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D. W. Herrin, Ph.D., P.E. University of Kentucky
Department of Mechanical Engineering
Measurement of Sound Absorption and Impedance
Impedance Tube Measurements
Noise and Vibration Short Course
ASTM E1050-95 (ISO 10534-2) Test Method
Transfer function
sample driver (loudspeaker)
microphones
Measurement of Sound Impedance
2 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Sound source Material sample
x
x1
x2
Coordinate System and Microphone Locations
3 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
+x traveling wave -x traveling wave
The transfer function between points 1 and 2:
Total sound pressure at any point in the tube:
Plane Wave Theory
( ) jkxjkx BeAexP += −
ABR = is the pressure reflection coefficient of the material
( )( ) 11
22
11
22
1
212 jkxjkx
jkxjkx
jkxjkx
jkxjkx
ReeRee
BeAeBeAe
xPxPH
+
+=
+
+== −
−
−
−
4 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Solving for R:
Sound absorption coefficient of the material for any angle of incidence :
Normalized specific boundary impedance:
Solving for Material Properties
R = e− jkx2 −H12e
− jkx1
H12ejkx1 − e jkx2
zρoc
=1+ R1− R
α ϕ( ) = 4 !r cosϕ1+ !r cosϕ( )2 + !x cosϕ( )2
where !r = rρoc
!x =xρoc
5 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Two-Microphone Standards
1. ISO 10534-2, Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes - Part 2: Transfer-function method
2. ASTM E1050-10, Standard Test Method for Impedance and
Absorption of Acoustical Material Using a Tube, Two Microphones and a Digital Frequency Analysis System
6 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Sound Absorption Measurement
Sample holder with rigid piston
7 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Cutting with Rotating Blade
Stanley, Internoise 2012
Inexpensive and accurate if kept sharpened
8 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Water Jet Cutting
Stanley, Internoise 2012
Expensive but accurate
9 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Stamping Press System
Stanley, Internoise 2012 Used for low-density fibrous materials
10 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
“Bad” and “Good” Specimens
Stanley, Internoise 2012
11 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Stanley, 2012 Specimen Preparation
Very important step for good results Gravity aided insertion and self centering Face uniformly flush with cell lip Front surface even across lip of test cell No facing protrusions or wrinkles interacting with cell wall Extremely small ( at most) and consistent gap between
specimen and cell wall No specimen compression in cell
12 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Comparison of 5 Samples (Water Jet)
Stanley, Internoise 2012
13 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Absorption Response of One Die Cut, and Three 99 mm Water Jet Cut Foam Samples of Slightly Different Diameter.
Comparison of 4 Samples
Stanley, Internoise 2012
14 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Effect of Impedance Tube Size
0
0.2
0.4
0.6
0.8
1
0 500 1000 1500 2000
Abs
orpt
ion
Coe
ffici
ent
Frequency (Hz)
1.375 inch tube 3.875 inch tube
8 Samples of 2 inch fiber
15 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Effect of Cutter Size 8 Samples of 0.75 inch thick 0.6 lbs/ft³ Melamine
0
0.2
0.4
0.6
0.8
1
0 1000 2000 3000 4000 5000
Abs
orpt
ion
Coe
ffici
ent
Frequency (Hz)
1.375 inch Diameter Cutter
1.360 inch Diameter Cutter
16 Dept. of Mech. Engineering University of Kentucky
Impedance Tube Measurements
Noise and Vibration Short Course
Summary
“While the use of an impedance tube system to measure acoustic absorption is not an extremely precise and repeatable process due to unavoidable variations of specimen cutting and cell fit, the disciplined use of the guidelines stated in this paper will help to insure that test results maintain a consistent level of accuracy and validity. The experience gained with repeated preparation and testing will also contribute to a better feel for more subtle aspects of preparation and specimen fitting for testing.”
Stanley, Internoise 2012
17 Dept. of Mech. Engineering University of Kentucky
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