Absorption is the act of turning acoustical energy into some other form of energy, usually heat. The unit of acoustical absorption is the sabin, named after W.C. Sabine (1868–1919), the man considered the father of modern architectural acoustics.
It is beyond the scope of this treatment to tell the story of Sabine’s pioneering work on room acoustics, but it should be required reading for any serious student of acoustics. Theoretically, 1.0 sabin equates to one square meter (m2) of complete absorption. Sabine’s original work involved determining the sound absorbing power of a material. He posited that comparing the performance of a certain area of material to the same area of open window would yield its absorbing power relative to the ideal.3 For example, if 1.0m2 of a material yielded the same absorbing power as 0.4m2 of open window, the relative absorbing power—what we now call the absorption coefficient—would be equal to 0.4.4
How absorption is used depends on the application and the desired outcome. Most of the time, absorption is used to make rooms feel less live or reverberant. Absorber performance varies with frequency, with most working well only over a relatively narrow range of frequencies. In addition, absorber performance is not necessarily linear over the effective frequency range.
Measuring or classifying absorbers is not as straightforward as it may seem. There are two main laboratory methods: the impedance tube method and the reverberation chamber method.
There are three broad classifications of absorbers: porous, discrete, and resonant. While it is not uncommon for people to design and build their own absorbers (indeed, there has been something of a resurgence in do-it-yourself absorber construction in recent years as a result of the proliferation of how-to guides and Internet discussion forums—this information may or may not be reliable, depending on the reliability of the online resource and the relative expertise of the “experts” offering guidance), many useful porous and resonant absorbers are available commercially. Fundamental information about the design of absorbers is included here for two reasons: there may be those who want to build their own absorbers, and more importantly, these absorbers are sometimes inadvertently constructed in the process of building rooms. This is especially true of resonant absorbers.
Excerpt from Handbook for Sound Engineers, 5th Edition edited by Glen Ballou © 2015 Taylor & Francis Group. All Rights Reserved.
About the Author
Glen Ballou is a graduate of General Motors Institute, now Kettering University, with a bachelor’s degree in Industrial Engineering and a minor in Electrical Engineering. He has been a Syn-Aud-Con representative, has served as governor, convention chairman, papers chairman, and facilities chairman of the Audio Engineering Society (AES), and has been a member for the Society of Motion Picture and Television Engineers (SMPTE). He has been a contributor to S&VC, Sound and Communications, and Church Production magazine on a variety of subjects. Glen also wrote the chapter on capacitors and inductors for the CRC Press publication The Electrical Engineering Handbook. Glen is owner of Innovative Communications, a company that specializes in room acoustics and sound system design.