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Automotive Q & A

Exhaust System

Question: How does a muffler work?

Answer: Mufflers are part of your vehicle’s exhaust system and are located at the rear, bottom of your vehicle. They aid in dampening vehicle emissions and engine noise. They are made of steel and are coated with aluminum to provide protection from the heat and chemicals released from the exhaust system. Mufflers are used mainly to dissipate the loud sounds created by the engine’s pistons and valves. Every time your exhaust valve opens, a large burst of the burnt gases used during your engine’s combustion is released into the exhaust system. This release of gases creates very powerful sound waves. To understand how a muffler dissipates the sound waves created by your engine, one must understand how sound is produced. Sound is a pressure wave formed by vibrations. These vibrations are pulses of alternating high and low air pressure. So, every time your exhaust valve opens, a very high-pressured gas enters into the exhaust system. These high-pressure gases will collide with low-pressure molecules, create pressure waves (sound), and travel through the exhaust system. Now, how exactly does a muffler dissipate these loud sound waves? Sound can actually be cancelled out. If you can introduce a pressure wave that is the exact opposite of the initial sound wave, meaning their wavelengths, or high- and low-pressure points, are opposite, they cancel each other out, and there is no sound. Another way to describe what happens is when one sound wave is at its maximum pressure, the other sound wave is at its minimum pressure; so, they cancel each other out. This is called destructive interference and is what occurs inside your muffler. A muffler design is very simple yet very precise. Inside a muffler there are tubes with perforations that direct the sound waves through the inside of the muffler and out the end. Sound waves will enter through a central tube, hit the back wall, pass through a hole and enter the center chamber. Then the sound wave will travel through another hole and enter the resonator chamber, which is back towards the front of the muffler where the sound waves first entered. Now, some of the sound wave will reflect off the center chamber’s wall, while the rest will pass through the hole and into the resonator chamber. The resonator chamber has a very specific length in order to produce sound waves that will cancel out other waves. The resonator chamber’s length is designed so that when the sound wave hits the back wall of the resonator chamber and travels back through the hole in which it came, it will meet with the next sound wave exactly when it hits off the center chamber’s wall. So, the high-pressure sound wave that traveled through the resonator will join with the low-pressure sound wave that was reflected off the center chamber’s wall and cancel each other out. Every aspect of the muffler is designed to aid in cancelling out noise. Even the walls of a muffler are specifically designed; they are actually able to absorb some of the pressure waves. Now, back to the tubes with perforations, these perforations allow thousands of tiny pressure waves to escape into the center chamber, bounce off the walls and cancel each other out. Basically, a muffler is specifically designed to control how sound waves bounce off its walls so they cancel each other out.

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