Introduction to Acoustics

390 Part C Architectural Acoustics
Part C 11.1
SPL (dB)
Near field
Free
field
–6dB/
doubling
of distance
Fig. 11.6 Sound fields in rooms
Far field
Reverberant
(diffuse) field
Distance
near field is the region within 1/4-wavelength (of the
lowest frequency of interest) of a sound source or large
reflective surface. Sound pressure levels can fluctuate
dramatically in the near field of a source and sound
pressures cancel and enhance each other near large reflective
surfaces, so sound pressure level measurements
should be avoided in near fields.
The far field is the region beyond the near field. It
is contrasted with the near field to designate the region
that is appropriate for recording sound pressure level
measurements from a sound source. As one moves out
of the near field and into the far field, sound pressure
levels drop off at a rate of 6 dB per doubling of distance,
for a point source, in accordance with the inverse square
law that is simplified in the following equation:
SPL2 = SPL1 −[20 × log(d2/d1)] , (11.2)
where SPL1 is the sound pressure level at the location
closer to the sound source, SPL2 is the sound pressure
level at the location farther from the sound source, d1 is
the distance from the source at which SPL1 is measured,
and d2 is the distance from the source at which SPL2 is
measured.
The region in which (11.2) is valid is known as
the free field. Free-field conditions exist in large open
outdoor spaces or in rooms having highly absorptive surfaces,
in which there are no obstructions in the sound
travel path between the source and listener. The free field
is sometimes referred to as the direct field when source
measurements are taking place that only consider the
sound wave traveling from the source to the listener
with no influence by reflected sound coming from room
surfaces.
As sound pressure levels decay from sources within
a room, they eventually drop to a relatively constant level
which is determined by the amount of reflected sound
within a room. This lower limit is known as the reverberant
or diffuse field. The size of the reverberant field
depends on the size of the room and the size and characteristics
of its reflective surfaces. Within the reverberant
field, sound pressures are similar independent of location.
Figure 11.6 puts all of the fields described above
into perspective.
11.1.3 Sound Absorption
Absorption converts sound energy into heat energy. It is
useful for reducing sound levels within rooms but not
between rooms. Each material with which a sound wave
interacts absorbs some sound. The most common measurement
of that is the absorption coefficient, typically
denoted by the Greek letter α. The absorption coefficient
is a ratio of absorbed to incident sound energy. If
a material does not absorb any sound incident upon it,
its absorption coefficient is 0. In other words, a material
with an absorption coefficient of 0 reflects all sound
incident upon it. In practice, all materials absorb some
sound, so this is a theoretical limit. If a material absorbs
all sound incident upon it, its absorption coefficient is 1.
As with the lower limit for absorption coefficients, all
materials reflect some sound, so this is also a theoretical
limit. Therefore, absorption coefficients range between
0 and 1.
Absorption coefficients vary with frequency. Typical
absorptive materials are porous and characterized by absorption
coefficients that increase with frequency. They
therefore have limited effectiveness for lower frequencies,
especially below 250 Hz. There are absorbers that
have been designed to absorb these lower frequencies,
and these will be discussed shortly. However, for typical
cases, it is convenient to use a single number (incorporating
multiple frequency components) to describe the
absorption characteristics of a material. This value has
been defined by the American Society for Testing and
Materials (ASTM) in Standard C 423 as the noise reduction
coefficient (NRC). The NRC is the arithmetic
(as opposed to the logarithmic) average of a material’s
absorption coefficients at 250, 500, 1000, and 2000 Hz,
rounded to the closest 0.05.
Table 11.1 lists absorption coefficients and NRC values
for common materials. Note that the values listed in
Table 11.1 are for general reference purposes only, and
specific values should be based on manufacturers specifications.
Also note that absorption coefficients and NRC
values have no units associated with them. In general,
materials with NRC values less than 0.20 are considered