The intensity of sound is determined by wave frequency and the air pressure level. The higher the pressure level the higher is the intensity of sound that you feel. This intensity of sound is measured by Decibel level. As the decibel level goes up, go is the level of discomfort that you feel and the time for which you can tolerate the high intensity sound goes down with higher decibel level. At a level of 120 decibel level (the sound of jet engine exhaust), you can tolerate it just for few seconds. The distance from the source also plays a part. The range of frequencies a human ear can hear is 20 cycles/second to 20,000 cycles /second.
Obviously, with all those automobiles working around you and the mufflers working, you do not appreciate their working. A tiny hole in one of pipe leading to the muffler of just one car is enough to make you realize the importance of mufflers in the automobiles.
By now you understood how noise and sound travels. You want to cancel the noise and not the sound. Fortunately, the noise has a characteristic and it is not varying as the sound. The noise can be cancelled by sending out a pressure wave equal and opposite to that of noise.
In case of a muffler, this pressure wave is generated automatically by providing baffles that generate the opposing wave. This is done through use of resonator. The high velocity gas is let off inside the muffler. A part of this chamber is closed and the end of closed chamber is in communication with portion where the incoming gas from engine is allowed to expand. As it expands, a pressure wave is generated and the closed chamber wall starts vibrating. Since the chamber is closed, the air trapped inside also starts vibrating and generate opposing wave. This effectively cancels the noise, coming out from the exhaust gases. The exhaust gases are then allowed to escape through another set of tubes, so that the velocity of gases is reduced immediately.
Therefore, you see that the muffler consists of two tubes with holes and a resonating chamber attached to the expansion chamber. The expansion chamber creates sound pressure wave, the resonation chamber cancels it and the tubes provide the pressure drop required to reduce the noise.
In some types of mufflers, instead of using the pressure wave cancellation, absorption techniques is used. In such mufflers, you will find the walls of mufflers filled up with sound absorption materials like glass wool or similar product, behind a perforated plate. The incoming exhaust comes in an expansion chamber and the pressure wave is absorbed by the material in the chamber. This method is generally not be used in automobiles as the resultant sound is generally higher than other technique.
The muffler also decreases the power of engine to some extent by increasing the backpressure of the engine. For jet engines, this is not acceptable and hence there is no muffler to a jet engine used for aviation.
The new technique that is being developed is the anti noise technique. In this case, a microphone picks up the noise in the pipe and a computer sends anti noise pulses to cancel the noise from the exhaust effectively. The technique has not yet reached a level where the commercial designs are available at an affordable price. Some of audio companies Like Bose Incorporated have come up with noise cancellation for use in houses.