SOUND MANAGEMENT Washington DC

Manufacturing is a noisy business, and stamping along with other metal-manufacturing processes are among the noisiest. About 20 million in the U.S. are exposed daily to intermittent dangerous sound levels that could cause some level of hearing damage. Noise-induced hearing loss can occur at levels as low as 85 decibels (dB), which is a little quieter than a lawn mower.

Longer exposure to high sound levels increases the damage to hearing. At 90 dB, the maximum safe exposure for unprotected ears is about two and one-half hours. Once hearing is impaired, the damage cannot be corrected because the inner ear cells required for hearing cannot be regenerated or repaired.

GOVERNMENT REGULATION

Since the 1970s, the Environmental Protection Agency (EPA) has regulated testing and labeling of all hearing protectors sold in the United States. This regulation was supplemented in 1983 by the OSHA Hearing Conservation Amendment, which requires factories with average noise at or above 85 dB for an eight-hour day to have hearing conservation programs. Annual audiograms must be done for each worker to see if there has been any hearing loss. Records of any work-related hearing losses must be kept, and workers in these plants must be supplied with earplugs and earmuffs at no charge. If subsequent hearing tests show a worker has lost 10 dB or more of hearing at higher pitches in either ear, the person must be informed and must be required to wear ear protection on the job.

The Noise Reduction Rating (NRR) measures effectiveness of personal hearing protectors. It has traditionally been calculated using idealized laboratory testing.

"The EPA is currently considering changes that would test the hearing protectors under conditions that are less than ideal, but more reflective of real-world usage," explains Brad Witt, audiology manager for Howard Leight/Bilsom Hearing Protection, part of the Bacou-Dalloz Hearing Safety Group, Smithfield, R.I. "Right now, a user may look at a hearing protector package and assume that an earplug with a NRR of 30 dB will block 30 decibels of noise. But we know that there are many factors that prevent users from achieving an optimal fit, like lack of training or improper sizing or insertion. In a real-world workplace, a user of a 30 NRR earplug may only be achieving 20 dB of protection. We expect the new EPA rating method to indicate a range of protection rather than a single value."

WAYS TO REDUCE THE EFFECT OF NOISE

The amount of noise workers experience can be decreased either by changing the physical environment through engineering controls or by wearing ear-protection devices. If engineering controls are insufficient to reduce noise to safe levels, hearing-protection devices are required by OSHA.

Different noise-control methods are used for low frequency and high frequency noise. In determining what type of physical changes to make to a stamping plant, for instance, a noise-control consultant would begin by studying the nature of the noise itself. "Because stamping impact noise is typically low frequency (from 63 Hz through 500 Hz), a high level of mass is important," explains John D. Sofra, commercial (Airside) and industrial U.S. market manager at Kinetics Noise Control, Inc. (www.kineticsnoise.com), Dublin, Ohio. In stamping plants it may not be possible to enclose each individual process since workers must be able to observe and interact with the machines. If enclosure is an option, noise reduction of 25 to 35 dB can be achieved with a rigid panel barrier wall system that incorporates warning lights while the machines are operating.

Failing that, "it is better to isolate the noisy part of the facility from others using clear view, mass-loaded vinyl curtain systems that can be hung from overhead factory trusses," notes Sofra. "The system can be manufactured entirely of the clear-view material, or clear view windows can be incorporated into quilt-mass-loaded vinyl enclosure and barrier-wall systems. Noise control offered by curtain systems is limited to 12 to 20 dB. Remember, a 10-dB drop in sound level is perceived by the average human as sounding half as loud."

These curtain systems can either contain the noise or serve as barriers between noisy and quiet areas, and are a good choice for use around industrial equipment because they are more versatile than rigid systems. KNC's curtain systems weigh from 0.5 to 2 pounds per square foot. They come with a self-supporting track and have Velcro nylon closures along the edges for opening.

"KNC's quilt can also be used to line the walls of an area to reduce the reverberant noise levels with the space," adds Sofra. This type of solution reduces noise only 5 to 8 dB. If noise is being reflected off walls, high-density fiberglass absorbers, which are especially useful for low to mid-range frequencies, can be attached directly to the walls.

PLUGS OR MUFFS?

Both earplugs and earmuffs can provide effective sound attenuation. Which to use is largely a matter of personal choice. "Assuming 'bigger must be better,' many people mistakenly believe earmuffs block more noise than earplugs," Witt notes. "But this is not the case. As a group, earmuffs generally have lower but more consistent and repeatable attenuation than most properly fit earplugs.

"The advantage of earplugs is that they are comfortable for extended use and are cooler in hot work environments," Witt points out. "Earmuffs can feel hot and heavy during extended use. The downside is the attenuation of earplugs is highly dependent upon a good fit."

Individual training and motivation are crucial to proper use of earplugs. Witt says repeated studies have found that group training in earplug use is ineffective, with workers using the plugs incorrectly no matter how many group sessions they had attended. "But one-on-one training by a safety professional was a very positive predictor of earplug effectiveness," he emphasizes. "An ideal time to perform that individual training is in conjunction with the annual audiometric testing.

"Earmuffs have an advantage in their ease of use," notes Witt. "It's easy to achieve a proper fit, and they are excellent for intermittent noise. There are also electronic options available in earmuffs, such as wireless communication headsets, level-dependent circuitry and AM/FM radio, but these come at a premium price. Even if earplugs are the predominant hearing protector in a noisy workplace, it is advisable to keep earmuffs on hand as an option for workers who may be temporarily unable to use the earplugs due to ear infections or irritations."

Because many workers feel they are invulnerable to workplace injury, particularly one like hearing loss, which has no obvious symptoms, it is necessary to provide effective demonstrations of the future risk they are taking.

"In training sessions, I often use recorded simulations of hearing loss to demonstrate how hearing is affected by loud noise over time," Witt says. "It can be an eye-opener for that young worker to understand what it means to have a noise-induced hearing loss."

Editor's Note: Jean Feingold is a freelance writer based in Gainesville, Fla. Photos are courtesy of Howard Leight/Bilsom Hearing Protection, www.hearingportal.com.