= Electroacoustics
<Electroacoustics> studies conversion between electrical signals and <sound>, together with the measurement and control of that conversion. A <microphone> produces an electrical response to an acoustic input; a <loudspeaker> performs the reverse conversion. Their <frequency responses>, loading and directional behavior matter as much as a single sensitivity number.
For a receiving <microphone>, one can define the complex <frequency response> $M_p(f)=V(f)/p(f)$ in volts per <pascal>, with the electrical termination and acoustic conditions specified. Equal outputs do not imply equal incident <sound pressures> if the two <microphones> have different sensitivities. Calibration supplies the link between the electrical recording and an acoustic measurement.
A motivating primary method is <microphone reciprocity calibration>. In the NIST system reported by <Randall P. Wagner> and collaborators, three laboratory <microphones> are measured in pairs using acoustic coupling cavities. One microphone transmits while another receives; known coupling properties and reciprocity relations allow the individual pressure sensitivities to be determined without starting from a separately calibrated reference microphone. Comparing results with different cavity lengths tests the acoustic corrections and associated <measurement uncertainty>.<New Measurement Service for Determining Pressure Sensitivity of Type LS2aP Microphones by the Reciprocity Method by Randall P. Wagner and others>[2011, measurement-system description, reciprocity method and comparison of long and short couplers]{ref}
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