Transmission loss, sound

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). 

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 MethodforEaboratoy Measurement of Airborne Sound Transmission Loss of Building Partitions, ASTM Designation E90-90, ASTM, Philadelphia, Pa., 1990. 

Sound transmission loss ratio of the sound energy emitted by an acoustical material or structure to the energy incident upon the opposite side. 

ASTM E90-09 provides a method covering the laboratory measurement of airborne sound transmission loss of building partitions such as walls of all kinds, operable partitions, floor-ceiling assemblies, doors, windows, roofs, panels, and other space-dividing elements (ASTM E90 2009). Laboratories are designed so the test specimen constitutes the primary sound transmission path between the two test rooms, and so approximately diffuse sound fields exist in the rooms. 

It is important to know the sound transmission loss of walls and floors in order to be able to compare different constructions, to calculate acoustic privacy between apartments or noise levels from outdoor sources such as road traffic, and to engineer optimum solutions to noise control problems. Laboratory measurements can be made for many different types of partitions, but it is impractical to test every possible design, and so it is necessary to have reliable methods for predicting the sound transmission loss of typical building constructions. 

Particle boards (PB) from jute stick (JS), date palm leaf, and their blends offered higher sound transmission loss, higher thermal insulation, and lower swelling compared to plywood. Increase of JS in blend with date palm leaf increased sound loss as well as thermal insulation. Sound loss increased with increase in thickness of PB. Relationship between sound loss and thickness is found to be nonlinear. Sound loss reached maximum at board thickness of 19 mm for PB (Ghosh et al. 2010). 

When sound reaches a surface, the surface absorbs some of the energy. Some passes through the surface to the other side. When designing walls and other sound barriers, a focus is on the reduction or sound transmission loss, TL. Sound transmission loss is the reduction in sound energy across a barrier... 

TABLE 23-3 Sound Transmission Loss in Decibels of Various Materials... 

Figure 23-4. Effect of enclosure leaks onbarrier sound transmission loss (TL). Enter the chart with thebarrier transmission loss for a fully enclosed noise source. Move vertically to intersect with the curve representing the percentage of the barrier area presented by cracks or openings. From the intersection, move left to read the noise attenuation actually achieved.

Sung Soo, J., K. Yong Tae, L. Yong Bong, C. Seung II, and L. Jong Kyu. 2008. Measurement of sound transmission loss by using impedance tubes. Journal of the Korean Physical Society 53P) 596-600. 

Vigran, T.E. 2012. Normal incidence sound transmission loss in impedance tube—measurement and prediction methods using perforated plates. Applied Acoustics 73(4) 454-459. 

The following table presents typical results selected from approximately 100 tests in the National Research Council of Canada Building Research series. The tests were conducted in accordance with ASTM E90-66T, Recommended Practice for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions. The gypsum board samples represented extremes of density and thickness (as allowed under CSA Standards) they included the fire-resistant and the vinyl-covered types, with no significant differences in performance. 

To calculate in detail the sound transmission loss for even simple constructions is difficult. As a result, engineers usually rely on laboratory test data. The transmission loss of a particular material has to do with the material only and is only part of the equation to determine what noise level will occur on the opposite side of a barrier. See the paragraph on noise reduction on the following page. 

Mass law For each doubling of the weight of a single-material construction, the average sound transmission loss increases by about 6 dB. Figure 4.99 shows a graphic presentation of the transmission loss of a single wall. The empirically determined curve (solid) is lower than the theoretical curve (dashed). 

Composite sound transmission loss The composite sound transmission loss of a multielement barrier is estimated from the transmission coefficients for each part. The sound power transmitted by the elements with a common incident sound power is ... 

Enclosures around noise sources or receiving personnel can provide effective noise reduction. They consist of materials with substantial sound transmission loss and are arranged to surround the noise source or receiver. 

The transmission coefficient is the fraction of incident energy transmitted through a partition or a textile layer when a sound wave is incident upon it, and the transmission loss is the logarithm of the reciprocal of the transmission coefficient. Two types of transmission loss are commonly measured with a layer of textile material. The random incidence transmission loss is measured in a reverberation room using the reverberant room method, while the normal incidence sound transmission loss is measured in an impedance tube using the transfer matrix method. 

Fig. 5.5 Setup for normal incidence sound transmission loss measurements with an impedance tube.

This measurement of the normal incidence sound transmission loss standardized in ASTM E2611-09 uses a plane wave tube instrumented with four microphones and a termination consisting of an adaptable acoustic load (four-microphone-two-load). The two loads are typically a minimally reflecting termination (eg, anechoic termination). 

ASTM E90-09, 2009. Standard test method for laboratory measurement of airborne sound transmission loss of building partitions and elements. American Society for Testing and... 

Sahssou, Y, Panneton, R., Doutres, O., 2012. Complement to standard method for measuring normal incidence sound transmission loss with three microphones. J. Acoust. Soc. Am. 131 (3), EL 216. 

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