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Impulse noise may be as quiet as the pop of a can opening. The phenomenon ranges through the snap of a pencil, the beat of a snare drum, the crack of a bullwhip, and the blast of a rifle. What these events share is rapid change in sound pressure within a very short period, often less than a hundred milliseconds. The high peaks and very brief sampling windows make impulse noise difficult to measure with current dosimeters and sound level meters. Since impulse noise is more likely than normal noise to cause noise-induced hearing loss, this difficulty is cause for concern among occupational and environmental health and safety professionals.

Robert Agnew, PhD, first noticed the challenge of accurately measuring impulse noise when he encountered a drop hammer while working as a field industrial hygienist. When the hammer let 15 tons of mass drop from 30 feet, "my sound level meter just pegged out," Agnew said. "It couldn't accurately respond."

Outside of his OEHS work, Agnew is a range safety officer for the Boy Scouts and teaches firearm safety to young people. Hearing protection is difficult for children and teenagers, he noticed, because their ear canals are too small for standard earplugs, or inserts. His students also struggle to wear protective earmuffs on the shooting range because the earmuffs are displaced when wearers place the stock of a rifle against their cheek. These observations led him to investigate new ways to measure impulse noise from both industrial sources and firearms and implement effective controls.

Problems Relating to Impulse Noise

Agnew focuses on impulse noise exposures for two broad exposure groups: industrial workers and members of the military and law enforcement. He faces "difficulties in accurately measuring it and then converting those measurements into something meaningful when determining, for lack of a better term, dose," he said. While current digital monitoring technology is helpful for monitoring noise over time, compared to older analog technology, it is less able to measure sudden changes in sound pressure. That is, digital devices often can't capture the sudden "peak" of sound during an impulse noise event. During firearm discharge—an occupational exposure experienced by law enforcement and military personnel, among other groups—several different noises occur within hundredths of a second. A monitoring device would have to sample up to a million times per second to accurately capture the peak. Most devices aren't able to do so.

"We know impulse noise is harmful, and we want to avoid it, but when it comes to measuring it, the instrumentation we have is not up to the task," Agnew said.

He's also concerned with the difficulty of communicating risks posed by impulse noise to exposed groups. Sound pressure is measured starting at the threshold of human hearing, or about 0.00002 pascals (Pa), up to 101,325 Pa, when sound becomes shockwave. This range, representing eleven orders of magnitude, is difficult to convey to human beings. For easier comprehension, sound levels are often converted to decibels, a logarithmic scale ranging from 0 to 194.1. The downside is that the decibel scale doesn't effectively convey how much louder sounds at the higher end of the spectrum are compared to sounds at the lower end. OSHA's standard for occupational noise exposure (29 Code of Federal Regulations 1910.95) sets the limit for impulse noise exposure at 140 dB (about 200 Pa). When represented on the decibel scale, the sound of a balloon popping (116 dB or 13 Pa) and the sound of a shotgun firing (about 168 dB or 5,000 Pa) appear to be comparable distances below and above the exposure limit, respectively. But because the scale increases logarithmically instead of linearly, the sound of the shotgun is much higher above the limit than it seems at first glance.

"That scale is convenient to talk about and put on a chart, but it fails to communicate how dangerous firearm noise is compared to the more conventional noises that we're used to," Agnew explained. "So this is the difficulty with impulse noise—communicating this kind of risk to people so that they can have that impetus to protect themselves."

Even a person planning to fire a single shot from a hunting rifle should wear hearing protection, Agnew added. Even if exposure to very loud noise doesn't lead to permanent hearing loss, someone can experience a temporary threshold shift when their eardrum tightens to protect itself. Controls are even more crucial for law enforcement personnel. If an officer must discharge their weapon, everyone nearby, including the officer, could experience temporary threshold shifts that will impede communication in these dangerous circumstances.

