How does the silencer of bike work? The Principle.

The silencer is an important part of the motorcycle. If you start or operate a silencer, 

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and you will understand its importance. It will be too noisy without a silencer. So silencer helps to reduce the noise volume level drastically using an interesting technique. So we are going to understand the complete science behind modern silencers. First, understand how sound is generated and propagates. The air particles around the speaker are at normal pressure. When the diaphragm moves one layer of air particles will be pushed and so collide with the next layer. This process continues next to the next layer and so on. Now you can see a compressed layer of air particles. 

However, the air is elastic in nature and always wants to come back to its original position. Due to this after traveling some distance the first layer of air particles comes back to its original position. When the sound particles continue this process you can see a beautiful phenomenon in the animation. See a layer of compressed air moving forward this is how the sound wave is generated and propagated. A stream of airborne particles never reaches your ears. The air particles are just oscillating in their mean position and the amplitude of the sound wave is clearly marked here.

In the case of a bike engine, you might think the combustion process creates noise. During

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combustion, the expanding fireball with high speed hits the nearby air particles. This kick is sufficient enough to start a sound wave propagation as we saw earlier. However, the majority of sound that we hear is not from this combustion process. But from the exhaust surprisingly right, the reason is that during combustion the cylinder is completely closed due to this sound waves are mostly absorbed by the cylinder making it vibrate.

Read more: How does gun Silencer Work?

The question is how the exhaust generates noise. The exhaust gas pressure is almost four times that of the surrounding air. That is, when the exhaust valve opens, these gases suddenly flow into the manifold and collide with air particles.  Due to this, a sound wave will be generated at this instance. So the engine case is different from firecrackers or guns where the source of the sound is combustion. These sound waves then strike your eardrum and it vibrates. These vibrations are sent to your brain and you hear the sound. Here's an interesting question. What do you think changes in the sound wave make it louder or quieter? Yes, it's the amplitude.

When the biker accelerates you hear an even louder noise this is because as more fuel is burned the exhaust gas pressure increases. The higher the exhaust pressure more will be the movement of the nearby air particles. When the exhaust valve is open, the sound wave is generated with a higher amplitude. Now let’s employ porous fiberglass to absorb the sound waves in the exhaust flow path. Some of the sound energy will be absorbed in each stage by the porous fiberglass and part of the force will be transferred. The distance traveled by each swing then. The sound waves are the motion transfer between the swings and the porous fibers are the rubber walls these fibers absorb the pressure waves and at the same time do not block the exhaust flow.  As the flow continues the amplitude of the sound keeps decreasing silencing the sound. After absorbing energy from these particles the fibers converts this energy into heat and radiate the heat through the silencer's outer cover. However, this fiberglass is not used directly, the reason for the back pressure on the engine. These dense fibers still partially block the exhaust gases reducing the engine's efficiency. Thus to avoid this the fiberglass sheet is wrapped around a

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perforated tube. Now the question is how the sound waves will get absorbed. Will the sound wave escape through the central hole? Here is the trick. Observe the path the exhaust gases take inside this muffler. The exhaust flows through the tube holes into the fiberglass. It is important that the sound waves always travel with the liquid. Thus the sound waves pass in this fashion and are silenced to a greater extent inside the fiber material as explained earlier. This kind of muffler design is known as the absorption type and is best for motorcycles. If your bike has gotten noisy it's probably time to change the fiberglass material.