Impulsive Noise: A Challenge to Measure

Bang! Crack! Pop! Impulsive noise is generated by firearms, hammer strikes and even balloons. Technically, gunshots and explosions (including firecrackers or balloons) produce “impulse” noise, while objects that collide (such as hammers, punch presses and drums) generate “impact” noise.

While the manner of sound generation is different for impulse and impact noise, there are commonalities in the actual waveforms. Collectively, “impulsive” sounds are short in duration yet high in peak amplitude. The total duration of impact noise is generally less than one second, while the duration of an impulse may be less than 10 microseconds. Characteristic of impulsive noise is an initial rising component of the wave, which may only last for a fraction of a millisecond, yet be of extremely high amplitude (see  Figure 1).

These signal characteristics make measurement of impulsive noise challenging. To accurately capture impulsive noise, a measurement system must meet demanding technical specifications. Such specifications include an extremely fast rise time (optimally less than 10 microseconds to peak); a wide dynamic range (with a very high maximum sound pressure level [SPL]); and a frequency response maximum of at least 80kHz.

Commercial sound level meters typically have technical limitations that can result in underestimation of peak SPLs (see chart, below). For example, a measurement microphone of ½” diameter will have a lower maximum SPL and a limited frequency range as compared to a ¼” microphone that is not typically specified for most commercial sound level meters. For an impulsive waveform where an 1/8” microphone yielded a peak level of 184 dB SPL, a ¼” microphone may yield a peak level of 180dB SPL, while a ½” microphone may show a severely underestimated peak level of only 168dB SPL.

Figure 1. Impact Waveform Generated by a Steel Shot Put Ball Hitting a Metal Plate

Failing to meet technical specifications for measuring impulsive noise can result in underestimation of peak SPLs, yielding inaccurate conclusions related to noise exposure.

• Donald S. Finan, University of Northern Colorado
• Gregory A. Flamme, Stephenson and Stephenson Research and Consulting
• Researchers and National Hearing Conservation Association (NHCA) Experts

Visit NHCA at: https://www.hearingconservation.org/