Abstract: In order to determine the acoustic impedance of a human ear at various static pressures, a probe tightly fitted to the pinna of the ear transmits a short acoustic pulse with a wide frequency spectrum (such as white noise) from an electroacoustic transducer to the ear canal and receives back an acoustic response signal which a microphone converts into an electrical wave. The latter is digitized and transformed into terms of a Fourier series fed to a processor which, on the basis of similar terms stored in a memory and previously obtained with the same probe fitted to two different cylindrical calibrating cavities, derives therefrom the acoustic ear impedance at a particular air pressure. The processor also triggers an electric pulse generator, working into the electroacoustic transducer, and may progressively adjust a generator of static air pressure connected to the probe.

Abstract: A subscriber's telephone set, provided with a ringer connected in series with a capacitor across a line loop, is provided with a unit for the suppression of spurious ringing currents including an antiparallel connection of a diode and a transistor in series with the ringer/capacitor combination. The transistor has a base/emitter circuit connected across the line loop by way of a Zener diode and an R/C network. The Zener diode breaks down in response to oscillations whose frequency and amplitude are characteristic of genuine ringing currents from the associated central office, thereby driving the transistor into conduction and enabling the ringer to operate; other oscillations or pulse trains, such as those due to dialing pulses generated by another telephone set connected across the same line loop, remain ineffectual with the transistor cut off.