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

186 9. Sound-Measuring Instrumentationneed arise to prove the accuracy of the instruments at the time of its questionedusage.One of the simplest case of noise measurements is that of steady noise, say, amotor running at steady speed in a factory. In this situation it would be necessaryto take only a few sound level readings in dB(A) near the worker’s ear and to checkthe measured values with established permissible levels. In the more common caseof fluctuating levels, moving noise sources or receivers, etc., it may be necessaryto take more readings over longer time intervals. Other types of meters or measurementprocedures may be more suitable for sustained noise exposures, such asthe use of the integrating sound level meter and the dosimeter which are describedin the following sections.9.9 The Integrating Sound Level MeterIn Chapter 3 the equivalent sound level was defined asL eq = 10 log 1 10 L/10 dtT 0or in terms of N sound level measurements taken during Nequal intervals:()1N∑L eq = 10 log 10 L 1/10N∫ Ti=1(9.5)Integrating sound level meters are based on the application of Equation (9.5) inmeasurements of fluctuating noise over a considerable interval. For example, ameter may be preset to measure in intervals of 1 s over a total time period of15 min, thus calling for 900 individual sets of measurements. A measurement over12 h may be programmed for 720 intervals of 1 min each. More elaborate metersdesigned for 24-h surveillances can compute the day–night equivalent sound levelin which a 10-dB penalty is automatically added to noise levels occurring betweenthe hours of 10 p.m. and 7 a.m. The integrating sound level meter is also capableof measuring the sound exposure level (SEL), which characterizes a single eventon the basis of both the pressure level and the duration. This parameter is definedby(∫ TSEL = 10 log0prms2pref2) (∫ T)dt = 10 log 10 L/10 dt0(9.6)where T is measured in seconds. If we consider a 2-s burst of sound at therms pressure of 1 Pa, use of Equation (9.6) will indicate an SEL of 97 dB.Comparison of Equations (9.5) and (9.6) leads to the relationship between SELand L eq :SEL = L eq + 10 log T (9.7)