Calibration in Acoustics
To obtain precise results, the acoustic measurement chain must be calibrated. The measurement chain consists of the microphone, a test amplifier and an A/D converter. This chain converts the sound pressure into a voltage signal and digitizes it.
In acoustics, sound levels are specified in dB. This is a logarithmic measure which is related to a reference magnitude. For an amplitude A, A in dB = 10 * Log (A² / Aref²) = 20 * Log(A / Aref) and for a power P, P in dB = 10 * Log(P / Pref). By definition, the acoustic sound pressure 2 * 10-5 Pa is assigned to the level 0 dB. This sound pressure is considered the threshold of human hearing.
A correctly calibrated acoustic measurement chain has a sensitivity of 50 mV/Pa. Therefore, for a level of 0 dB, 2 * 10-5 Pa * 50 mV/Pa = 100 * 10-5 mV = 1 μV should be measured. The RMS value (root mean square) of the measured wave is applicable to this voltage.
The calibration usually is the result of measuring an acoustic reference level generated by a calibrator. Usually, an acoustic calibrator provides a reference level of 94 dB, which corresponds to a sound pressure of 2 * 10-5 Pa * 10(94/20) ≈ 1.0023 Pa. The measurement chain should provide an RMS value of approx. 50.11 mV for this level.
The calibration of the measurement chain provides a calibration value that is specified in dB. Since we are dealing with a logarithmic measurement, this calibration value must be added to the measured level in order to compensate for the deviation of sensitivity of the measurement chain from the required 50 mV/Pa. For the measured RMS voltage, this addition corresponds to multiplication with a calibration factor. For a measurement chain with a sensitivity of exactly 50 mV/Pa, the calibration value is equal to 0 dB. If the sensitivity of the measurement chain is lower, then the result is a positive dB value, which corresponds to a factor greater than one. For a measurement chain with sensitivity that is too high, the result is a negative dB value, which corresponds to a factor less than one.