114 Chapter 5
on wavelengths that large, most likely one could move a
wall, improving, if not optimizing, the modal distribution
and obviating the need for trapping.
Nonetheless, the preceding sections have provided
many examples of products that could be designed to
trap bass without taking up much space. Additionally,
there are good broadband absorbers on the market that
extend down into the low-frequency region. The prac-
tical limits of size and mounting usually lead to a natural
cutoff between 50 and 100 Hz for many broadband
absorbers. These products exhibit performance that is
highly dependent on placement, especially in small
rooms.
5.2.6 Applications of Absorption
In large rooms (see Chapter 6 for the definition of large
vs small rooms) where there is a statistically reverberant
sound field, absorption can be used to actually modify
the reverberant field, and the results are predictable and
fairly straightforward. The whole concept of reverbera-
tion time (RT—discussed in detail in Chapter 6) is a
statistical one that is based on the assumption of uniform
distribution of energy in the room and complete random-
ness in the direction of sound propagation. In large
rooms, both conditions can prevail.
In small rooms—particularly at low frequen-
cies—the direction of propagation is by no means
random. Because of this, the propriety of applying the
common RT equations to small rooms is questioned. For
small rooms (nonreverberant spaces), absorption is
useful for control of discrete reflections from surfaces
located in the near field of the source and listener. In
rooms the size of the average recording studio or home
theater, a true reverberant field cannot be found. In such
small rooms, the common RT equations cannot be used
reliably. Further, predicting or trying to measure RT in
small spaces where a reverberant field cannot be
supported is typically less useful relative to other anal-
ysis techniques. The results of RT measurements in
small rooms will neither show RT in the true sense, nor
will they reveal much of value regarding the behavior of
sound in a small room relative to the time domain. Typi-
cally, it is more useful to examine the behavior of sound
in the time domain in more detail in small rooms. Deter-
mining the presence of reflections (wanted or unwanted),
the amplitude of those reflections, and their direction of
arrival at listening positions is typically a better
approach. For low frequencies, Chapter 5 provides some
small room analysis techniques that are more beneficial
than the measurement of RT.
5.2.6.1 Use of Absorption in Reverberant SpacesIn large rooms, the common RT equations can be used
with reasonable confidence. When absorptive treatment
is not uniformly distributed throughout the space, the
Sabine formula is typically avoided in lieu of other
formulas. RT is covered in detail in Chapter 6.
In reverberant spaces, the selection of absorbers can
be based on the absorption data collected in accordance
with ASTM C423, as described in Section 5.2.1.1. Care
should be taken, however, to somehow account for
effects not directly evident from laboratory measure-
ment methods. As one example, consider a
fabric-wrapped, mineral fiber panel that is tested in a
Type A mounting configuration. The test specimen is
placed in the reverberation chamber directly against a
hard (typically solid concrete) surface, often the floor.
The absorption coefficients then represent only the
absorption provided by the panels. In practice, panels
such as these might be directly applied to a GWB
surface having absorption characteristics of its own that
are significantly different than the solid concrete floor of
a reverberation chamber! Applying an absorptive panel
to a GWB wall or ceiling not only changes the acous-
tical behavior of the GWB surface (by changing the
mass), but the panel itself will not absorb as measured in
the lab, because of the mounting, the size of the room
relative to the laboratory test chamber, the proportion of
absorptive material relative to the total surface area of
the room, etc. This is one example of why the predictive
modeling for the acoustics of large spaces can be—like
many aspects of acoustics—as much art as science. All
acousticians are likely to have methods they use to
account for idiosyncrasies that can neither be measured
in a laboratory, nor modeled by a computer.
In addition, it is generally agreed among acousticians
that reverberation time is no longer considered the
single most important parameter in music hall and large
auditorium acoustics. Reverberation time is understood
to be one of several important criteria of acoustical
quality of such halls. Equal or greater stress is now
placed on, for example, the ratio of early arriving energy
to total sound energy, the presence of lateral reflections,
the timing of the arrival of various groups of reflections,
and other parameters discussed in detail in Chapter 6.5.2.6.2 Use of Absorption in Nonreverberant SpacesIn rooms where there is not enough volume or a long
enough mean free path to allow a statistical reverberant
field to develop, one must view the use of absorption in a
somewhat different manner. As alluded to previously, the