Transmission, sound

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). 

Two usehil measures of the performance of a sound-isolating constmction are sound transmission loss (TL) and noise reduction (AIR). Sound transmission loss is defined as follows, where IH is the incident sound power (Watts) on the source side of the specimen, and W is the transmitted sound power on the receiving side (7). 

Fig. 4. Typical STC rating curves. Numbers on curves designate sound-transmission class (STC).

L bor toiyMethods. The laboratory test method for determining the sound-transmission loss performance of constmctions is defined in ASTM E90-90 (11). The sample is installed in an opening between two highly reverberant rooms that are acoustically well isolated from each other. 

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 and die average sound... 

Table 4. Sound-Transmission Loss of Some Common Materials...

Increase Sound- Transmission Loss. The only significant iacreases ia sound-transmission loss that can be achieved by the appHcation of dampiag treatments to a panel occur at and above the critical frequency, which is the frequency at which the speed of bending wave propagation ia the panel matches the speed of sound ia air. AppHcation of dampiag treatment to 16 ga metal panel can improve the TL at frequencies of about 2000 H2 and above. This may or may not be helpful, depending on the appHcation of the panel. 

Standard Test Methodfor Laboratory Measurement of Impact Sound Transmission Through Thor—Ceiling Assemblies Using the Tapping Machine, ASTM E492-90, ASTM, Philadelphia, Pa., 1990. 

The subject of acoustics involving sound transmission is of prime importance in industrial ventilation. Correct system design will ensure that the designer provides a system that will not give rise to complaints regarding noise levels. 

The simplest insulator is a sheet of material placed in the sound-transmission pathways. Sound energy reaches the surface in the form of a pressure wave. Some energy passes into the partition and the rest is reflected. 

Grayish metal hexagonal close-packed crystal system, lattice constant, a=2.286 A and c=3.584 A density 1.85 g/cm permeable to x-rays highly ductile modulus to weight ratio very high, elastic modulus 44.5 x 10 at 25°C (for hot-pressed block and sheet) melting point 1,287°C vaporizes at 2,471°C sound transmission velocity 12,600 m/sec reflectivity (white hght) 55% thermal neutron absorption cross-section 0.0090 barns/atom electrode potential, Be/Be2+(aq) 1.85 V electrical resistivity 3.36 x 10-i° ohm.m (at 20°C). 

In addition to their decorative appeal, glass blocks are claimed to provide better energy conservation (solar rcfiectivc blocks are available), lower sound transmission, aesthetic flexibility, minimal maintenance, and enhanced security. In addition to plain blocks, they are available with various decorative designs. See Fig. 8. An effective use of glass blocks... 

To enable a smooth transition between two-channel stereo and discrete surround sound transmission, different matrix systems have been proposed. One system is... 

Schroeder, 1970b] Schroeder, M. R. (1970b). Digital simulation of sound transmission in reverberant spaces . J. Acoust. Soc. Am., 47(2) 424-431. 

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