Post on 26-Dec-2015
transcript
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)
Outline
Description Measurement System (Impedance Tube)
Functionality of Measurement System
Measurement Results
Limitations of Measurement System
Impedance Tube
Kundt’s Tube:
Observation of longitudinal standing waves in air
Sound Speed in air: Clear tube with cork dust
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
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
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
Impedance Tube
Overview of System in Operation
Mic PowerSupply
PowerAmplifier
Computer with Digitizer
Excitation signal
Impedance Tube
Measurement using Melamine Foam
Sample cross-section: Cross-section of tube
Thickness: Limited by depth of tube
Speaker
Microphones
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:
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
Questions?