Abstract: A gas flow rotor cyclically connects a microphone to a dynamic flow press and then a reference static flow pressure. The microphone converts the pressures to a resultant alternating differential signal providing a first input to a phase-locked amplifier. A second amplifier input is derived from an optical pickup mounted on the rotor. Only the AC component of the converted microphone signal, which has a fixed phase relationship to that of the rotor, is amplified. A readout connected in circuit with the amplifier indicates the dynamic flow pressure.