Sound-Absorption Coefficients

Units. The unit of sound absorption is the metric sabin, which is equivalent to one square meter of "perfect" absorption, eg, one square meter of a material with a = 1.0. The Knglish unit of sound absorption is the sabin, which is equivalent to one square foot of perfect absorption. In order to avoid confusion, the designation metric should always be used when referring to metric sabins. The number of metric sabins of absorption provided by an area of material is calculated by multiplying its area by its sound-absorption coefficient. For example, 10 m of material having a sound-absorption coefficient of 0.75 provides 7.5 metric sabins of absorption. 

AH materials, even those considered to be sound-reflecting, absorb some small fraction of the sound energy impinging on them. Table 1 provides sound-absorption coefficients for some common building materials. 

Table 1. Sound-Absorption Coefficients (a) for Some Common Building Materials...

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. 

Products. There is a large number of commercially available sound-absorbing products for use on ceilings, walls, and for other special appbcations. Sound absorption coefficients and NRC values for some sound-absorbing products and treatments ate indicated in Table 2. 

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. 

A linear relation exists between the sound-absorption coefficients in the melts and their volumes V, cm ) as a function of temperature ... 

The sound absorption coefficient, a, is increased when the dynamics of the chemical system are of the same order of magnitude as the frequency of the sound wave,41 and experimentally this quantity is measured as a function of frequency of the ultrasonic sound wave (Fig. 4). When the frequency of the sound wave is of the same order as the frequency for the relaxation process, effects due to relaxation of the equilibrium give rise to characteristic changes in the quantity a//2, where a is the sound absorption coefficient measured at frequency /40 The variation of a with frequency, /, has an inflection point at the relaxation frequency of the system, fr, which is related to 1/t, where r is the relaxation time (1/t = 27i/r).40,41 The expression relating the quantity... 

When a sound wave strikes a material a fraction of its energy is reflected and a fraction is dissipated, or absorbed, by the material. The fraction of sound energy absorbed by a material is designated by its sound-absorption coefficient (oc). The sound-absorption coefficient of a given material is between zero and one if it is zero all the impinging energy is reflected and none absorbed if it is one all the eneigy is absorbed and none reflected. 

The variation with frequency of the sound absorption coefficient, p, which rises to a maximum in the centre of the dispersion zone, is given by... 

Here, a is the total (amplitude) sound-absorption coefficient, in units of cm-1, less the classical (viscothermal) absorption [equation (84)]. The frequency at which equation (83) has its maximum value is given by a>max = r 1. This relation is frequently utilized in reducing experimental data from sound-absorption measurements. 

The condenser microphone has been shown by Bauer and others [70, i 10, Mi] to be a very useful device for obtaining sound absorption coefficients at extremely high values of JJp. First described by Sell [H2] in... 

Noise reduction coefficient (NRC) arithmetic average of the sound absorption coefficients of a material at 250,500,1000, and 2000 Hz. 

The normal incidence sound absorption coefficient of a material can be determined using the method described in BS EN ISO 9614 [69] (BS 4196. Part 5 [70]). In this type of determination, a plane sound wave is made to be reflected by the sample mounted in a standing wave tube. The single-frequency wave hits the sample at normal incidence, and the amplitudes of the incident and reflected waves are measured over a series of chosen frequencies. Usually the frequencies of interest lie between 50 and. 00 Hz, although some automotive specifications will cover much wider ranges (sec Ford Engineering Specification WSK-M2D406-A). 

The sound absorptive power of a given material sample is characterized by the sound absorption coefficient a defined as the ratio of the acoustic wave energy Ea absorbed by the sample to the total energy Et incident on the sample ... 

Fig. 1. Frequency dependences of sound absorption coefficient a for the samples No. 1 - PP, No. 2 - PP + 40 wt. % of crumble hemp plant. No. 3 - PP + 40 wt. % of long hemp fibres...

The effect of the modification of the fibre surface consists in the shift of the sound absorption maximum towards higher frequency range and is accompanied by the decrease of the sound absorption coefficient a. The reduction of the coefficient a is not large. It amounts 2.5 % in the case of composites with pine wood and rapeseed straw Californium. For the samples with beech wood filler the coefficient a remains unaffected after modification. The exception is the composite containing the rapeseed straw Kaszub as a filler that shows the decreasing in the sound absorption of 7%. The reduction of the a coefficient value at the frequency related to the maximum of the sound absorption can be associated with the increase in the density of the composite after modification (Table 1) due to better adhesion between filler particles and polypropylene matrix. The fact implies that the specific acoustic impedance of... 

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