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

12.11 Noise Insulation Ratings 299shells. This method is based on statistical studies, and +3 dB per octave was chosenas the standard SIR contour. The STC contour method is generally preferred fordescribing the noise attenuation effectiveness of building structures in the presenceof non-transportation sounds, viz. speech, television, and radio. The SIR systemis favored for the evaluation of noise reduction provided by building shells againsttransportation noise.An element SIR (or member SIR) is an estimate of the difference in the A-weighted sound levels when a structural element or member is placed between thetransportation noise source and the receiver. It is assumed that all noise transmissionoccurs through the subject element. A room SIR (or composite SIR) refers to theestimate of the A-weighted sound level difference caused by the presence of allmembers that act as noise transmission paths between the source and the receiver.An estimate of the member SIR may be obtained from a set of transmission lossmeasurements in one-third octave or octave bands in the following manner:1. The transmission loss is plotted against frequency.2. The +3 db per octave SIR reference contour is plotted on an overlay sheet,which is then superimposed on the TL versus f plot in the highest position sothat the sum of the deficiencies is less than twice the number of test frequencies.A deficiency is defined as the number of decibels by which the SIR referencevalue exceeds the measured TL of the member.3. The value of TL at 500 Hz on the SIR reference curve constitutes the value ofthe SIR of the member.The statistical quality of SIR prediction is improved by using a larger numberof TL laboratory measurements, and the one-third octave band measurements willgenerally yield more reliable results than those based on octave bands. Manufacturers’catalogues frequently report the one-third octave TL measurements usedto evaluate STC. An example of an SIR determination is given in Figure 12.13for the TL plot of Figure 12.12. Much of the tedium of SIR contour fitting can beeased through the use of computer plotting.The field incidence mass law, expressed by Equation (12.13), can be utilized toextrapolate data. If the transmission loss is known for all applicable frequenciesfor a structural element 1, and it is desired to find the TL of a similar element witha different surface mass, then( )m2TL 2 = TL 1 + 20 log .m 1If the relationship holds in the applicable frequency range, the values of SIR aresimilarly related.Example Problem 6A shell constructed of 5-in. thick concrete with 55 lb/ft 2 surface mass carries arating of SIR 43. Estimate the SIR rating for an 8-in. thick concrete with a surfacemass of 92 lb/ft 2.