TEST METHOD TO DETERMINE THE ACOUSTIC PROPERTIES OF BUILDING MATERIALS BY USING FOUR MICROPHONE IMPEDANCE TUBE

Abstract

The paper discusses a simple and low-cost method to design four microphone impedance tube of measuring the acoustic
properties of building materials. The acoustic properties of the material are defined by the reflection coefficient, absorption coefficient,
and transmission coefficient. The experimental setup follows the ASTM-E2611 standard of four microphone impedance tube with two
load boundary conditions to measure these coefficients. The setup consists of four microphones around a brass tube with the speaker
at one end and termination at the other. Raw data from the four microphones is obtained through a Virtual Instrument (VI) program
developed in LabView. The novelty in the design is the tapered connection between the two pipes connected via the sample holder. The
mathematical equation involved in estimating acoustical properties is solved in MATLAB 2019a. The reflection and absorption coefficient
data of ephony fibbrette of 15 mm thickness are compared with the data provided by an accredited laboratory. The experimental
results of the in-house designed impedance tube are in good agreement with the lab results. This material is used in the auditorium,
theatres for hearing comfort. Further, two new samples of ephony fibbrette along with wood fibre cement and damper has been analysed.
It has been found that adding a layer of wood fibre results in an increase in the absorption coefficient whereas the addition of
the damper results in an increase in the reflection coefficient.