What is Noise?
The Oxford Dictionary defines noise as:
“A sound, especially one that is loud or unpleasant or that causes disturbance”
Noise is subjective – what is noise to one person isn’t necessarily noise to another. Believe it or not, some people would classify this as noise:
Is it loud? Yes. Unpleasant? I don’t think so.
Essentially, noise is unwanted sound. So what is sound? Most people would define it as something you hear, but in terms of physics, it’s a vibration going through matter. Now most people can tell you that sound travels in waves, which most people would picture as this:
How sound is transmitted
That isn’t how sound waves work, though. In fact, sound waves take on different forms depending on whether they’re travelling through a solid, or gas/liquid. Put another way, the sound waves emanating from your engine take on a different form when they transition from the air around the engine into the structure of your vehicle. We’ll cover the means of noise transmission in vehicles in more depth in another article.
When sound waves travel though air (or liquids, or plasma), they look like this:(2)
When these waves meet a solid surface, they induce this pattern of wave in the material:
So, that’s what sound looks like at the particle level. In the first sound wave animation, the sound wave is being generated by the moving grey bar on the left. The bar could be your engine, probably the biggest source of vibration and noise in your vehicle. If you watch the red dots, you can see that even though the wave is only moving one way, each individual particle moves backwards and forwards, mimicking the vibration of the grey bar.
So instead of a being like a sine wave, sound is a pressure wave, which causes the affected air (or metal/fibreglass) to be at either higher-than-normal pressure or lower-than-normal pressure. You will see sounds being represented as a sine wave, like the first illustration of this article, but that’s a representation of pressure and time, not the actual path of the particles. It is the effects of pressure and time that affect what the ear hears. It is this vibration that Hushcoat reduces, so less of it reaches the inside of your vehicle.
In some circumstances noise can be seen. This isn’t totally relevant, but who could resist the lure of an explosion. Explosions are cool, as long as you’re not on the receiving end of one.
In this example (a reconstruction of a 950kg van bomb) you can quite clearly see the pressure wave from the explosion travel outwards from the detonation. If you were to be hit by this wave before it dissipated, it would travel through you, likely affecting your bones and internal organs in a very bad way. In any explosion, it’s the pressure wave that does the damage.
A less extreme way to see noise is by Schlieren flow visualisation – a clever technique that distorts light to allow the air disturbance to be recorded by a video camera. More details on this video:
Other than reducing vibration, or soundproofing, the only other way to reduce noise is to create a vacuum around the noisy object. There is no sound in outer space, as it is virtually a perfect vacuum with very low density and pressure. As the particle density is so low, waves can’t travel through it in the way that you’ve seen in the previous animations. This is just as well as, according to one Solar Physicist, if there was air between us and the Sun, and all of the noise made by the Sun was transmitted to Earth, there would be a constant background noise of around 100 decibels. That’s equivalent to a motorbike, or a pneumatic drill.
Soundproofing, then, is essentially about reducing the amount of vibration transmitted to your ears. The most effective way of doing this is to provide damping to the structural elements of the car, or van. This damping reduces the vibration transmitted through the vehicle structure. Adding mass and thickness is one way of reducing this vibration. What’s better than just adding mass and thickness is to apply a visco-elastic material to the surface. Not only does this add mass and thickness, it also works in a more effective way due to the way that the molecules within the material behaves. Hushcoat is one of these visco-elastic materials and also contains additives that convert vibration to low grade heat.
More details of how this product works will follow in a later post.
Thanks for reading, I hope you have found this brief introduction to noise of interest.
(4) Excerpt from https://youtu.be/g2jEUHyQ2Qw