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?