RPG SPECIALTY
ABSORBERS
Dr. Peter D’Antonio
Dr.
Peter
DSystems,
’Antonio
RPG
Diffusor
Inc.
RPG Diffusor Systems, Inc.
Absorbers: Parametrix
NRC
1
Absorption Coefficient
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
Frequency, Hz/10
350
400
450
500
Acoustic Absorbers
Helmholtz Resonators
∅=2a
D
Perforated sheet
Porous absorbent
t
d
Rigid backing
Surface Impedance
z 1 = rm + j [ω m − ρ c cot( kd ) ]
The resistance or real
term, which is associated
with energy loss
The acoustic mass or imaginary
term is associated with phase
change or resonant frequency
k = 2p l the wavenumber in air;
d the cavity depth;
m the acoustic mass per unit area of the panel;
wr 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
f =
2p
r
md
This is the basic design equation for resonant absorbers, i.e.
Helmholtz, Membrane and Plate resonators
Helmholtz Acoustic Mass/Unit Area
2 ’
È
˘
8n Ê
t ˆ
rD
rD t
m=
t + 2d a +
ÁË1 + ˜¯ ˙ =
2 Í
2a ˙˚ p a 2
p a ÍÎ
w
2
- 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
R
A
D
Acoustical CMU
Acoustical Properties
Slotted/Unsealed Slotted/Sealed
Absorption Coefficient
Unslotted/Unsealed Unslotted/Sealed
Transmission Loss
High School Auditorium
Gymnasium
Music Rehearsal
Band Room
Gymnasium
Noise Control Chiller
Hybrid LF Diffsorber
•
The Helmholtz resonator slots provide low frequency absorption and the
reflection phase grating provides mid-high frequency diffusion
When Your Project Calls for
Visibility/Light Transmission
R
A
D
Absorption Mechanism
When surface perforations are the same
size as a boundary layer of air.
Viscous Losses
Reflected Sound
Incident Sound
Microperforated Panel
0.5 mm diameter holes
Air Cavity
NEW
Glass
Microperforated Absorbers
2
rD
m=
2
pa
È
8n Ê
t ˆ˘
Ít + 2d a +
ÁË1 + ˜¯ ˙
2a ˙˚
w
ÍÎ
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.
The end correction δ is increased by the boundary layer effect and
resonant frequency is reduced due to an increase in acoustic mass .
Losses
2wrh z 1
j1.7wra
zh =
+ - j rc cot(kd ) +
e
2e
e
•
•
•
Generally the resistive term in Helmholtz absorbers is very small and to get
good absorption it is necessary to add porous material to the cavity.
However, when the holes are sub-millimeter the resistive term (in red above)
is very large
Consequently, no porous material is needed in the cavity
Sound Absorption Data
1 Layer of Foil
30 mm off Glass
2 Layers of Foil
50 mm off Glass
30 mm off Glass
50 mm between layers
100 mm off Glass
30 mm off Glass
100 mm between layers
Effect of Layers/Backing
Foil & Mounting
Polycarbonate Foils
Botanical Gardens
Atria
Courtyard
Laser Micro-Slotted Panels
Scenery
Overlook
Partitioning
Custom Design
Translucence
Absorptive Glazing
Light Transmitting Microslit
Microslit Advantages
•
•
•
•
•
•
Fiber-Free (i.e. does not require porous fiberglass backing)
Light-transmitting
Can be fabricated in large panels
Slit pattern can be linear, sinusoidal, triangular, custom
Thickness ranges from 2 mm to 20 mm
Can be directly attached to window mullions with velcro or
suspended in front of glazing with standard decorative
hardware
• Can be fabricated from acrylic, PETG or recycled PETG
(slightly cloudy)
• Very absorptive in mid and low frequency regions which
are typically difficult to treat
Absorptin Data
Micro Perf vs. Micro Slit
Deamp Microslit Baffles
Acoustical baffle PETG
Variation1.
2743mm long
457mm Height
Upper width: ~227mm
Lower width: 100mm
Slot c-c 10mm, slot lengths 3x
112mm
Deamp Microslit Baffle
Acoustical baffle PETG Variation2.
2743mm long
457mm Height
Upper width: ~187mm
Lower width: 60mm
Slot c-c 10mm, slot lengths 3x 112mm
Limp Membrane Resonator
Membrane
Porous absorbent
ta
d
Rigid backing
c
f =
2π
ρ
md
60
f =
md
r = 1.21 kg / m3
c = 340 m / s
LF Band Cut Absorbers
•
Membranes- are essentially pressure transducers. They operate
where the pressure is high and the particle velocity is low- I.e. near a
boundary. They convert pressure fluctuations into air movement in
a frequency range determined by the mass and compliance of the
membrane and the air cavity depth.
Low Frequency Absorption
Impedance Tube Measurements
1
Absorption
efficiency
decreases with
frequency,
because the
impedance of
the porous
material moves
further from the
characteristic
impedance of
air at low
frequencies.
2" Cavity
0.9
4"
6"
0.8
6"+Damp
8"
10"
Absorption Coefficient
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
40
60
80
100
120
Frequency, Hz
140
160
180
Plate Resonators
High Pass
Broadband
Plate Resonators
Mechanisms
Pistonic Resonance
Steel Plate
Perf Metal
Damp Bending Modes
Polyester
Diffraction
High Pass Broadband
Above these frequencies absorption occurs
from diffraction of the sound around the plate
into the porous absorber
Plate Parameters
F A
E=
x H
Δr R
υ=
x H
F
Young’s Modulus Stress/Strain
Units N/m2 or psi
x
H
R
Δr
Poisson’s Ratio Transverse
Strain/Vertical Strain
steel density
melamine density c in m/s
E steel, Pa
Kg/m3
Poissons ratio
Kg/m3
melamine L, m W, m T, m n m
2.06E+11
7850
0.3
9.5
340
1
1.5 0.001 1 1
0.0025 2 2
3 3
4 4
fnm bending fnm bending
1mm, Hz
2.5, Hz
f piston 1mm, Hz f piston 2.5mm, Hz
3.52
8.79
188.25
119.06
14.07
35.17
31.66
79.14
56.28
140.69
Modex Plate & Broadband
In-wall installation
Performance
1.6
Absorption Coefficient
1.4
Broadband
1.2
1
0.8
0.6
Plate
0.4
0.2
0
50
160
500
Frequency, Hz
1600
5000
A/V Conference Room
1,2
Reverberation time [s]
1
0,8
0,6
0,4
0,2
0
32
63
125
250
500
1000
2000
4000
Frequenzcy [Hz]
no Absorber
with Absorber
8000