How mufflers work(一)

- Jul 23, 2018-

If you've ever heard that a car engine is running without a muffler, you know what a huge difference a muffler can make to the noise level. Inside the muffler, you'll find a set of seemingly simple tubes with holes in them. These tubes and chambers can be finely adjusted like a musical instrument. They are designed to reflect the sound waves generated by the engine in such a way that they partially cancel themselves out.

The muffler uses some very neat technology to eliminate the noise. In this article, we'll introduce a real car muffler and learn the principles that make it work.

But first, we need to understand the sound.

Where Does the Sound Come From?

Sound is a pressure wave formed by alternating high and low pressure pulses. These pulses makes their way through the air at - you guessed it - the speed of the sound.

In the engine, a pulse is generated when the exhaust valve opens and a high pressure gas suddenly enters the exhaust system. The molecules in this gas collide with low-pressure molecules in the pipeline, causing them to stack on top of each other. They in turn stack up on the molecules a little further down the pipe, leaving an area of low pressure behind.In this way, the sound waves travel down the pipeline much faster than the actual gas.

When these pressure pulses reach your ear, the eardrum will vibrate back and forth. Your brain interprets this action as sound. The two main characteristics of the wave determine how we perceive the sound:
Sound Wave Frequency - A higher wave frequency means that the air pressure fluctuates faster. The faster the engine runs, the higher the pitch we hear. Slower fluctuations sound like lower pitch.
Air Pressure Level - The amplitude of the wave determines the size of the sound. Sound waves with larger amplitudes make our eardrums more, and we record this feeling as a higher volume.
It turns out that it is possible to add two or more sound waves together and get less sound. Let's see how.