THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

12.10 Combined Sound Transmission Coefficient 295Figure 12.10. Examples of flanking.the values of the sound transmission coefficients of the individual elements andtheir respective areas. Assuming the incident sound to be fairly uniform over theindividual elements, we can compute the combined sound transmission coefficientfromn∑A i τ ii=1τ combined =(12.24)n∑A iwhere i designates the i-th element of the subdivided wall area A and the correspondingvalue of τ. For cracks and open areas, τ = 1, which is to say the soundtransmission through such crevices may be considered to be virtually unimpeded.The corresponding transmission loss of the composite wall then becomes⎛ n∑⎞( )A1iTL combined = 10 log= 10 log ⎜i=1⎟τ combined⎝ n∑⎠ . (12.25)A i 10 −TL i /10Example Problem 4A 6.0 m × 2.5 m wall is for the most part constructed of 8-cm thick dense pouredconcrete with a transmission loss of 52 dB at 500 Hz. An opening in the wall isprovided for a 2.2 m × 1.0 m wood door with a transmission loss of 25 dB at500 Hz. There is a crack across the width of the door that is 1.0 cm high. Estimatethe effective transmission loss of the composite wall at 500 Hz.SolutionThe total area A of the wall is 6.0 × 2.5 = 15.0 m 2 , but the concreteportion of the wall is 15.0 − 2.2 × 1.0 − 0.01 × 2.2 = 12.778 m 2 . Applyingi=1i=1