Acoustical Treatment for Indoor Areas 975.1 Acoustical Treatment Overview
It is possible that there is no area in professional audio
where there is more confusion, folklore, and just plain
misinformation than in the area of acoustical treatment.
Everyone, it seems, is an acoustical expert. Of course,
like most disciplines, much of acoustics is logical and
intuitive if one understands the fundamentals. As Don
Davis wrote, “In audio and acoustics the fundamentals
are not difficult; the physics are.”^1 The most fundamental
of all rules in acoustics is that nothing is large or small.
Everything is large or small relative to the wavelength of
the sound under consideration. This is one of the realities
that makes the greater field of audio so fascinating.
Human ears respond to a range of wavelengths covering
approximately 10 octaves, as compared to eyes, which
respond to a range of frequencies spanning about one
octave. Even though the bandwidth of visible light is
obviously much larger than that of audible sound because
of the much higher frequencies involved, the range of the
wavelengths in this 10 octave bandwidth poses some
unique challenges to the acoustician. We must be able to
deal with sounds whose wavelengths are 17 m (56 ft) and
sounds whose wavelengths are 1.7 cm (0.6 in).
Getting rooms to sound good is an art as much as it is
a science. In some situations, concert halls, for example,
there is a reasonable agreement as to what makes for a
good hall. In other applications, such as home theaters,
recording studios, or houses of worship, there is little
agreement among the users, let alone the consultants, as
to how these rooms should sound. Considerable research
must be done before we are able to trace all of the
subjective aspects of room acoustics back to physical
parameters. However, some fundamental rules and prin-
ciples can be noted. The acoustician has very few tools.
In fact, there are only two things one can do to sound. It
can either be absorbed or redirected, Fig. 5-1. Every
room treatment, from a humble personal listening room
to the most elaborate concert hall, is made up of mate-
rials that either absorb or redirect sound. Room acoustics
boils down to the management of reflections. In some
situations, reflections are problems that must be
removed. In other situations, reflections are purposely
created to enhance the experience.
This chapter will address general issues of modifying
the way rooms sound. Absorption and absorbers will be
covered in detail, as well as diffusion and diffusers, and
other forms of sound redirection. Additionally, some
discussion on the controversial topic of electroacous-
tical treatments, and brief sections that touch on life
safety and the environment as they pertain to acoustical
treatments are provided. The information will be thor-
ough, but not exhaustive. There are, after all, entire
books dedicated to the subject of acoustical treatments.^2
The intention here is to be able to provide a solid under-
standing of the fundamentals involved. Specific applica-
tions will be dealt with in subsequent chapters.5.2 Acoustical AbsorptionAbsorption 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 early 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 (m^2 ) 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
1.0 m^2 of a material yielded the same absorbing power as
0.4 m^2 of open window, the relative absorbing
power—what we now call the absorption coeffi-
cient—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 straight-
forward as it may seem. There are two main laboratory
methods: the impedance tube method and the reverbera-
tion chamber method, both of which will be discussed in
detail below. Field measurement of absorption will also
be discussed below. Absorber performance can also be
determined theoretically; discussions of those methods
are beyond the scope of this chapter. (The reader is
referred to the Bibliography at the end of this chapter for
advanced absorber theory texts.)
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”