Page 244 - The Master Handbook Of Acoustics

219
ABSORPTION OF SOUND

The frequency of resonance of perforated panel absorbers backed
by a subdivided air space is given approximately by:

p
f 200 (9-4)

o (d) (t)
in which
f frequency, Hz
o
p perforation percentage,
hole area divided by panel area 100
t effective hole length, inches, with correction factor applied,
(panel thickness) (0.8) (hole diameter)
d depth of air space, inches
There is a certain amount of confusion in the literature concerning
p, the perforation percentage. Some writers use the decimal ratio of
hole area to panel area, rather than the percentage of hole area to panel
area, introducing an uncertainty factor of 100. This perforation per-
centage is easily calculated by reference to Fig. 9-33.
Equation 9-4 is true only for circular holes. This information is pre-
3
sented in graphical form in Fig. 9-34 for a panel thickness of 16″.
3
Common pegboard with holes 16″ in diameter spaced 1″ on centers
with the square configuration of Fig. 9-33 has 2.75% of the area in
holes. If this pegboard is spaced out from the wall by 2 4s on edge,
the system resonates at about 420 Hz and the peak absorption appears
near this frequency.

d
d
S r
w
S
S

S
PERF % = 78.5 ( ) 2 PERF % = 90.6 ( ) 2 PERF % = 100 ( )
d
d
r
s
s
w r
FIGURE 9-33
Formulas for calculating perforation percentage for Helmholtz resonators.