Absorption Coefficient

Topics: Sound, Acoustics, Wavelength Pages: 6 (2009 words) Published: October 10, 2011
Measuring the Absorption Coefficient for Some Common Materials Used in Functional Rooms through Standing Wave Ratio Adonis Cabigon1, Alaiza Tangaha2
Department of Physics, University of San Carlos, Nasipit, Talamban, Cebu City 6000 1adoniscc@yahoo.com
2mayalaiza92@yahoo.com

Abstract
In this paper, we present the measurement of the absorption coefficient α of MDF (Medium Density Fiberboard) and Fiber Cement through an improvised standing wave apparatus consist of an enclosed tube with a speaker on one end and the sample material on the other end. The maximum and minimum sound pressure developed inside the tube is measured by a moving microphone probe and the Standing Wave Ratio SWR and the absorption coefficient of the samples is calculated. The absorption behavior of each sample was also investigated in response to varying frequency. It was found out that the fiber cement was more absorbing than MDF at our chosen frequency rage (100 Hz to700 Hz).

1. Introduction
Explaining the acoustical behavior of large and small rooms requires knowledge of the absorbing properties of materials which cover the surfaces of the room under analysis. The sound absorption coefficient of these materials is ideally defined as the fraction of the randomly incident sound power absorbed by the surface which also depends on the frequency of the emitted sound [1-2]. Measuring the sound absorption of a room is part of the procedure for acoustical measurement, such as determining the sound power level of noise source or the sound transmission loss of a partition. It is also used in certain calculations such as predicting the sound pressure level in a room when the sound power level of a noise source in the room is known [2]. The standing wave method for measuring the acoustic absorption coefficient is by means of a standing wave formed in a tube. This method has contributed greatly to the development of effective acoustic absorbents, which are important in minimizing noise, thus play a significant role in architectural acoustics such as the design of recording studios and listening rooms, automobile interiors and many more [2-3]. In this experiment, we demonstrate the method by using an improvised standing wave apparatus to measure the absorption coefficient of some acoustic samples commonly used in rooms and evaluate its response to varying frequencies.

2. The Experiment
The experimental set-up of the improvised standing wave apparatus is schematically shown in Figure 1. Sound of pressure amplitude A is directed through the length of the tube by means of a loudspeaker at one end, to strike the sample placed at the other end. When the sound waves encounter the sample, part of the sound energy is absorbed

Figure 1.Experimental set-up consist of (A) computer with Waveform Analysis LabView program, (B) frequency generator, (B) loud speaker, (D) improvised standing wave tube, (E) movable microphone probe and (F) absorbing material. and another part is reflected back through the tube at an amplitude B. As a result of interference with the incident wave a standing wave pattern is formed in the tube, whose pressure maximum and minimum can be measured by means of a movable microphone probe. The microphone is connected to a computer with a LabView program that is used to read the frequency and the amplitude of the signal inside the tube. In Figure 2, it is shown how the sound pressure inside the tube develops as a function of position for both complete reflection and for partly absorbing material. On the left side, shown is the pressure amplitude in the tube with a rigid a rigid termination at x = L, where L is the length of the tube. The entire sound energy incident upon the termination is reflected with the same amplitude. However, there may be some absorption along the walls as the waves travel back and forth along the tube. The figure on the right side represents the case when the pipe is terminated with some acoustic absorbing...