valve begins to open, a high flow-velocity is caused. To examine the processes closely, we are considering an open pipe, and not a pre- or a post silencer. In theory, this should be so long, that the first, intense exhaust gas flow is reflected at the open end and reaches the exhaust valve shortly before it closes. When it turns around again, in the most favourable case, it takes any gas remaining in the combustion chamber with it, at the same time providing for a little more vacuum when intaking.
Staying with the open pipe for a moment, as far as multi-cylinders are concerned, the important question arises of when the individual pipes are brought together. In theory, one cylinder could, in addition, help another one to get rid of ist own exhaust gas, in the practice, the wide RPM-band of the motorcyle, where in each area the demands are different, must be considered. Apart from that, the two-wheeler generally has fewer cylinders. However, there is one, almost even more decisive argument. If each cylinder has ist own exhaust-pipe, the individual silencers can be much smaller, and still be almost more effective.
The silencer is sometimes still concieved being a multiplication of the cubic capacity and although it's not easy to accomodate six (!) silencers, they do look great. In motorcycles, the design, particularly of the exhaust system, is also a lot more important than it is in motor cars. Whereby, one does get the impression that even standard systems exploit the performance completely. One can buy expensive add-on parts, which either overstep the legal requirements or provide only very little higher performance, which brings us to the question of how this development will come to terms with the new, stricter legal noise emission limits (80 dBA).
In particular, also as far as exhaust gas decontamination is concerned, the pre- and the main silencing in one common unit does make sense. The distribution of the weight and the amount of room available may also play a part here. The exhaust collector underneath the engine has a sort of crease, which is laid out exactly according to the ground clearance, also taking extreme banking angles into consideration. It would seem that electrically actuated flaps in the exhaust, which are regulated by the engine electronics, are nowadays, also inevitable.
Perhaps this technology will make it easier for the engine developers to solve the dilemma of modern exhaust gas decontamination and high revving engines, which by the way, are not all that much different from the motor car, and the requirement of keeping the exhaust system as compact as possible. It used to be more difficult to combat unpleasant noises and torque losses which appeared in hidden operational areas, perhaps nowadays, simply a differently regulated flap-opening may be enough. Perhaps an even better and much smaller solution would be a loudspeaker system which counters the exhaust noise. 08/10