Sound weighting scales

A weighting scale, dBA The unit of sound intensity expressed as a logarithmic scale, related to a reference level of 10 W m"-. The A weighting scale is the most commonly used scale, as it reduces the response of sound meters to very high and low frequencies and emphasize those within the range audible by the human ear. 

Human loudness perception depends in a complex manner on both frequency and the overall loudness of sound. (For example, bass is more difficult to hear in music played at low volume than in the same music played at high volume.) To capture this behavior, two weighting scales have been developed for use in sound hazard analysis. The most common of these is the A weighting scale, which is commonly used to assess occupational and environmental noise. The A scale weights sounds in the 1000-6000 Hz range much more heavily than low-frequency sounds. The A-weighted intensities (dBA) of some common sounds are listed in Table 5. By contrast, the C weighting scale is used for very loud sounds and is a much flatter function of frequency. 

The term noise refers to any unwanted sound, either a pure tone or a mixture of frequencies. Since the sensitivity of the ear is frequency dependent, as illustrated by the above table, noise level is expressed in a frequency-weighted scale, known as A-weighting. Decibel readings on this scale are designated as dBa. Typical noise levels from various sources are illustrated in this table ... 

What is the significance of the A weighting scale when measuring sound ... 

Although this approach would have generally been preferred, it seemed impossible to come up with a sound model within the given time and financial limits. Therefore, an experiment-based relationship was finally developed directly from the available data. Simply stated, observed debris mass density stributions from accidents and full-scale tests were taken and correlated with the four main parameters explosives weight, scaled cover depth, chamber loading density, and slope angle of the overburden. 

Critical pan-coating parameters such as charge weight, process air, temperature, spray rate, and pan speed can be established and maintained based on sound scientific principles. These parameters can be scaled to commercial and economic requirements of a process. 

A device to measure instant noise levels. It is comprised of a microphone, amplifier, output meter, and frequency-weighting networks, which are used for the measurement of noise and sound levels. Sound-level meters are often made with various filtering networks that measure the sound directly on A, B, C, etc., scales. Sound-level meters may also incorporate octave-band filters for measuring sound directly in octave bands. Since sound levels are specific to the areas being measured, the sound levels are called area sampling. Sound level meters used for measuring noise for compliance requirements must meet American National Standards Institute (ANSI) Standard S1.4, Specifications for Sound Level Meters. See also Noise Dosimeter Weighted Measurements. 

TABLE 23-5 Frequency Weighting Adjustments for Converting Sound Pressure Levels to A Scale Readings... 

Make the maximum noise level of the equipment 80 dBA, 8 Hour Time Weighted Average when measured on the A scale of a standard sound level meter or noise dosimeter within 3 feet of the equipment. 

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