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

9.6 Vector Sound Intensity Probes 181SolutionApplying Equation (9.4)C T,P = 10 logC T,P = 10 log(√10 + 460528(√110 + 460528( ) ) 30=−0.25 dB (for 10 ◦ F)30( ) ) 30=+0.17 dB (for 110 ◦ F)30At the lower temperature the reading would be (92 − 0.25) = 91.8 dB and at thehigher temperature, (92 + 0.17) = 92.2 dB. Since the deviations are within theinstrument error, the application of such corrections is not very meaningful.The presence of reflecting surfaces affect measurements with the use of microphones.For example, the presence of a person near the microphone will disturbthe sound field, and it would be advisable to place the microphone on a tripod andmonitor the instrumentation from a distance. Also when a long cable is used toconnect the microphone to other instruments, care must be taken to avoid noisegenerated by the motion of different segments of the cable with respect to eachother. The cable should be constrained from moving and isolated from vibrationto the greatest degree possible. The cable should also be well shielded from strayelectromagnetic fields; but it is preferable that the tendency to noise generation becut down by incorporating the preamplifier and the microphone into a single unit,thereby yielding a considerably greater signal-to-noise ratio (S/N ratio). As a rule,stronger signals are more impervious to outside influences than would be the caseif the signals were not preamplified.9.6 Vector Sound Intensity ProbesA sound intensity probe that is used to measure vector sound intensity is illustratedin Figure 9.6(a), and a block diagram in Figure 9.6(b) shows the components of thisvector sound-measuring system. The device contains two microphones mountedface to face. Other modes of mounting are possible, including side by side andback to back. The probe measures sound pressure levels at two different pointssimultaneously. With the microphone spacing constituting a given factor, the soundpressure gradient can be determined and the particle velocity in a given directioncan be calculated. The intensity vector component in that direction may then beestablished. The microphone spacing should be considerably smaller than thewavelength of the sound being measured in order to yield valid results from twopointmeasurements. For example, in order to sustain an accuracy of ±1 dB, theupper frequency limit for 1-cm microphones set apart 12 mm from each otheris approximately 5 kHz. If the microphones are spaced apart by only 6 mm, thecorresponding upper frequency limit is 10 kHz.