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

176 9. Sound-Measuring Instrumentationthe appropriate impedance for connecting the microphone to a measuring system.Because of their long-term stability, superb high-frequency response, insensitivityto vibration excitation, condenser microphones are generally preferred for precisionmeasurements. They function well in extreme temperature and pressureenvironments, but they are affected adversely by high humidity (which causeselectrical leakage) and their diaphragms are fragile.9.4 SensitivityIn addition to frequency response, sensitivity constitutes one of the principal characteristicsof a transducer. In general there is a trade-off between sensitivity andfrequency response. Small microphones tend to have lower sensitivity but operate atboth low and higher frequencies, whereas large microphones possess high sensitivitybut are useful mainly at lower frequencies. Microphone diameters are typically1, 2, and 4 cm ( 1 / 4 , 1 / 2 , and 1 in.) in diameter. The frequency response of a 4-cmcondenser microphone is virtually flat to 20 kHz, while that of a 1-cm microphoneis fairly flat to approximately 100 kHz. We define microphone sensitivity S byS =electrical outputmechanical inputThe microphone sensitivity level L s , also called simply “sensitivity,” is defined as( / ) 2Eout pL s = 10 logdBV/μbar = 20 log ( / )E out p dBV/μbar (9.1)E rewhereE out = output voltage into the instrumentE re = reference voltage (1 V for an incident sound pressure of 1.0 μbar)p = rms pressure on the microphoneOne microbar (μbar) is equal to 0.1 Pa. Equation (9.1) can be rearranged to obtainthe output voltage:E out = p 10 L s/20(9.2)We recall the definition of Equation (3.6) for sound pressure level (SPL), L p andrecast Equation (9.2) asE out = 0.0002 × ( 10 L p/20 )( 10 L s/20 ) = 0.0002 × 10 (L p+L s )/20 V (9.3)Example Problem 1A microphone has sensitivity rating of L s =−50 dB V/μbar. Find the outputvoltage for a sound pressure level of 85 dB.