Sound Absorption Test

Test Methodfor Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM C423-90a, ASTM, Philadelphia, Pa., 1990. Standard Practices for Mounting Test Specimens During Sound Absorption Tests, ASTM E795-92, ASTM, Philadelphia, Pa., 1992. 

Standard Practices for Mounting Test Specimens During Sound Absorption Tests, ASTM E795-92, ASTM, Philadelphia, Pa., 1992. 

In addition to mechanical tests, sound absorption test and flammability test were also carried out. The aim of the sound absorption and the flammability tests was classification of the panels according to the available standards. 

The sound absorption tests, performed according to the ISO Standard No. 354, showed that the absorption capacity of the panels would fall in the range of comparable systems. 

Preliminary sound absorption testing of foam samples, Figure 13, showed that, sound absorption is achieved at relatively high frequency. Further investigation of all the samples is currently in progress. Full characterization of the sound absorption of all the produced ceramic samples and study of the effect of various stractural and morphological effects on the absorption behavior will be presented upon completion. 

Test Methods. Two basic types of test methods are commonly used to measure sound-absorption in test laboratories the reverberation room method and the impedance tube method. 

Because the reverberation room test method approximates many real-world conditions, it is used to derive sound-absorption coefficients for evaluating the effect of most actual appHcations of sound-absorbing treatments. Sound-absorption coefficients pubflshed in acoustical textbooks and by manufacturers of acoustical materials are almost exclusively from reverberation room tests, and this may be assumed unless specified otherwise. 

ASTM E1050-90 also makes use of a tube with a test specimen at one end and a loudspeaker at the other end, but iastead of a single movable microphone there are two microphones at fixed locations ia the tube. The signals from these microphones are processed by a digital frequency analysis system which calculates the standing wave pattern and the normal iacidence sound-absorption coefficients. 

One advantage of the impedance tube test methods is the small (usually <10 cm (4 ia.) dia) size of the test samples. For these tests sound impinges on the test sample only at normal iacidence to the surface, and the sound-absorption coefficients derived ia this manner are vaUd only at this angle. 

When sound waves come in contact with a boundary obstacle, such as a wall or door, a portion of the sound wave energy is reflected, a portion is transmitted through the obstacle, and the rest is absorbed by the obstacle. One of the standard methods of measuring the effective sound absorption coefficient of an acoustical material is by finding its effect on the reverberation time, or decay rate, of the sound pressure level in the sound chamber. The total sound absorption in the receiving room is required to determine the noise reduction of the specimen being tested. The key here is to rule out the absorption of sound waves within the chamber that may be attributed to the door being tested. 

Techniques for measuring the complex sound speeds and moduli of polymers are described in the section on test methods. The data shows that the real and imaginary components of the elastic moduli are frequency dependent. The frequency dependence is strongest for materials with high values of the loss factor r. Materials with frequency-dependent elastic moduli are called dispersive, and measurements and theory show that sound absorption mechanisms lead to dispersion. The real and imaginary part of an elastic modulus are related by the Kramers-Kronig relations, which are presented in the next section. 

In the measurements, the surface area of the test material should be large enough to measurably affect the reverberation time, but not so large as to seriously affect the diffusivity of the sound field. The ASTM and ISO standards recommend that the area should be between 10 and 12m with a lenglh-to-breadth ratio between 0.7 and 1.0. In many cases, sound absorption in a reverberation room is affected by things other than the room walls, such as loudspeakers, at low frequencies stationary and rotating diffuser surfaces, at low and mid frequencies and air absorption, at high frequencies. All these need to be considered in formal tests. 

ASTM C423-09, 2009. Standard test method for sound absorption and sound absorption coefficients by the reverberation room method. American Society for Testing and Materials. ASTM C522-03, 2009. Standard test method for airflow resistance of acoustical materials. 

Rotating vanes are provided in the rooms to ensure diffuse sound fields. Sound is introduced into the source room, the average sound pressure level is measured in one-third octave bands in both rooms, and the sound-transmission loss is calculated as follows, where L2 and L2 are the average sound pressure levels in the source and receiving rooms S is the area of the test sample, m2 andis the absorption in the receiving room, metric sabins. 

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