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

200 9. Sound-Measuring Instrumentationthe averaging time T , according to1ε =2 √ bw × TThe measurement uncertainty is twice that given by Equation (9.19).(9.19)Example Problem 4A normal distributed noise is to be analyzed in 20 Hz bands, with the measurementuncertainty not to exceed 4%. Find the necessary averaging time T .SolutionUsing Equation (9.19) modified to give the measurement uncertainty, we obtainT =9.17 Sound Power1bw × ε = 12 20 × 0.04 = 31.3s2Sound power denotes the rate per unit time at which sound energy is radiated. Thisrate is expressed in watts. The sound power L w level defined by( ) WL w = 10 logW 0is given in decibels. W is the power of the sound energy source in watts and W 0 =1 pW is the standard reference power in watts.The principal advantage of using sound power level rather than the sound pressurelevel given by Equation (3.22) to describe noise output of stationary equipmentis that the sound power output radiated by a piece of equipment is independent ofits environment. The sound energy output of the machine will not change if thatunit is moved from place to place, provided it operates in the same manner. Soundpower is primarily used to describe stationary equipment, but it is not generallyused to rate mobile equipment since the operational situations may be too highlyvariable, as is the case with construction equipment.Sound power may be measured directly using spund intensity instrumentationor indirectly, either determined from the rms sound pressures at a number ofmicrophone locations spatially averaged over an appropriate surface enclosingthe source in a free field over a reflecting plane (as exemplified by the use ofa semianechoic chamber) or in a totally free field (full anechoic chamber), oraveraged over the volume of a reverberation chamber in which the measurementsare conducted.Some sources are omnidirectional, i.e., they radiate sound uniformly in all directions.Most sources are highly directional, radiating more sound energy in somedirections than in others. Hence, the directivity,ordirectional characteristic, constitutesan important descriptor of a sound source. In a free field or anechoic