Impact And Impulse Magnitudes

The brief noise pulses (lasting only a fraction of a second) caused usually by impacts or explosions require special means of measurement. Even the fast setting of a standard sound level meter usually requires about 0.2 second to reach its deflection, and an overshoot also occurs.

Analysts can ideally measure true impulse or impact magnitudes by recording the wave form on an oscillograph for detailed analysis, and then determining frequencies and total energy as well as peak magnitudes. Most impulse noise meters are somewhat simpler.

A typical impact-noise measuring instrument can determine the peak pressure (by a sample-and-hold circuit or by a peak-reading voltmeter) and the time-average (the average voltage over a chosen period of time). The result is indicated on a pointer-type instrument, where the reading is retained long enough to permit accurate readings. The time-average mode of operation can average over any selected period of time from 2 milliseconds to 1 second; this average provides a simple means of approximating the pulse length. The instrument should respond to rise times as short as 100 microseconds.

Analysts have devised simplified techniques for approximating peak values of impact noise based on the dynamic characteristics of the sound level meter. One method for checking at the 140 dB peak level sets the sound level meter on its C or flat weighting, fast response and the 130 dB range. If the pointer swings no higher than a 125 dB indication for impulsive noise, the peaks are probably no higher than 140 dB. This method assumes that the sound pulse is perhaps 25 to 50 milliseconds long; shorter pulses read too low; longer pulses read too high since the amount of energy in the pulse as well as the peak value affects this type of indication. The length of impact and explosive noise pulses is often increased by reflected sound; indoors, it is increased by reverberation.

MONITORING DEVICES (NOISE DOSIMETERS)

Since noise exposure involves both duration and noise level, continuous observation is required to evaluate exposure if durations and levels vary.

Noise monitors or noise exposure meters respond only when a preset level is reached; some include several circuits with different preset values. They are available in small portable packages to be worn by a worker or in larger (and more accurate) models which are not restricted to the within-limits or exceeding-limit indication. Figure 6.4.7 shows a block diagram of a typical dosimeter that complies with the Occupational Safety and Health Administration (OSHA) criteria.

Where variations in level are wide and not easily predictable, integrating devices are used. The Occupational Safety and Health Act (OSH Act) of 1970 (and others)

halves the permissible exposure for each 5 dB increase in level above 90 dB; this computation can be made by a simple circuit.

Monitoring devices may simply record the duration of exposure above a preset level for daily examination; they may signal hazardous levels with a flashing light; for airport use, they record times of occurrence of all levels above a preset threshold and sound some alarm when another (and higher) preset limit is reached.

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