The Catholic University of America

1. 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

2. Outline • Description Measurement System (Impedance Tube) • Functionality of Measurement System • Measurement Results • Limitations of Measurement System

3. Impedance Tube • Kundt’s Tube: • Observation of longitudinal standing waves in air • Sound Speed in air: Clear tube with cork dust

4. 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

5. 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

6. 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

7. Impedance Tube • Overview of System in Operation Computer with Digitizer Power Amplifier Mic Power Supply Excitation signal

8. Impedance Tube • Measurement using Melamine Foam Sample cross-section: Cross-section of tube Thickness: Limited by depth of tube Microphones Speaker

9. Impedance Tube • Absorption and Reflection Coefficients 200-2200Hz Range Tube: Fig1: Absorption and Reflection coefficient of Melamine foam.

10. 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

11. Questions?