Walk outside on a cold winter day just after the first big snowfall and you can hear the hush in the air. Everything sounds different because that hush in the air is the snow absorbing sound. Now, compare that to walking into a gym where the sound bounces around and lingers in the room. Also, have you wondered why some hotel rooms are “soundproof” and some seem to have walls that are paper thin?
When sound hits a material three things can happen. First, the sound can be reflected like in the gymnasium where it bounces off the hard walls and is redirected. Secondly, the sound can be transmitted through the material like in the thin walls of a hotel. Finally, the sound can be absorbed. What happens to the sound when it is absorbed? It gets trapped in the material and converted into a very small amount of heat. Imagine a college football stadium filled with screaming fans charged up for the state rivalry game. If all of the sound from the screaming fans could be absorbed into one cup of coffee in the middle of the field, there might just be enough to energy to heat the coffee.
The types of materials that absorb sound are porous like the snow. The air gets trapped between the little snow crystals or fibers and turned into heat. One of the most common materials used to absorb sound is fiberglass. We also often get asked about Styrofoam. Although Styrofoam may act as a good thermal insulator, it is very poor acoustical absorber.
A very important thing to note is that acoustical absorption is frequency dependent. That means that two materials may look the same but one might absorb high frequencies and another may absorb mid or low frequencies. Typically, as the thickness of the material increases, so will the absorption of low frequency sound.
At Acoustics By Design, it’s our job to engineer acoustical solutions that control how the sound is reflected, transmitted, or absorbed based on the type of space. If maximum absorption were the goal, we’d treat every project like a movie theater or even like an anechoic chamber. But in the real world, different spaces have different acoustical needs, and our job is to engineer acoustical solutions that are custom fit to meet those needs.