The prequel to the ‘A Quiet Place’ saga got me thinking.
spoiler alert!
There is a scene in which many humans march towards a safety point. Each individual human would have been relatively quiet, but because there are a lot of them (potentially hundreds), they end up being, as a whole, loud enough to alert the monsters so they get all killed.
This would suggest that many sources of noise which are near to each other and generate more or less the same amount of noise end up adding up so that the end result in dB is more or less the sum of the individual dB levels.
But then again, it’s fiction.
Back to reality, I work in a room full of different servers which have also very different levels of noise. I have noticed that from my standpoint, the noise of the quietest server seems to disappear whenever the loudest is running, so it kind of does blow my mind how our perception of noise works…
So lets talk about constructive versus destructive interference.
First off, waves are waves. When you have something like a fan, its generating a series of waves. These waves are typically coming at a constant frequency. The frequency here matters.
Sound travels in waves. These waves are pressure waves in a fluid. Those waves don’t just get absorbed by hard surfaces; they reflect or refract. But those waves are both moving through the space, and themselves have a frequency.
This is important because of this you get patterns of constructive and destructive interference in space.
In some areas of a space there will be amplification, because the frequencies are lining up. In some spaces, the waves will cancel out. Those areas will be quieter.
This phenomena is true for all waves.
OP: Destructive interference as illustrated here is the basis of active noise cancellation (ANC) in headphones.
In theory it is a microphone on the outside of the headphones that has the outside sound inverted and mixed into whatever is playing from the speakers inside the headphones. The undesired outside noise reaches inside the headphones at the same time as the inverted signal is played so that they cancel each other out.
In reality there is some more complexity, as always, but it’s the core principle.
Very nice, though one thing to note for readers is the pictures are of transverse waves like you’d see in a vibrating string. Sound waves are longitudinal pressure waves which propagate outward in expanding spheres from the source of the noise. The loudness of a sound you hear corresponds to the pressure and frequency it imparts on your eardrum at the point of intersection between that expanding sphere and your ear.
This pressure is directly proportional to the surface area of your eardrum on the surface of that sphere. As you may or may not recall from high school geometry, the surface area of a sphere is 4pi*r^2. If you consider the pressure of a sound wave as being evenly spread out over the surface of that expanding sphere (assuming an ideal gas), then doubling the distance from the sound source will quadruple the surface area of the sphere, thus decreasing the pressure your eardrum experiences by a factor of four! Sounds from very far away rapidly lose pressure (so are quieter) without the aid of constructive interference to boost them.
If you’ve ever heard an echo, you know that sounds can bounce off solid surfaces. Combined with the phenomenon of constructive interference, sound reflections can achieve a great deal of amplification (and attenuation with destructive interference). This is the principle upon which architectural acoustics is based.
This is nice!