Absorption sound

During the propagation of a plane sound wave through a medium the intensity of the wave decreases as the distance from the radiation source increases. The intensity, /, at some distance, d, from the source is given by Eq. 2.14. 

The total loss, or absorption, caused by both viscosity and thermal conductivity is called the classical absorption coefficient a j, and is given by Eq. 2.15c. 

However when values of the calculated absorption coefficient are compared with those obtained experimentally, the agreement is often poor. For example if we take water at 20 °C for which = ICp, p = 1 g cm and c = 1500 m s and we pass a sound wave of 20 kHz, then a can be calculated to be approx. 3.5 x 10 cm . Experimentally a is found to be 8.6 x 10 cm i. e. approx, two and a half times larger. In fact only in the case of monatomic gases is the observed absorption, equal to the classical absorption. In all other cases the observed absorption is greater than the classical absorption by an amount called the excess absorption, (given by the expression 2n i1b/p complete accuracy, Eq. 2.16 should be further modified to take 

The bulk viscosity referred to here (pg) should not be confused with the so-called bulk viscosity of polymers which refers to the steady flow shear viscosity of the bulk undiluted polymer. Here it represents all the causes of sound absorption other than those produced by shear viscosity or thermal conductivity. Typically these may be  

According to the above expressions the value of a/f is a constant for a given liquid at a given temperature. Any increase in sound frequency,/ must result in a compensatory increase in a and thus a more rapid attenuation of the sound intensity with distance (Eq. 2.14). This has important consequences. Consider for example the passage of sound through water at room temperature. According to Fox and Rock [10] the value of a// for a wide variety of frequencies in water is 21.5 x 10 cm. Using this value the absorption coefficients at 21.5 kHz and 127.0 kHz can be deduced to be 

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Some of the problems often encountered during ultrasonic inspection of plane specimens are also found on cylindrical specimens. For example, problems associated with the directional characteristic of the ultrasonic transducer. Furthermore, the discontinuity influences the shape and propagation direction of a reflected pulse, causing wave mode transformation. In addition, the specimen influences the shape and amplitude of the reflected pulse by sound absorption. 

The first example refers to the detection of a 1mm side drilled hole at a depth of 45 mm in a polyethylene plastic material. Due to the high sound absorption in plastics, a low operating frequency is chosen. A probe having a 1 MHz element of 24 mm diameter was selected for this example. The echo pattern of a conventional probe with a PZT transducer is pre-... 

Miscellaneous Properties. The acoustical properties of polymers are altered considerably by their fabrication into a ceUular stmcture. Sound transmission is altered only slightly because it depends predominandy on the density of the barrier (in this case, the polymer phase). CeUular polymers by themselves are, therefore, very poor materials for reducing sound transmission. They are, however, quite effective in absorbing sound waves of certain frequencies (150) materials with open ceUs on the surface are particulady effective. The combination of other advantageous physical properties with fair acoustical properties has led to the use of several different types of plastic foams in sound-absorbing constmctions (215,216). The sound absorption of a number of ceUular polymers has been reported (21,150,215,217). 

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. 

The sound absorption of materials is frequency dependent most materials absorb more or less sound at some frequencies than at others. Sound absorption is usually measured in laboratories in 18 one-third octave frequency bands with center frequencies ranging from 100 to 5000 H2, but it is common practice to pubflsh only the data for the six octave band center frequencies from 125 to 4000 H2. SuppHers of acoustical products frequently report the noise reduction coefficient (NRC) for their materials. The NRC is the arithmetic mean of the absorption coefficients in the 250, 500, 1000, and 2000 H2 bands, rounded to the nearest multiple of 0.05. 

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...

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. 

The reverberation time in a room is direcdy proportional to the volume and inversely proportional to the amount of sound absorption in the room. For most practical purposes the reverberation time is determined by the Sabine equation ... 

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. 

Metal Pan Assemblies. These units consist of tiles and panels formed from perforated aluminum or steel with pads of fiber glass or mineral wool inserted into the pans to provide the sound absorption. They are used primarily for ceilings in a similar manner to acoustical tiles and panels. The pads are sometimes sealed in plastic film to prevent absorption of moisture, dirt, and odors. The perforated metal is relatively sound transparent and functions as the finished ceiling and the support for the sound-absorbing material. The perforated metal by itself has no acoustical value. 

Draperies. Draperies of light weight or open-weave fabrics are ineffective for sound-absorbing purposes. Heavy draperies, such as flannel and velour, can provide useful sound absorption if properly installed. For best results they should be hung with 100% fullness, ie, 2 nC for every nC of wall or window surface covered. The sound-absorbing properties also are affected by the amount of space between the draperies and the surface behind them. 

Noise reduction (AIR) is the difference in the average sound pressure level between the source room and the receiving room. When the receiving room is relatively reverberant and the measurements are made in the reverberant fields of the two rooms the relationship between TL and AIR is as follows, where S is the surface area of the sound barrier between the two rooms and is the amount of sound absorption in the receiving room (7). 

Sound-Absorptive Blankets. Sound-absorptive blankets of fiber glass or mineral wool are not usually considered damping materials, but when fastened to sheet metal machine enclosures they can provide some useful damping in addition to sound absorption. 

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. 

The polymers combine a high level of flame retardancy with good thermal insulation and sound absorption characteristics. Densities are somewhat high (16-20 kg/m ). 

Resonant frequency The sound frequency for which a particular system provides the maximum absorption. The amount of sound absorption in a system depends on the degree of damping achieved this depends on the mass and the associated air space. 

In Eq. (4-29) jc is the distance traveled by the wave, and a is the absorption coefficient. Sound absorption can occur as a result of viscous losses and heat losses (these together constitute classical modes of absorption) and by coupling to a chemical reaction, as described in the preceding paragraph. The theory of classical sound absorption shows that a is directly proportional to where / is the sound wave frequency (in Hz), so results are usually reported as a//, for this is, classically, frequency independent. 

Generally used with mineral wool products where, in its decorative forms, it gives attractive facings to ceilings and wall tiles and enhances their sound-absorption characteristics. PVC is also used as a vapor control layer facing. 

Glass-fiber tissue or non-woven fabrics are used for decorative purposes on many insulants. They also give improved strength to foam plastics and enhanced sound-absorption characteristics to mineral wools. 

Interesting phenomena are observed by increasing the concentration of reversed micelles, changing the temperature or pressure, applying high electric fields, or adding suitable solutes, In some conditions, in fact, a dramatic increase in some physicochemical properties has been observed, such as viscosity, conductance, static permittivity, and sound absorption [65,80,173,233,243,249,255,264-269],... 

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