Microphone reciprocity calibration

Microphone reciprocity calibration obtains absolute pressure sensitivities by measuring pairs of reciprocal microphones, rather than starting with a microphone of already known sensitivity. One microphone transmits into a known acoustic coupler while another receives. Reciprocity relates transmitting and receiving behavior, so the electrical transfer measurement constrains the product of their receiving sensitivities.

Schematically, after correcting each pair measurement for its coupling and electrical factors, write $P_{12}=M_1{M}_2$, $P_{13}=M_1{M}_3$ and $P_{23}=M_2{M}_3$. Then $M_1^2=P_{12}{P}_{13}/P_{23}$, with analogous expressions for the others. The actual frequency-dependent calibration includes phase conventions, acoustic losses, diaphragm impedance and cavity geometry.

The NIST apparatus uses three laboratory-standard microphones in opposed ports of cylindrical air-filled couplers. The reported long and short coupler lengths are 9.4 mm and 4.7 mm; a sinusoidal source and electrical measurements determine pair transfer responses while temperature and static pressure are recorded. Comparing results from both couplers checks corrections that would be invisible in a single geometry.^[ref]