ASA - ANSI/ASA S12.9 PART 7
American National Standard Quantities and Procedures for Description and Measurement of Environmental Sound, Part 7: Measurement of Low-frequency Noise and Infrasound Outdoors and in the Presence of Wind and Indoors in Occupied Spaces
|Publication Date:||25 April 2016|
This standard provides requirements and methods for measuring low-frequency sound and noise levels outdoors in the presence of wind and indoors in occupied spaces. The most common application anticipated is the measurement of outdoor immission levels either near or far from sound emission sources or emission levels near a source.
A repeatable method for measuring low-frequency levels and spectra indoors is given so that results can be compared from site to site or for repeated measurements at the same site under differing operating scenarios or time periods.
The value of this method to indoor measurements is that wind effects on the microphone are eliminated. However, two new variables are introduced: wind impinging on a structure creates significant low-frequency noise that is difficult to quantify, and measuring indoors with room sizes comparable to or smaller than low-frequency wavelengths requires an accounting of room resonance modes. Nevertheless, following the prescribed measurement technique in low-wind conditions should give a uniform repeatable method of measurement that accounts for differing room modes from space to space.
The standard does not address all factors to be considered when measuring the emissions of sources or background sound levels in the environment that are covered in other standards. Such factors may be measurement metrics, duration of measurements, background corrections, time of day, and countless others. Usually, wind speeds in these specific standards are limited to low values (about 2 m/s or 5 mph) to minimize wind-induced noise effects. Measured data in the annexes show clearly that wind, even at these low wind speeds, significantly affects the measurements at very low frequencies. This standard is limited to measurement techniques, principally microphone wind-mitigation measures that can be used to quantify such wind effects. Hence, the wind-mitigation techniques for microphone protection described herein are supplemental to existing measurement standards.
Advanced signal processing techniques in the time domain that can detect ILFN signals in the presence of wind are discussed briefly and referenced.