Many accident investigations now include consideration of possible noise interference with the detection of warning signals, moving vehicle alarms and speech communication. Comprehending speech in the presence of loud background noise can be challenging at best, and at times nearly impossible.
This article reviews some basic principles affecting a worker’s ability to understand spoken messages in a noisy environment and provides practical tips for improving communication in noise.
Key factors affecting speech communication
Signal-to-noise ratio (SNR): The most basic concept of speech detection (realizing someone is speaking) and speech intelligibility (understanding what is said) is the relative sound pressure level (loudness) of the speech compared to the level of the noise. This relationship can be somewhat complicated, depending on the frequency (pitch), timing, and complexity of both the speech and background noise.
For most listeners, the level of a speech signal must be significantly higher than background noise in order to be understood.
In industry, achieving an adequate signal-to-noise ratio can be a challenge in many work areas. There is a physiologic limit to how loud and how long talkers can raise their voice, or shout, to be heard. Even if workers have the benefit of a 2-way radio, phone or public announcement (PA) system, cranking up the volume to be heard over the background noise only adds to workers’ noise exposures.
Inner ear distortion: Simply making sounds louder through amplification may not make those sounds easier to understand. Another consideration is the physiologic limit of the normal auditory (hearing) system. Sensory cells and nerve fibers in the inner ear work most effectively at low to moderate sound levels. At high sounds levels, signals within the inner ear become distorted, and auditory function is degraded.
Hearing loss: According to the National Institutes of Health, approximately 17 percent of American adults report some degree of hearing loss.4 There are many different types of hearing loss that can affect listeners in varying ways.
Some people have trouble hearing even in quiet situations. Others get by fine in quiet, but experience significant difficulty distinguishing sounds, particularly speech, in the presence of competing background noise. A listener with hearing impairment is likely to say making sounds louder doesn’t necessarily make them clearer or easier to understand.
Hearing protector complications: Conventional hearing protectors may degrade speech audibility for some workers, particularly those with hearing-impairment. Conventional earplugs and earmuffs provide more sound reduction for high pitch (frequency) sounds than lower pitches.
In speech, vowel sounds (a, e, i, o, u) are fairly low in pitch, while consonants (such as k, s, t, f, sh, etc.) are higher pitched. Missing out on the consonants of speech can leave the listener struggling to interpret vowel sounds that come across as mere mumbling or gibberish. For a worker with high frequency hearing loss (very common), wearing hearing protectors can exacerbate the high frequency deficit, and in turn, speech intelligibility is further diminished.
Options for improving communication
A company concerned that noise may be interfering with communication and warning sounds should conduct a detailed noise evaluation. Engineering controls may be needed to reduce noise at its source. When noise control is not possible, take into account the following potential solutions:
Develop visual cues: Where obstructed vision is not a problem, consider developing a system of hand signals or written text to clearly convey messages without relying on auditory communication. For emergency alerts, investigate alarm systems with flashing lights or vibrating pagers.
Create pre-coded messages: In advance, develop a predetermined set of short, predictable code words and key phrases for your most common operations. Expected messages are typically easier to recognize than novel communications.
Improve the speech-to-noise ratio: In some cases, amplification/PA systems may be warranted.
Another solution is to provide employees with specialized hearing protectors that block out background noise without compromising the speech or warning signal. Although these protectors are not hearing aids, by their design they often improve communication capabilities for all, including those with hearing loss. These systems typically consist of a standard hearing protector (earplug or earmuff) that attenuates or blocks the noise, in combination with a small speaker imbedded in the earplug or earmuff to deliver the communication signal directly to the ear. Systems may be designed for one-way or two-way communication.
There are a variety of products in the marketplace with pros and cons to consider, including comfort and compatibility with other personal protective equipment.
When it comes to communication effectiveness, there are three important features to consider:
Binaural (both ear) listening provides a significant advantage over monaural (one ear) listening. The human auditory system is designed to work with input from both ears. Most research studies agree that the binaural advantage is most robust the more complex the signal, the more complex and loud the competing background noise, and the fewer visual cues available.
As a result, signals such as radio transmissions do not need to be as loud if a listener is using both ears. The practical result of this phenomenon is that comfortable volume settings are typically lower if using a binaural headset compared to one with a monaural listening configuration.
Output-limiting promotes safer listening levels. Whether a worker has normal hearing or hearing loss, it is important that signals delivered to the ear do not add to a noise hazard on the job. When purchasing a communication headset for any employee, make sure the device has an electronic feature to limit outputs to safe levels.
Adequate seal of hearing protector ensures best results. Delivering audio communications directly to the ear(s) at a safe but sufficient level can be strongly dependent on a suitable seal of the hearing protector. If the hearing protector is not sealed properly, background noise leaks in and competes with the speech signal. The user may be inadequately protected from background noise, and worse yet, may try to compensate by increasing the volume control on the communication device resulting in added noise exposure.
The key to high-quality communication is an optimal signal-to-noise ratio. When a proper hearing protector seal is achieved, excessive background noise will not be competing with the desired speech signal.
Because job requirements vary and workers’ hearing abilities differ, consider building job-specific protocols for hearing-critical jobs and individualized plans for any hearing-impaired workers. See OSHA’s targeted Safety and Health Information Bulletins.5-6