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How Does Acoustical Absorption Work?

by Melinda Miller on December 8, 2011

Acoustic Engineering QuestionWalk 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.

Melinda Miller

Melinda Miller

Melinda Miller, PE, LEED AP BD+C, EDAC, INCE. Bd.Cert., has been working in acoustical engineering since 2001. Her expertise includes diagnosing and preventing noise problems, as well as designing acoustically optimized environments using 3D computer modeling, and evidence-based design practices. Melinda earned her Bachelor's Degree in Mechanical Engineering from the University of Idaho in 1998, and Master's from the University of Illinois, Chicago, in 2003. Melinda is the Director of Acoustics for Acoustics By Design, and the Senior Acoustical Consultant in Portland, Oregon.

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william December 6, 2014 at 10:21 pm

Our club house is 36 ft. X 40 ft. 9ft.high. What sq ft. of sound board will we need?
Thank You, William

Brian Atkinson Brian Atkinson December 12, 2014 at 8:43 am

William,
Figuring that out is exactly what we do!
In order to determine the acoustical treatments necessary to achieve a desired result, we would need to collect a few pieces of information. While the size (volume) of the room is one factor, the materials are another. We would also meet with you to talk about your desired results. Then, we would perform acoustical measurements in the space, and provide a report that would tell you how many square feet of which types of treatment to use, along with their placement, and descriptions of alternates.

Please give us a call or send an email, and we will be happy to provide a proposal for your project.

http://www.acousticsbydesign.com/acoustical-consulting/contact-us.htm

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