rpg specialty absorbers

50 Pages·2009·1.82 MB·English

RRPPGG SSPPEECCIIAALLTTYY
AABBSSOORRBBEERRSS
DDrr.. PPeetteerr DD’’AAnnttoonniioo
DDrr.. PPeetteerr DD’’AAnnttoonniioo
RRPPGG DDiiffffuussoorr SSyysstteemmss,, IInncc..
RRPPGG DDiiffffuussoorr SSyysstteemmss,, IInncc..
Absorbers: Parametrix
NRC
1
0.8
t
n
e
i
c
i
f
f
e
o 0.6
C

n
o
i
t
p
r
o
s 0.4
b
A
0.2
0
0 50 100 150 200 250 300 350 400 450 500
Frequency, Hz/10
Acoustic Absorbers
Helmholtz Resonators
∅=2a
D
Perforated sheet
t
Porous absorbent
d
Rigid backing
Surface Impedance
[ ]
z = r + j ωm − ρc cot(kd )
1 m
The acoustic mass or imaginary
The resistance or real
term is associated with phase
term, which is associated
change or resonant frequency
with energy loss
2p
k = the wavenumber in air;
l
d the cavity depth;
m the acoustic mass per unit area of the panel;
r
w the angular frequency = 2p f
the density of air, and
c the speed of sound in air
Resonant Frequency
At resonance, the imaginary term goes to zero
w m = 2p fm = rc cot(kd )
The cavity size is much smaller than the acoustic wavelength, i.e.
kd<<1, so that cot(kd)→1/kd
c r
f =
2p md
This is the basic design equation for resonant absorbers, i.e.
Helmholtz, Membrane and Plate resonators
Helmholtz Acoustic Mass/Unit Area
2 È ˘ 2 ’
rD 8n Ê t ˆ rD t
m = t + 2d a + 1+ =
Í ˙
Á ˜
2 Ë ¯ 2
p a w 2a p a
Í ˙
Î ˚
- The last term in the equation is due to the boundary layer effect, and
ν is the kinematic viscosity of air. This last term is often not significant
unless the hole size is small, say sub-millimetre in diameter.
- δ is the end correction factor (not allowing for mutual interaction),
which to a first approximation is usually taken as 0.85 and derived by
considering the radiation impedance of a baffled piston. Other more
accurate formulations exist.
- t’ includes the end correction factor
When Your Project Calls
for CMU Finish
RR
AA DD
Acoustical CMU
Acoustical Properties
Slotted/Unsealed Slotted/Sealed Unslotted/Unsealed Unslotted/Sealed
Transmission Loss
Absorption Coefficient
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