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

178 9. Sound-Measuring Instrumentationstandards apply here, one by the American National Standards Institute (ANSI)which calls for microphones with random-incidence response and the other bythe International Electrotechnical Commission (IEC) which specifies free-fieldmicrophones.The free field occurs as a region that is not subjected to reflected waves, as is thecase in an open field or in an anechoic chamber. The presence of a microphone inthe sound field disturbs the field. A microphone designed to compensate for thisdisturbance is called a free-field microphone. In order to obtain maximum accuracyin measurements, the free-field microphone should be pointed toward the noisesource [0 ◦ incidence, as shown in Figure 9.4(a)]. Microphone sensitivities are alsostated in terms of mV/Pa.The disturbance of the sound field by the presence of the microphone dependson the sound frequency, the direction of propagation, and the size and shape of themicrophone. At higher frequencies, where the wavelength of the sound is smallcompared with the principal dimensions of the microphone, reflections from themicrophone cause the pressure acting on the microphone diaphragm to differ fromthe actual free-field sound pressure that is supposed to be measured. Becausea wavelength of 1 inch corresponds to 13,540 Hz, a 4-cm microphone will notprovide accurate free-field measurements of noise in the frequency range in theneighborhood of 13 kHz and above. Even at 6 kHz, the error for a 4-cm microphonecan exceed 2 dB.The converse of the free field is the diffuse field which occurs as the resultof multiple reflections. A random-incidence microphone is utilized in measuringsound in diffuse fields; it is omnidirectional in that it responds uniformly to soundarriving from all angles simultaneously [cf. Figure 9.4(d)].Pressure microphones are designed to yield a uniform frequency response tothe sound field including the disturbance produced by the microphone’s presence.This type of microphone may also be used in diffuse fields. Using a free-fieldmicrophone in a diffuse field will result in lessened accuracy unless special circuitryin the measurement system provides compensating corrections. As shown inFigure 9.4(b), when a random-incidence microphone is used to measure sound in afree field, the unit should be placed at an incidence angle of 70 ◦ −80 ◦ to the source.A pressure microphone should be positioned at an incidence angle of 90 ◦ (oftenreferred to as the grazing incidence) in a free field [Figure 9.4(c)]. Microphoneplacement becomes more critical as the increasing sound frequencies approach theupper limits of accuracy.For a windy environment, special precautions should be taken to obtain theproper data. The wind rushing past a microphone produces turbulence, generatingpressure fluctuations resulting in low-frequency noise. In the use of A-weightedmeasurements, no precautions are necessary for winds below 8 km/h (5 mph),because the A-weighting network attenuates greatly at low frequencies. But forC-scale or linear sound level measurements, a windscreen should be employed forany sort of wind. The device should also be used for wind speeds above 5 mph inA-weighted measurements. Windscreens are typically open-celled polyurethane