Acoustic measurements: Tutorial on decibels, microphones

The decibel is widely misunderstood. Fortunately, stating the sensitivity of microphones in V/Pa (volts/pascal) is gaining popularity. Article demystifies acoustic measurements. See equation.

08/27/2008


The decibel is widely misunderstood. Its definition came from acoustic studies in the early 1900s and has been widely adopted by many disciplines, most notably electronics. Fortunately, stating the sensitivity of microphones in V/Pa (volts/pascal) is gaining popularity. A white paper written by Endevco engineer George A. Pender entitled “Microphone talk”
In the beginning was the Bel (named in honor of Alexander Graham Bell). The Bel was first defined in terms of acoustic power, but it can be generalized as the logarithm of the ratio of two quantities with the same units. Since logarithms are extraordinary compressors, the magnitude of the Bel was deemed inconvenient for typical acoustic calculations, so it was subdivided into 10 (deci) parts and abbreviated dB.
In acoustic research, the power level of sound was defined as 10 log 10 of measured sound power divided by reference sound power. Because it is more convenient to measure sound pressure than sound power, the pressure level of sound was defined as 10 log 10 of measured sound pressure squared divided by reference sound pressure squared. Since log 10 ( N 2)= 2 log 10 N , the sound pressure level (in dB) can be rewritten as 20 log 10 of measured sound pressure divided by reference sound pressure.
Fortunately, stating the sensitivity of microphones in V/Pa (volts/pascal) is gaining popularity. Since the output of any type of microphone is linear with pressure, the units of V/Pa fall out naturally. As an indication of the confusion still prevalent in defining microphone sensitivity, the Endevco model 2510 piezoelectric microphone has several numbers and units listed for sensitivity (the electrical output from piezoelectric microphones is commonly expressed in picocoulombs [pC], rather than volts).
Although sensitivity units of V/Pa (or pC/ssure. Since no one (to our knowledge) has yet marketed a microphone with such a high output, microphone sensitivities in dB are negative ( e.g ., -60 dB re 1V/Pa).
C.G. Masi , senior editor
Control Engineering
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