2.4. Sound absorption coefficient measurement
A sound wave incident on a material can be absorbed, reflected and/or transmitted by the material. These three
phenomena are all possible depending upon the types of material. The absorption of the incident sound wave is an
effective way to control noise.
Acoustic performance of sound absorbing material is valued by the sound absorption coefficient. An impedance
tube was used to determine the sound absorption coefficient of sound absorbing materials analyzed.
The absorption coefficient is the ratio of the absorbed energy to the total incident energy and the reflection
coefficient is the ratio of the reflected energy to the total incident energy.
The measurements were based on a two-microphone transfer-function method according to ISO 10534-2,
international standard for horizontally mounted orientation sensitive materials [24]. A medium Type 4206 A
Brüel&Kjaer tube kit was used to measure acoustic parameters for a frequency range of 100÷3200 Hz.
The testing apparatus is part of a complete acoustic material testing system (Fig. 4), featuring a Brüel&Kjaer
Type 4206 A medium impedance tube (1), two 4187 Brüel&Kjaer microphones (2), an Analyser PULSE 3560-B-
030 acoustic signal generator (3), a 2716 Brüel&Kjaer signal amplifier (4) and a PC (5) for recording and processing
results, connected with the Brüel & Kjaer PULSE interface [25].