Employees at your company need to wear hearing protectors, so you give them the highest level of protection available, right? Not always.
If you believe that one ear plug or muff is pretty much like the next, with the noise-reduction rating (NRR) as the only difference, you may be putting employees at risk if they need to communicate with co-workers or hear warning signals. Audiologist Chris Dixon-Ernst, CIH, manager of North American Industrial Hygiene Services for Alcoa, has had to overcome the misperception that more is better among many of the 70,000 employees spread across the continent.
"The biggest challenge is working with supervisors and even safety professionals to help them understand that putting workers in the most protective hearing protectors is not always the best solution," Dixon-Ernst said. "In fact, many times it's not."
In many cases, wearing a hearing protection device (HPD) with too high of an NRR could cause overprotection, which is too much attenuation (decibel reduction in sound power and pressure levels) of a specific noise caused by inadequate hearing protector selection.
Whether an individual wearing hearing protection can hear speech and signals in noisy environments depends greatly on the attenuation characteristics of the HPD, the hearing threshold of the wearer, and the level and spectrum of the noise and the target speech or signal, according to John G. Casali, Ph.D., CPE, director of the Auditory Systems Laboratory at Virginia Tech University. Casali is co-author of Chapter 14, "Speech Communications and Signal Detection in Noise," in the fifth edition of The Noise Manual (AIHA Press).
There are situations where a particular hearing protector may attenuate high frequencies (speech and signals) substantially more than low frequencies (ambient or background noise), said Dennis P. Driscoll, PE, principal consultant for Associates in Acoustics Inc. The result is that important consonant sounds of speech may be unintelligible, and warning signals such as backup alarms on forklifts might not be heard.
Workers with too much hearing protection may not be able to communicate with a co-worker and might remove their HPDs to talk or, if they wear ear plugs, not fully insert the plug into their ear, both of which could cause noise-induced hearing loss.
When Katherine Armstrong, a hearing protection technical specialist for Bilsom (Bacou-Dalloz) hearing products, visits with plant employees about their HPDs, "the first thing they do, almost every time, is take out the hearing protector to talk to me," Armstrong said.
In industrial settings, low-frequency sounds tend to mask or obscure mid- and high-frequency sounds, which leads to decreased speech intelligibility, according to Driscoll. The part of the speech spectrum critical to understanding is above 1,000 hertz (Hz). Employees who need to communicate with co-workers are overprotected if they wear an HDP with high attenuation in that frequency range, Casali said.
As with speech, the ability to hear warning signals, alarms and machinery operating noise can be diminished when wearing the wrong type of HPD. With machinery noise, for example, the operator needs to be able to hear a signal that would indicate a malfunction.
The ability to hear warning signals, generally in the range of 1,000 to 4,000 Hz, can be affected by the masking sound of ambient noise. Masking becomes a problem when a signal or alarm is rendered inaudible by the surrounding noise. This can be especially problematic if the background noise is at a similar decibel level or frequency, because attenuating the undesired sound will reduce the ability to hear the warning signal as well.
When employees are involved in their job tasks and are not specifically listening for sounds that may warn them of danger, there also may be a concern whether the signal will get their attention, according to Casali.
Hearing-impaired workers. Overprotection is most prevalent for people with sensorineural hearing loss. This type of loss is common among industrial workers because it occurs with noise exposure and aging. For these individuals, hearing loss usually starts in the higher frequency range, where traditional HPDs are most effective, according to Driscoll. Hearing protectors may attenuate speech and warning signals to the point where hearing-impaired employees are unable to distinguish these sounds.
What is the proper type of HPD that allows the wearer to hear necessary speech and signals? The simple answer is a protector that has moderate attenuation of mid- and high-frequency sounds and the proper NRR for the situation. Arriving at a complete answer, however, is not so simple.
Choosing hearing protection based on NRR only does not take into account the spectrum, or frequency characteristics, of the protector or noises. For situations where proper HPD selection is critical, Casali suggests using a spectrum analyzer to perform spectral matching, which determines what spectral attenuation characteristics of an HPD are needed to match the spectral characteristics of specific noises.
"The safety professional or industrial hygienist concerned about a worker hearing a forklift backup alarm or an evacuation signal needs to determine the spectrum of attenuation," he said.
A similar solution, one used more often by Dixon-Ernst at Alcoa, involves workplace observations and data from dosimetry measurements, combined with attenuation data from HPD manufacturers. She also will review job tasks to determine whether there are communication requirements that call for a more specialized hearing protector.
EN 458, a European guidance document that gives hearing protector recommendations, suggests selecting HPDs that reduce the level of exposure to the ear to between 70 and 85 decibels A-weighted sound level (dBA). If a protector with too high an NRR attenuates noise to below 70 dBA, the wearer also is overprotected.