Impulse noise exposures may be detrimental even well below the OSHA limit. Agnew provided the example of a farrier, a person employed to make horseshoes and fit them to horses' hooves. Throughout their workday, a farrier will be exposed to the noise of a hammer striking an anvil. At about 118 dB, this noise is well below the 140 dB limit, but because the farrier's exposure is chronic, they will be at risk of hearing loss.

Controls and Solutions

Hearing protection is necessary for anyone exposed to impulse noise, but for the intensely loud noise of firearm discharge, Agnew recommends moving up the hierarchy of controls. "There is an engineering control for firearm noise. It's called a suppressor, or some people call it a silencer, but they're very highly regulated, not very accessible," he said. In popular culture, firearm suppressors are often associated with action movie villains. "One of the concerns I have is getting suppressors to be more accepted as a safety device," Agnew continued.

He also advocates that OEHS professionals follow impulse noise exposure limits developed by other organizations, such as ACGIH and the World Health Organization. "I'm really advocating for the WHO limit, which is 20 decibels less or one-hundredth of the OSHA limit," he explained. "WHO's limit is 120 decibels. And it's probably still not protective, but it's better."

A shift away from using decibels as a unit of measurement for impulse noise may also help OEHS professionals better communicate risk, Agnew believes. The decibel unit does not account for differences in sound waveforms, which may have broader or narrower peaks. "It's kind of like the difference between a Prius and a Mack truck," he said, explaining that a speed limit set at 85 miles per hour is a concept similar to the impulse noise limit set at 140 dB. "As long as you stay below the speed limit, you're fine. But if you get into a wreck, and you get hit by a Prius, how much damage is that going to do versus if you get hit by an 80,000-pound truck? They're both going 85 miles an hour. They're both complying with the law, but the damage that happens is a lot different, and that's the same thing with only looking at peak decibels."

"That's why we want to use units like Pascal-seconds or convert that to an auditory risk unit, which takes into account the whole shape rather than just the peak," he continued. The auditory risk unit is a metric used by the U.S. Department of Defense to calculate the risk posed to human hearing by impulse noise exposures.

Educating on Impulse Noise

However, for these solutions to be adopted, OEHS professionals must become more familiar with impulse noise. According to Agnew, impulse noise is "a different animal from the typical, what we call Gaussian or continuous noise that we typically measure in a workplace."

At AIHA Connect in May, he plans to bring attendees up to speed with the nuances of impulse noise with both an educational session and a professional development course. The educational session, titled "The Impulse Noise Inventory," will outline challenges for measuring impulse noise, alternative metrics and exposure limits, and an inventory of impulse noises ranging in loudness. Agnew's PDC, "A Hands-On Exercise in Firearm Impulse Noise Measurement," will offer participants a more active experience. Participants will create impulse noises, such as by popping a balloon and triggering a mousetrap, then measure the noise with microphones and investigate conditions that can affect measurements. Under instructor supervision at a shooting range near the convention center, they will also measure impulse noise produced by firearms and explore controls. "It's going to be a very hands-on, very interactive PDC," Agnew said.

He intends that participants who work with the military or law enforcement will gain a better understanding of suppressors as a control for firearm noise. He also hopes that participants will come to appreciate the need to control impulse noise more generally. "If we're ignoring industrial impulse noise, then we are really doing a disservice to workers because it is far more damaging than continuous or Gaussian noise, which we normally evaluate," Agnew said. He added that new technology is beginning to provide better measurements. "We gotta start addressing it."

Robert Agnew will present Session M7, "The Impulse Noise Inventory" on Wednesday, May 22, 2024, from 10:30 to 11:30 a.m. and PDC 803, "A Hands-On Exercise in Firearm Impulse Noise Measurement" on Thursday, May 23, from 7 a.m. to 6 p.m., all times Eastern. AIHA Connect 2024 will be held May 20–22 in person at the Greater Columbus Convention Center, Columbus, Ohio, and virtually. To learn more about the keynote sessions, view the conference agenda, or register, visit the conference website.