This part of a motorcycle, the silencer, is so important. Let’s operate the motorcycle, without the silencer to understand its importance. It is too noisy. Silencer helps to reduce the volume level drastically. Using an interesting analogy we are going to understand the complete physics of modern silencers in this video.
Let’s first understand how sound is generated and propagates. The air particles around this speaker are at normal pressure. When the diaphragm moves to the right, one layer of air particles will collide with the next layer and this process continues. Now you can see a compressed layer of air particles. However, the air is elastic in nature. This means it always wants to come back to its original position. Due to this after travelling some distance towards the right the first layer of air particles come back to its original position. When the sound particles continue this process, you can see a beautiful phenomenon. You can see a layer of compressed air moving forward. This is how the sound wave propagates. There is never a flow of particles to your ears or the air particles are just oscillating in their mean position. Amplitude of the sound wave clearly marked here. Let’s replace these air particles with the pressure bands for the ease of visualisation.
In the case of a bike engine, you might think the combustion process creates noise? Let's check it. During combustion, the expanding fireball with high speed hits the nearby air particles. This kick is sufficient enough to start a soundwave propagation as we saw earlier. However, the majority of sound that we hear is not from this combustion process, but from the exhaust. Surprising, 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. The question is: how does the exhaust generate noise? Let’s have a look.
The pressure of the exhaust gas is almost 4 times that of the surrounding air. This means when the exhaust valve opens, these gases suddenly rush into the manifold and hit the 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 to 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. Higher the exhaust pressure more will be the movement of the nearby air particles when the exhaust valve is open or generation of a sound wave with higher amplitude. According to the Motor Vehicle Act every vehicle’s noise should be limited to 80 decibels.
Let’s first understand how sound is generated and propagates. The air particles around this speaker are at normal pressure. When the diaphragm moves to the right, one layer of air particles will collide with the next layer and this process continues. Now you can see a compressed layer of air particles. However, the air is elastic in nature. This means it always wants to come back to its original position. Due to this after travelling some distance towards the right the first layer of air particles come back to its original position. When the sound particles continue this process, you can see a beautiful phenomenon. You can see a layer of compressed air moving forward. This is how the sound wave propagates. There is never a flow of particles to your ears or the air particles are just oscillating in their mean position. Amplitude of the sound wave clearly marked here. Let’s replace these air particles with the pressure bands for the ease of visualisation.
In the case of a bike engine, you might think the combustion process creates noise? Let's check it. During combustion, the expanding fireball with high speed hits the nearby air particles. This kick is sufficient enough to start a soundwave propagation as we saw earlier. However, the majority of sound that we hear is not from this combustion process, but from the exhaust. Surprising, 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. The question is: how does the exhaust generate noise? Let’s have a look.
The pressure of the exhaust gas is almost 4 times that of the surrounding air. This means when the exhaust valve opens, these gases suddenly rush into the manifold and hit the 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 to 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. Higher the exhaust pressure more will be the movement of the nearby air particles when the exhaust valve is open or generation of a sound wave with higher amplitude. According to the Motor Vehicle Act every vehicle’s noise should be limited to 80 decibels.
To achieve this, let's employ porous fibreglass in the exhaust flow path to absorb the sound waves. To understand how this fibreglass reduces sound, first look at these swing examples. If you push the first child, it pushes the second child and so on, transferring the energy without leaving their place, similar to sound propagation. Ideally all these swings travel the same distance, there is no energy loss. Suppose you want to reduce the amplitude of their swing’s oscillation. If we introduce concrete walls between them, the motion of the swings is completely obstructed. We don’t want to obstruct their motion. Now, let’s introduce a slightly flexible rubber wall between them. As you can see, some of the swing’s energy will be absorbed in each stage by the rubber wall and part of the force will be transferred. The distance travelled by each swing then keeps decreasing. This is exactly what we want - reduction in amplitude without obstructing the movement.
The same thing happens in the case of fibreglass. Here, the sound waves are the motion transfer between the swings and the porous fibers are the rubber walls. These fibres absorb the pressure waves and at the same time do not block the exhaust flow. As the flow continues, the amplitude of sound keeps decreasing, silencing the sound. After absorbing energy from these particles, the fibres convert this energy into heat and radiates the heat through the silencer's outer cover.
However this fibreglass is not used directly as depicted. Do you know why? The reason is backpressure on the engine. These dense fibres still partially block the exhaust gases, reducing the engine efficiency. Thus, to avoid this, the fiberglass sheet is wrapped around a perforated tube. You might be thinking, with this design how will the sound waves 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. This exhaust form eddies flowing through the tube holes into the fiberglass. The important thing is that the sound waves always travel with the fluid. Thus the sound waves pass in this fashion, and are silenced to greater extent inside the fibre 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.
The same thing happens in the case of fibreglass. Here, the sound waves are the motion transfer between the swings and the porous fibers are the rubber walls. These fibres absorb the pressure waves and at the same time do not block the exhaust flow. As the flow continues, the amplitude of sound keeps decreasing, silencing the sound. After absorbing energy from these particles, the fibres convert this energy into heat and radiates the heat through the silencer's outer cover.
However this fibreglass is not used directly as depicted. Do you know why? The reason is backpressure on the engine. These dense fibres still partially block the exhaust gases, reducing the engine efficiency. Thus, to avoid this, the fiberglass sheet is wrapped around a perforated tube. You might be thinking, with this design how will the sound waves 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. This exhaust form eddies flowing through the tube holes into the fiberglass. The important thing is that the sound waves always travel with the fluid. Thus the sound waves pass in this fashion, and are silenced to greater extent inside the fibre 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.
