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The Catholic University of America . Measurement Of the Acoustic Properties Of Materials (Absorption and Reflection Coefficients). Aldo Glean. ENGR 518 Experimental Techniques for Graduate Students December 04, 2012.

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The catholic university of america
The Catholic University of America

Measurement Of the Acoustic

Properties Of Materials

(Absorption and Reflection Coefficients)

Aldo Glean

ENGR 518 Experimental Techniques for Graduate Students

December 04, 2012


Outline
Outline

  • Description Measurement System (Impedance Tube)

  • Functionality of Measurement System

  • Measurement Results

  • Limitations of Measurement System


Impedance tube
Impedance Tube

  • Kundt’s Tube:

  • Observation of longitudinal standing waves in air

  • Sound Speed in air:

Clear tube with cork dust


Impedance tube1
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

Sample

Piston


Impedance tube2
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 tube3
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 tube4
Impedance Tube

  • Overview of System in Operation

Computer with

Digitizer

Power

Amplifier

Mic Power

Supply

Excitation signal


Impedance tube5
Impedance Tube

  • Measurement using Melamine Foam

Sample cross-section: Cross-section of tube

Thickness: Limited by depth of tube

Microphones

Speaker


Impedance tube6
Impedance Tube

  • Absorption and Reflection Coefficients

200-2200Hz Range Tube:

Fig1: Absorption and Reflection coefficient of Melamine foam.


Impedance tube7
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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