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

174 9. Sound-Measuring Instrumentationthe response curve of a high-quality loudspeaker may deviate ±5 dB over its ratedfrequency range.Dynamic range defines the range of signal amplitudes that an instrument iscapable of handling in the process of responding and measuring accurately. Asound level meter (SLM), for example, which can measure a minimum of 10 dBand a maximum of 150 dB, covers a dynamic range of 140 dB.Crest factor capability denotes an instrument’s capacity to measure and distinguishinstantaneous peaks. Crest factor itself is the ratio of the instantaneous peaksound pressure to the root-mean-square sound pressure.Response time refers to the rapidity with which a measuring instrument respondsto abrupt changes in signals. An oscilloscope display of a square-wave responsewill result in a trapezoidal display if the relayed signals came from a loudspeakerthat requires a longer reaction time to respond to a square-wave pulse.9.3 MicrophonesMicrophones serve as transducers by receiving and sensing pressure fluctuationsand converting them into electrical signals that are relayed to other electronic components.The quality of a microphone determines the accuracy of a measurementsystem. A top-caliber measuring (or a sound reproduction) system can be underminedby the use of a microphone that is of a lesser caliber. Four principal typesof microphones are used in measurement procedures, namely, dynamic, ceramic,electret, and condenser microphones.Dynamic microphones produce an electric signal through the motion of a coilconnected to a diaphragm in a magnetic field. They are in effect loudspeakersworking in reverse, accepting an acoustic signal, and converting it into an electricpulsation rather than the other way around. Because of their low impedance,they can be used in applications entailing the use of long cables connected toauxiliary instrumentation. But they cannot be used in the vicinity of devices thatemit magnetic fields (e.g., transformers and motors). Also they generally havelonger response times, more limited frequency response, but can be constructed towithstand rough handling and high humidity.Ceramic microphones consist of a piezoelectric (ceramic) element attached tothe rear of a diaphragm. Sound pressure causes the diaphragm to vibrate, exertinga varying force on the ceramic element. The piezoelectric crystal generates anelectric signal from the oscillating strains imparted by the diaphragm. These microphonesare rugged, relatively inexpensive and reliable, have high capacitanceand good dynamic range, and do not require a polarizing voltage that electret andcondenser microphones need. But the high-frequency response may be lacking,and the operating temperature range may be limited.The electret (or electret-condenser) microphone, illustrated in Figure 9.1, consistsof a self-polarized metal-coated plastic diaphragm. Sound pressure causes thediaphragm to move relative to a back plate, varying the capacitance, and producinga signal. While relatively impervious to high humidity, this type of microphone