Acoustical Treatment for Indoor Areas 115common RT equations will not work satisfactorily in
these spaces.
Absorption is often used in small rooms to control
discrete reflections or to change the way the room feels.
Contrary to popular belief, the impression of liveness or
deadness is not based on the length of the reverberation
time. Rather, it is based on the ratio of direct to reflected
sound and on the timing of the early reflected sound
field, especially in the first 20 ms or so. Adjusting the
acoustics of nonreverberant spaces (sometimes referred
to as tuning the room) involves manipulating discrete
reflections.
To determine the suitability of a particular absorber,
the acoustician needs a direct measurement of the
reflected energy off of the product. In small
rooms—where a significant portion of the spectrum is
below f 1 (see Chapter 6)—field measurement of absorp-
tion, such as the techniques and methods presented in
Section 5.2.1.3, might be more appropriate in deter-
mining the applicability of a particular absorber to a
small room application.
5.2.7 Subjective Impact of Absorption
Sometimes it is useful to consider the extremes. It is
interesting to note that rooms with no absorption and
rooms with total absorption represent the most acousti-
cally hostile spaces imaginable. At one extreme, there is
the absorption-free space, also known as the reverbera-
tion chamber. A good real-world example of this is a
racquetball court. As anyone who has played racquetball
can readily attest to, racquetball courts are not acousti-
cally friendly! At the other extreme is the anechoic
chamber. This is a room that is totally absorptive and
totally quiet. Since an anechoic chamber has no reflected
sound and is isolated from sounds from the outside, a
good real-world example of this is the desert. Standing in
a part of the desert free of reflective surfaces, such as
buildings and mountains, located many kilometers from
any noise sources, such as highways and people, at a
time when there is no wind, the complete lack of sound
would be comparable to what one would experience in
an anechoic chamber. It is difficult to describe just how
disorienting spending time in either of these chambers
can be. Neither the reverberation chamber nor the
anechoic chamber is a place where a musician would
want to spend much time, let alone perform!
The use of absorption has a powerful impact on the
subjective performance of a room. If too much absorp-
tion is used, the room will feel too dead—i.e., too much
like an anechoic chamber. If too little absorption is used,
the room will feel uncomfortably live—i.e., too much
like a reverberation chamber. Additionally, the absorp-
tion of any material or device is frequency-dependent;
absorbers act like filters to the reflected sound. Some
energy is turned into heat, but other frequencies are
reflected back into the room. Choosing an absorber that
has a particularly nonlinear response can result in rooms
that just plain sound strange.
More often than not, the best approach is a combina-
tion of absorbers. For example, large spaces that already
have the seats, people, and carpet as absorbers may
benefit from a combination of membrane absorbers and
porous absorbers. In a small room, some porous
absorbers mixed with some Helmholtz resonators might
provide the best sound for the room. Both of these are
examples of the artistic (the aural aesthetic) being
equally applied with the science (the acoustic physics).
Experience is important when considering the appli-
cation of absorption and—more importantly—when
considering what a particular application will sound like.
The savvy acoustician will realize the aural differences
between a small room treated with 5.1 cm (2 in) acous-
tical foam and a room treated with 2.5 cm (1 in) glass
fiber panels. On paper, these materials are quite similar
(compare Figs. 5-6 and 5-14). However, the knowledge
that a room treated with 9.3 m^2 (100 ft^2 ) of foam gener-
ally sounds darker than a room treated with the same
area of 96 kg/m³ fabric-wrapped, glass fiber boards
comes only with experience. Likewise, the knowledge
that a room treated with a slotted concrete block wall
will sound much different than the same room with a
GWB wall that is treated with several well-placed perfo-
rated absorbers (even though RT predictions for each
scenario come out to be approximately the same) comes
only with experience.5.2.8 Absorption and Absorption Coefficients of
Common Building Materials and AbsorbersTable 5-3 gives the absorption coefficients of various
popular building materials.5.3 Acoustical DiffusionCompared to acoustical absorption, the science of acous-
tical diffusion is relatively new. The oft-cited starting
point for much of the science of modern diffusion is the
work of Manfred Schroeder. In fact, acoustically diffu-
sive treatments that are designed using one of the various
numerical methods that will be discussed below are often
referred to generically as Schroeder diffusers. In the most