Noise Measurements

In the early noise surveys^{35, 36, 37} both methods were employed. For the aural method, one arrangement of the apparatus is indicated in Fig. 20. The phonograph plays a special record producing a warbling or variable-frequency tone. The set is first calibrated in a sound-proof room for zero noise by adjusting the apparatus so that with no noise present the warble tone is just audible. Then, when the set is used to make noise measurements in a typical location, the attenuator is decreased until the warble tone is again just audible above the noise. The difference in the attenuator setting (measured in decibels) is therefore equal to the deafening effect, or audibility threshold shift, of the noise.

Figure 20. Arrangement of apparatus for the aural method of measuring noise.

Because of its practical advantages, the objective method of measuring noise has been generally adopted. Acceptable instruments for this purpose are designed following rigid specifications³⁸ based on the characteristics of the ear. A diagram of a typical sound- or noise-measuring instrument is shown in Fig. 21.

Figure 21. Schematic diagram of a sound-level or noise meter.

A high-quality microphone is used to intercept the noise to be measured, and its output is impressed on the preamplifier. A variable attenuator providing the various ranges is then included, after which the signal passes through a frequency-weighting network, a final amplifier, and to a suitable indicating instrument such as a vacuum-tube voltmeter.⁽¹⁾ Of course, provisions must be made for calibrating the circuit, either by using a standard tone, or by other means.³⁹

The frequency-weighting network weights the various frequency components of a complex noise wave in accordance with the characteristics of the ear. The ear does not have the same characteristics at different loudness levels. Thus, for sounds of a level of 40 db, the ear has the characteristics given by the 40-db loudness-level contour of Fig. 17. For a 70-db sound, the characteristics are as given by curve 70, and for very intense sounds of about 100 db, the characteristics of the ear are essentially flat. Noise meters are designed with three different characteristics:³⁸ first, with a weighting network giving the correct characteristics for measuring weak noises (curve 40, Fig. 17); second, with a weighting network for noises of intermediate intensity (curve 70, Fig. 17); and third, a flat characteristic for intense noises and for general sound-measurement purposes.

(1)

With sound-level meters used in the United States the sound level in decibels above a zero level of 10-16 watt per square centimeter is obtained. By international agreement (see Bell Laboratories Record, February, 1938, page 214), the decibel is to be used for sound-intensity-level measurements, and the phon is to be used for loudness-level measurements. Thus, it is proposed that, if loudness-level measurements are being made as explained in section The Loudness of Sounds and if the intensity level of the reference tone is n decibels, then the unknown sound has a loud-ness level of n phons.