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-26- <br />control on sound measuring devices. S1.4-1983 is accepted <br />throughout the acoustical field as the standard for specifying <br />noise instrumentation. <br />Subpart 3 provides that all sound level measuring devices <br />must, at a minimum, be recertified or cross-referenced according <br />to American National Standard Institute S1.4-1983 annually. The <br />rule also provides that the equipment must be externally field <br />calibrated before and after the monitoring event using a <br />calibration device of known frequency and sound pressure level. <br />It is reasonable to require certification and calibration of <br />instruments to assure that sound measuring devices continue to <br />meet original specifications and provide accurate and <br />representative measurements. <br />Subpart 4 sets forth measurement procedures that must be used <br />to obtain representative sound level measurements. This subpart <br />establishes reasonable criteria for the accurate measurement of <br />Found. Specifically, this subpart contains four items, which are <br />discussed below. <br />Subpart 4, item A requires that measurements be made between <br />three and five feet off the ground or surface and away from <br />natural or man-made structures which would diminish the sound <br />level. These specifications are needed in order to ensure that <br />the measurements are representative of the sound level being <br />measured. These specifications are reasonable in that they may <br />