The basic instrument for measuring noise levels is the sound level meter, sensitive to RMS sound pressures between about 20 and 20,000 Hz. It is equipped with weighting networks, fast and slow response, an attenuator with 10—dB steps, and an indicating meter which spans 16 dBs, from —6 to +10 dB. It operates over a total range of about 30 to 140 dB sound pressure level. [Minimum specifications for general-purpose sound level meters are in the
International Electrotechnical Commission (IEC) Recommendations 123 and 177, in American National Standards Institute (ANSI) S1-4-1971, and elsewhere.]
Most sound level meters have output terminals so that accessories can be attached; these accessories include impact-noise meters, octave-band and M octave-band filter sets, graphic recorders, and the like. Self-contained analyzers are also available, with all components housed in a single unit; these often have variable width settings.
Vibration meters are like sound level meters in that they contain a sensing element, amplifier, attenuator, and output meter. They do not have weighting circuits but do have integrating circuits, so that with a single pickup acceleration, velocity and displacement can be measured. (Sometimes a vibration-measuring accessory operates with a sound level meter; it consists only of a vibration pickup, integrating circuits, and a table of conversions; the electronic circuitry in the sound level meter operates with it.) Vibration meters are usually calibrated in acceleration (cm/sec2), velocity (cm/sec), or displacement (cm) and display either peak or average values.
Figure 6.4.5 shows simplified block diagrams of typical sound level meters, vibration meters, audiometers, and noise monitors; Figure 6.4.6 shows a block diagram of the elements of a sound level meter.
Primary calibration of microphones or vibration pickups is normally done in the best equipped laboratories; most manufacturers of the instruments maintain such laboratories. Their standards are traceable to those maintained by government standards agencies (e.g., National Bureau of Standards, U.S.A.). Such calibrations are made with pistonphones, shake-tables, electrostatic actuators, and the use of reciprocity methods. Until recent years, the Rayleigh disk was widely used. Reciprocity calibrators are now easily available, and small electronic generators are widely used for field checks.
International standards and recommendations are available from the International Organization for Standardization, Geneva, Switzerland; U.S. standards are available from the U.S. ANSI, New York City.
Audiometers are basically audio-frequency oscillators, adjustable in frequency and in output level, with headsets for the subject. The subject determines the existence of the threshold. Some audiometers are more complex, having recording devices and masking noise facilities.
Since loudness is a subjective magnitude, it cannot be measured with a simple instrument. Arbitrary systems have been developed for determining loudness by instrumental measurements, using rather complex empirical criteria and techniques. A self-contained instrument is available, which analyzes the sound as it is received, breaks it up into critical bands—actually Ad octave bands—performs a number of calculations on these data, and produces a single-figure output. This equipment has been incorporated into a system for continuously monitoring impulse and continuous noise and for printing out loudness levels and occurrence
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