Page 214 - The Master Handbook Of Acoustics

189
ABSORPTION OF SOUND

mizes the problem of reduced absorption. Under certain conditions, a
painted surface can reduce porosity but act as a diaphragm that might
actually become a fair absorber on a different principle, that of a
damped vibrating diaphragm.
In the first radio broadcasting studios, the acoustical treatment was
an overuse of carpeted floors and drapes, which emphasized a serious
shortcoming of most porous absorbers—that of poor low frequency
absorption. Tiles of cellulose fiber with perforated faces became the
next style of treatment, but they were also deficient in low-frequency
absorption. Overly enthusiastic use of porous absorbers, not only dur-
ing the early days but even today, causes overabsorption of the high-
frequency sound energy, without touching a major problem of room
acoustics, low-frequency standing waves.

Glass Fiber: Building Insulation
Great quantities of glass fiber materials are used in the acoustical
treatment of recording studios, control rooms, and public gathering
spaces. These glass fibers can consist of both special, high-density
materials, and ordinary building insulation. In wood or steel stud sin-
gle frame walls, double walls, and staggered stud walls thermal insu-
lating batts are commonly used. This material usually has a density of
about 1 lb/cu ft. Such material is often identified as R-11, R-19, or
other such numbers. These R-prefix designations have to do with
thermal insulating qualities, but are related to thickness. Thus R-8 is
2.5″ thick, R-11 is 3.5″, and R-19 is 6″. 3
Building insulation installed within a wall increases its transmis-
sion loss a modest amount, primarily by reducing cavity resonance
that would tend to couple the two wall faces at the resonance fre-
quency of the cavity. A certain increase in the transmission loss of the
wall can also be attributed to attenuation of sound in passing through
the glass fiber material, but this loss is small because of the low den-
sity of the material. Considering all mechanisms, the transmission loss
of a staggered stud wall with a layer of gypsum board on each side can
be increased about 7 dB by adding 3.5″ of building insulation. A dou-
ble wall might show as much as a 12 dB increase by adding 3.5″ and
15 dB with 9″ of insulation. As far as wall transmission loss is con-
cerned, using the denser, more expensive glass fiber between wall
faces offers only a slight advantage over ordinary building insulation.