Have you ever heard a car running without a muffler? If so, then you'll understand just how loud a car's engine really is. Before you can learn about how mufflers work, however, there are a few things that you should know about the nature of sound.
Sound travels in waves, much like waves in the ocean. As sound waves enter your ears, your eardrums sense the vibration and your brain interprets it as sound. Below is a visual representation of a sound wave.
Fig. 1 - A Sound Wave
One of the fundamental qualities of a sound wav e is frequency, or the number of pulsations per unit time. Lower frequency sounds have more of a bass quality to them while higher frequency ones have a high-pitched quality. Every sound source transmits its own unique sound wave, but by the time they reach your eardrum, different sound waves will combine in one of two ways. This is called interference.
The first type of interference that can occur between two or more sound waves is constructive interference. In essence, if the waves are in phase they will join to become one wave with larger amplitude. In the below example, two of the same waves have combined to create one larger (and louder) wave.
Fig. 2 - Constructive Interference
The other type of interference that can occur is called destructive interference. It occurs when two waves that are out of phase with each other meet. In the below example, the two waves will actually cancel each other out, making it noiseless. This principle is at work in both noise-cancelling headphones and car mufflers.
Fig. 3 - Destructive Sound Wave
So if two or more waves that are out of phase meet, there will be a reduction in the total noise. Now, it's rare that sound waves will cancel each other out completely, but this type of interference occurs all the time. A car muffler is specifically-designed to reduce the engine noise in this way.
A car muffler is actually just a hollow piece of metal that has a shape that concentrates the noise of the engine. Then, as the sound is reflected within the muffler, it becomes out of phase with itself and has a cancelling effect. To further reduce the engine noise, the muffler has a layer of insulating material surrounding it that keeps sounds from escaping.
If you notice that your car is unusually loud, then it may be a good to have you muffler checked out.
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