Importance

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Abstract

An audiometer is provided with attenuators which have two successive stages, a main attenuator providing a large range of attenuation and a second stage which is capable of providing silent switching and finely adjustable attenuation. The signal from the main attenuator in the first stage is provided through two signal paths in the second stage to two input terminals of the earphone, with the sound output from the earphone determined by the voltage across the two input terminals. The first of the signal paths includes a pad having fixed attenuation which provides a low level voltage output signal to the one input terminal, and the other signal path includes an adjustable attenuator which provides fine adjustment of the magnitude of the output signal across the earphones. The adjustable attenuator may include multiplying digital to analog converters which have an attenuation determined by their digital input word with the signal from the main attenuator provided as the voltage reference signal to the digital to analog converters. The attenuation may be controlled utilizing a microprocessor to provide the data words to the multiplying digital to analog converters. A switching circuit is provided on the output of the adjustable attenuator to disconnect the adjustable attenuator from the earphone when that attenuator's output is not required. This disconnection improves the signal to noise ratio for low level signal because the noise at the output of the adjustable attenuator does not contribute to the earphone signal. As a result, the available dynamic range is increased by the attenuation of the fixed attenuator. No clicks are produced when the adjustable attenuator is switched in or out.

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First Claim