The basic job of the silencer in an exhaust is to minimise the sound waves coming from the engine which is basically a series of very loud explosions. In some countries people associate noise with power, probably due to the fact that older cars had inefficient silencers. In countries where cars have relatively recently been introduced like Japan the silencer was more efficinet and the generally desired exhaust note is very quiet and more of a buzz, so this is reflected in aftermarket parts from different countries.
Do we really need exhaust silencers? The noise levels that even a standard small bore engine generates is incredibly loud and most of this exits the engine via the exhaust. The noise is too loud for the occuypants to be comfortable when driving and is a nuisance to everyone else around.
In most countries the law enforcement officials can take action if they deem a car exhaust to be too loud and causing a nuisance. For this reason then most countries have legislation requiring a silencer to be fitted to the exhaust and in some countries the noise levels are heavily regulated.
Most silencers around contain some kind of sound deadening absorbant material to dissipate the sound waves but exhausts work in different ways.
There are 2 primary exhaust types one contains a series of baffles (think of it like a maze with soft walls) and this reduces the exhaust flow as well as reducing the noise emitted.
The second type is a straight through pipe with perforations in it. The perforations take the sound energy and as this bounces back of the outside of the silencer it distorts and reduces the sound waves in the main exhaust charge, there is usually some form of sound absorbing material between the pipe and the outside.
Some manufactures have a primary and secondary resonating chamber to break up the sound and these can be of one or both types outlined above.
In a high performance engine or tuned engine you really want the exhaust gases to be vacated as quickly as possible. The baffle type of exhaust is too restrictive in this instance so we would need to look to a straight through or performance exhaust back box.
The benefits of the perforated pipe are obvious. The main exhaust stream is allowed to continue virtually unimpeded and the holes primarily allow the sound waves to penetrate and be absorbed.
The overall bore size of the entire exhaust system has an optimum setting. To small and it is restrictive, too large and the exhausts will flow more slowly. A larger bore silencer will not affect the airflow by a noticeable amount, and the larger the silencer the deeper the sound.
If you are performance tuning your engine, you need to choose a silencer that does not restrict the engine. A silencer alone will not add power to an engine but a restrictive silencer will rob you of power so all you are doing is maximising what the engines potential is.