The point of an exhaust silencer is to do exactly what it’s name suggests, make silence. Now In this article, we aren’t talking about mufflers and resonators in general. Those will come in a later post about how exhaust systems work. Here we’re just referencing those little end restrictors that you often see inserted at the track or sometimes on the road in the end can of the exhaust.
If you took physics in school, you may have learned all about sound waves and the difference between high and low frequencies. By inserting a bung into the main opening of the exhaust which makes the pipe exit smaller, and having a small length of pipe extend inside from the silencer, it cancels out sound waves. The waves hit the backside of the silencer (the flat face around the smaller exit). The reason there is a pipe behind this is because if the sound waves and farts and sharts coming out of your engine just hit this plate with a smaller exhaust hole in it, it would make a huge amount of turbulence and slow things down quite a lot. Because the pipe is extends and is smaller, it creates more pressure (pressure is force over area), forcing the exhaust out the pipe.
Lower frequencies also travel farther than higher ones. This is the reason you can hear the idiot with his subwoofers booming down the street half a mile away, but you can’t hear the lyrics so clearly. The higher frequencies are more easily absorbed because they have tiny wavelengths, compared to huge ones for low frequencies (remember the Hz test for your subwoofers? How low can they go). The silencer may cancel out the low frequency’s (making negative and positive waves meet, creating neutral) making the exhaust raspy and high-pitched, but that also makes them quieter as well.