NIOSH Impulsive Noise Measurement System (NIMS)

This paper describes how the National Institute for Occupational Safety and Health (NIOSH) developed a measurement and analysis tool for accurately capturing and measuring impulsive noise parameters. In the 1998 NIOSH Criteria for a Recommended Standard for Occupational Noise Exposure, impulsive noise and hearing protectors were identified as research needs. The paper discusses how the NIOSH Impulsive-Noise Measurement System (NIMS) was primarily developed to help evaluate the effect of impulsive noise on the auditory system. The NIMS tool uses a graphical user interface (GUI) built in MATLAB to display time domain waveform, frequency spectrum, and (1/1) and (1/3) octave band spectra of the captured impulse noise event. Additionally, parameters such as peak pressure level, equivalent average level, kurtosis, time duration, number of impulses, and temporal spacing between impulses are also calculated and displayed. A detection routine was developed to isolate and identify the location of impulsive events embedded in a waveform. The time-domain impulse analysis requires a high signal-to-noise ratio to identify events which exceed a user-defined threshold. The frequency-domain impulse analysis estimates the spectrum of a user-supplied background noise and then applies a spectral threshold to windowed samples of the target waveform. Once the impulsive events have been identified, the program uses the three major damage risk criteria that are in use today: (1) A-weighted equivalent 8-hour average level (LAeq-8hr); (2) Department of Defense MIL-STD 1474D; and (3) the Auditory Hazard Analysis Algorithm for Humans (AHAAH) based on Price/Kalb ear modeling technique to determine the limits for exposure to a particular impulsive noise event.