User Guide

Sound System Design Reference Manual
When dealing with the behavior of sound in an
enclosed space, we must be able to estimate how
much sound energy will be lost each time a sound
wave strikes one of the boundary surfaces or one of
the objects inside the room. Tables of absorption
coefficients for common building materials as well as
special “acoustical” materials can be found in any
architectural acoustics textbook or in data sheets
supplied by manufacturers of construction materiaIs.
Unless otherwise specified, published sound
absorption coefficients represent average absorption
over all possible angles of incidence. This is
desirable from a practical standpoint since the
random incidence coefficient fits the situation that
exists in a typical enclosed space where sound
waves rebound many times from each boundary
surface in virtually all possible directions.
Absorption ratings normally are given for a
number of different frequency bands. Typically, each
band of frequencies is one octave wide, and
standard center frequencies of 125 Hz, 250 Hz, 500
Hz, 1 kHz, etc., are used. In sound system design, it
usually is sufficient to know absorption characteristics
of materials in three or four frequency ranges. In this
handbook, we make use of absorption ratings in the
bands centered at 125 Hz, 1 kHz and 4 kHz.
The effects of mounting geometry are included
in standardized absorption ratings by specifying the
types of mounting according to an accepted
numbering system. In our work, familiarity with at
least three of these standard mountings is important.
Acoustical tile or other interior material
cemented directly to a solid, non-absorptive surface
is called “No. 1” mounting (see Figure 5-2). To obtain
greater absorption, especially at lower frequencies,
the material may be spaced out on nominal two-inch
thick furring strips and the cavity behind loosely filled
with fiberglass blanket. This type of mounting is
called out as “No. 2”. “No. 7” mounting is the familiary
suspended “T”-bar ceiling system. Here the material
is spaced at least 0.6 meter (2’) away from a solid
structural boundary.
Absorption coefficients fall within a scale from
zero to one following the concept established by
Sabine, the pioneer of modern architectural
acoustics. Sabine suggested that an open window be
considered a perfect absorber (since no sound is
reflected) and that its sound absorption coefficient
must therefore be 100 percent, or unity. At the other
end of the scale, a material which reflects all sound
and absorbs none has an absorption coefficient of
zero.
In older charts and textbooks, the total
absorption in a room may be given in sabins. The
sabin is a unit of absorption named after Sabine and
is the equivalent of one square foot of open window.
For example, suppose a given material has an
absorption coefficient of 0.1 at 1 kHz. One hundred
square feet of this material in a room has a total
absorption of 10 sabins. (Note: When using SI units,
the metric sabin is equal to one square meter of
totally absorptive surface.)
Figure 5-2. ASTM types of mounting (used in conducting sound absorption tests)
5-2