Managing Your Car’s Noise Pollution
February 20th, 2009 adminManaging Your Car’s Noise Pollution
Without a muffler, the modern automobile would be much louder. What may seem like a nondescript metal cylinder is actually a device that reduces engine noise by partially canceling out sound waves. In this article, I’ll describe the nature of sound in the context of how it is controlled under the hood.
What Is Sound And How Is It Controlled?
Sound is a form of mechanical energy, a vibration formed by alternating phases of compression and rarefaction (high and low pressure) in frequencies able to be detected by the ear. An internal combustion engine creates sound by the opening of exhaust valves, sending pulses of high-pressure gas into the exhaust system in rapid succession. The spaces between these pulses, in combination with the low-pressure environment in the exhaust pipe, create the distinct hum (or roar) of an engine.
If the engine is running faster, then the space between the pulses is shorter and the frequency rises, leading to a higher-pitched noise. If the gas pedal is depressed, then the pressure of the individual pulse rises, increasing the amplitude of the sound waves and volume.
Understanding Its Operation
A muffler operates on the principle of sound known as destructive interference. Destructive interference is the effect by which two opposing waves cancel each other out. As a muffler is tuned to cancel out a majority of the frequencies produced by the common engine, the noise level is partially reduced.
Originating at the inlet, the exhaust fumes and noise pass through a tube into the resonating chamber. The sound waves then reflect off the back wall of the resonating chamber and pass through a hole into the main chamber. From this point, the gases then travel through the outlet pipe and exit the vehicle.
The resonating chamber is designed to a specific length to match the majority of sound frequencies produced by the engine. When the they reflect off the back wall, they are inverted; when passing through the hole of the main chamber, the inverted waves collide with incoming waves and cancel each other out.
In addition to the resonator chamber, the shell of the muffler is made of three layers: a thicker layer sandwiched between two thin layers of metal. Secondly, the pipes entering and exiting the muffler are wrapped in thousands of tiny holes that serve to reflect and cancel a portion of the sound. In certain luxury automobiles, the muffler is supplemented by a resonator, a cylinder very similar to the resonator chamber that serves to cancel additional noise.
The science behind the compression of the noise that your car makes is complicated. What I’ve just described above is the framework through which this particular part manages it.
