the catholic university of america aldo glean engr 518 experimental techniques for graduate students...
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The Catholic University of America
Aldo Glean
ENGR 518 Experimental Techniques for Graduate StudentsDecember 04, 2012
Measurement Of the Acoustic Properties Of Materials (Absorption and Reflection Coefficients)
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Outline
Description Measurement System (Impedance Tube)
Functionality of Measurement System
Measurement Results
Limitations of Measurement System
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Impedance Tube
Kundt’s Tube:
Observation of longitudinal standing waves in air
Sound Speed in air: Clear tube with cork dust
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Impedance Tube
Standing wave tube:
Measure Absorption and Reflection Coefficient of materials (single and multilayer) Material use: Minimize reverberation in recording studios, interior of vehicles, classrooms & offices
Excitation (speaker)Microphones PistonSample
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Impedance Tube
Microphones measure acoustic pressure at two locations in the tube (Transfer function method)
Complex Transfer Function (H): Mic #2 to Mic #1
Excitation signal
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Impedance Tube
Measurement system parameters (ASTM: E 1050-98) Tube diameter (Plane wave propagation):
Microphone spacing:
Tube Length (Full development of plane wave):
Restriction of higher modes
Excitation signal
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Impedance Tube
Overview of System in Operation
Mic PowerSupply
PowerAmplifier
Computer with Digitizer
Excitation signal
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Impedance Tube
Measurement using Melamine Foam
Sample cross-section: Cross-section of tube
Thickness: Limited by depth of tube
Speaker
Microphones
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Impedance Tube
Absorption and Reflection Coefficients
200 400 600 800 1000 1200 1400 1600 1800 20000
0.2
0.4
0.6
0.8
1
Frequency, Hz
Ref
lect
ion
Coe
ffici
ent,
R
200 400 600 800 1000 1200 1400 1600 1800 20000
0.2
0.4
0.6
0.8
1
Frequency, Hz
Abs
orpt
ion
Coe
ffici
ent,
Fig1: Absorption and Reflection coefficient of Melamine foam.
200-2200Hz Range Tube:
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Impedance Tube
Measurement Limitations
• Measurements are made at Normal Incidence (compensation for other angles is possible)
• Upper frequency is limited by:
Plane wave requirement (tube diameter)
Microphone spacing
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Questions?
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