"You don't need to wear a hearing protector that's going to offer a noise reduction of 30 decibels when you're dealing with a noise exposure of 87 or 88 decibels," Dixon-Ernst said. "Workers will reduce how deeply they insert the plug because it's blocking their ability to communicate or to hear certain sounds. So they won't wear it properly or won't wear it at all, which is not what you want."
The effectiveness of a hearing protector is highly dependent on wearing time. According to EN 458, when someone removes an HPD for even a brief time in the noise exposure, the effective protection provided is significantly reduced. "The best hearing protector is the one that gets worn," Bilsom's Armstrong said.
Because attenuation of conventional HPDs increases with frequency, employees who need to hear higher frequency sounds such as speech and signals will be more willing to wear the protector 100 percent of the time if it has flat or uniform attenuation. Flat attenuation makes the environment sound more natural, said Elliott H. Berger, senior scientist for auditory research at E-A-R (Aearo Co.). "It's as though you turned down the volume instead of putting in a high-frequency filter."
Data suggests that an HPD with more high-frequency attenuation than low allows an imbalance of low-frequency noise that causes an upward spread of masking, which interferes with speech intelligibility or other sounds you want to hear, Berger said.
Because flat attenuation devices generally have a lower NRR than conventional HPDs, they may not be appropriate when maximum protection is needed, such as exposures of 100 dB and higher. "At those levels," Berger said, "you can't hear anyway, no matter what's going on. It's just too noisy. It's not an issue of improving your speech intelligibility. You just need protection."
For situations where communication is critical or hearing-impaired workers cannot hear necessary sounds with ear plugs or muffs, communication headsets may be the answer. These headsets, which also provide hearing protection, can range from acoustical devices that use a mechanical network to transmit necessary sounds to electronic devices that feature two-way communication. These types of headsets typically limit sounds to 82 dB.
Alcoa employees working near mobile equipment who might be vulnerable to accidents wear communication devices. Any accident investigation includes reviewing whether hearing or noise was a factor, Dixon-Ernst said. So far, the company has documented no incidents where hearing or noise contributed to, or caused, the incident.
Mike Cimino, marketing director for Peltor (Aearo Co.) communication devices, points out that amplified headsets should present sound in a stereo format for omnidirectional hearing, which helps wearers pinpoint the location of hazards such as vehicle backup alarms.
Communication. Casali and co-author Gary Robinson, Ph.D., a research associate professor at Virginia Tech, provide several recommendations in The Noise Manual for reducing unintelligible speech in addition to selecting an HPD that will not overprotect:
- Whenever possible, decrease the distance between the speaker and the listener and encourage the use of hand and facial cues, although these are ancillary aids that should not take the place of speech.
- Include material on speech intelligibility and detection of signals in employee training and stress the positive effects that the proper use of HPDs can have on the audibility of speech and signals.
- Encourage employees to speak more forcefully to overcome the tendency to lower the voice while wearing HPDs. Increased vocal effort, however, should not be continued for long periods because of the potential to irritate the speaker's vocal tract.
- Encourage input and feedback from employees regarding speech intelligibility (and the audibility of signals) so problem areas can be identified before an accident occurs.
- Improve message content by encouraging and implementing consistent sentence construction for standard messages. Workers should avoid the use of single letters and use whole words (phonetic alphabet) or complete sentences whenever possible.
Warning signals. While the detection of audible alarms and warnings will present a problem as long as excessive noise levels are prevalent in the workplace, several recommendations can help reduce the severity of the problem:
- Use the most appropriate standard for guidance in selecting an alarm or warning device. One such standard is the American National Standard Institute's S3.41, "Audible Emergency Evacuation Signal."
- Limit the number of discrete signals to seven or eight. More than this could cause confusion as to the meanings of the signals.
- Ensure that signals contrast well against background noise and that the various signals sound sufficiently different to avoid confusion.
- Preferably, signals should be at least 15 dB above their masked threshold, the level at which the signal is just audible in the presence of noise. Signals more than 20 to 25 dB above their masked threshold may elicit an undesirable startle response from workers in the area.
Hearing-impaired workers. While each situation likely is unique for employees with hearing loss, several steps can be pursued to help them hear speech and signals:
- If the loss is not too severe, following the speech and signal guidelines mentioned previously may be sufficient.
- Use of a special-purpose HPD or a communication headset may help the wearer hear necessary sounds.
- Auditory alarms can be supplemented with visual displays. Color-coded lights can substitute for nonverbal auditory displays, and text-based displays can be used instead of verbal communications.
- In special cases, tactile (vibratory) alarm displays should be considered. The primary disadvantage is that these are personal displays that must be worn against the body.
In the end, the burden falls on the employer to understand how to select the appropriate type of hearing protection for workers who need to communicate or hear specific noises. "Err on the side of safety," Casali said. "Be more precise about the selection of hearing protectors to avoid this issue of overprotection."