Portable Masking Meter for Measuring Impact Noise in Industry

Abstract

An instrument for evaluating loudness of industrial noises of the impact type must be portable, be convenient to use in the field, and yield results which may be interpreted in engineering terms. The development of such a device is described which enables an observer in the field to evaluate the magnitude of impact noise by finding the level of the masking threshold using an appropriate spectrum of masking noise. The instrument consists of a compact random noise generator, amplifier, filter set, attenuator, and mixer, and is normally used with a sound level meter and headphones. Six masking noise spectra of +9, +6, +3, 0, −3, −6 db per octave are available by means of a selector switch. The circuit is arranged so that the attenuator is calibrated in terms of phons. Metering circuits are provided for setting the output of the noise generator and the input from the sound level meter, as well as showing the condition of the batteries. In use, the microphone of a sound level meter is placed in the vicinity of an impact noise and the output of the sound level meter is connected to the masking unit. A spectrum of masking noise is selected which appears in the earphone simultaneously with the impact noise. The level of the masking noise is adjusted in the range of 70 to 110 db until the masking threshold is determined. This is repeated for each spectrum slope until the one with the lowest masking threshold is found. The attenuator dial then shows the level of the masking spectrum which most closely matched that of the impact noise. A demonstration using recorded industrial noises will show the operation of this equipment.