Tips to communicate in noisy industrial environment

Hearing protective devices can be effectively utilised for the preservation of hearing in high noise level environments with minimal effects on speech discrimination

We all are aware of the fact that hearing protective devices (HPDs) reduce user sound exposures when properly worn. This means that all sounds may be attenuated, both unwanted sounds (noise) and useful sounds such as speech and warning signals. Thus wearing HPDs may affect speech discrimination, and the perception of warning signals. This article discusses the magnitude and quality of these effects as a function of hearing level and hearing protector type.
 
Speech discrimination
Speech discrimination (SD) is a measure of one’s ability to understand speech. It is greatly affected by such factors as a person’s hearing acuity, the signal (speech) - to - noise ratio, the absolute signal levels, visual cues (lip and hand motion), and the context of the message set. SD is measured by presenting to subjects one of a number of prepared word lists, and determining what percentage correct responses they achieve. The effects of HPDs on SD can be evaluated by establishing a set of test conditions, and measuring SD with and without HPDs on the subjects. The results of such tests conducted by many investigators may be summarised as follows: 

• Hearing protective devices have little or no effect on the ability of normal hearing listeners to understand speech in moderate background noise at 80 dBA, but HPDs begin to decrease SD as the background noise is reduced even further. HPDs will decrease SD for hearing impaired listeners in low-to-moderate noise situations.
• At high noise levels >85 dBA HPDs actually improve SD for normal hearing listeners. For hearing impaired listeners the effect of HPDs on SD at these high noise levels is not unequivocal, but the results seem to indicate no significant effect.
• The study is not extensive enough to differentiate between the effects of earmuffs and earplugs on SD. Nevertheless it may be said that the higher attenuation devices, be they ear muffs or earplugs, offer greater potential for degrading speech discrimination at lower sound levels.

The beneficial effects of hearing protective devices on SD can be partially explained by referring to the spectrum where a male voice is superimposed upon a typical industrial noise spectrum of 91 dBA. Although the HPD’s attenuation increases with increasing frequency, at any one frequency both the speech and the noise are reduced equally. The signal to noise ratio is constant, but importantly the overall signal level is reduced. This prevents the ear itself from distorting the signal, a phenomenon which occurs even at levels well below 90 dBA. Thus as long as the speech signal is maintained above audibility, intelligibility can be improved by restricting signal levels to those that will not overload the ear.
 
The preceding generalisations may be modified in practice by three important factors. Typically, in real work environments, communications will be accompanied by visual cues and/or be limited in scope. Missed words can be “filled in” and intelligibility maintained. Studies haveshown that when the person speaking wears HPDs his speech quality is degraded and this will adversely effect communications. And finally, studies have demonstratedthat employees get accustomed to listening in noise and can perform better with respect to SD than do laboratory subjects with equivalent hearing levels. The interaction of these three effects has not been fully evaluated by any studies, but it has shown that visual cues do improve SD for hearing impaired persons wearing HPDs, especially in noise.

 
Localisation
Another effect that HPDs can have is to confuse one’s ability to locate the direction of origin of sounds. The data indicate that earmuffs, which necessarily cover the entire ear, can interfere with this localization accuracy whereas inserts, which generally leave virtually the entire outer ear exposed, do so to a much lesser extent. Furthermore, experiments with earmuffs indicate that subjects cannot adapt to this effect, ie, they cannot learn to compensate for the adverse effects of the muff.
 
Summary
HPDs can be effectively utilised for the preservation of hearing in high noise level environments with minimal effects on speech discrimination. For hearing impaired persons, the utilisation of HPDs in lower noise level environments should be carefully considered. If localisation capabilities are important then inserts should be chosen instead of earmuffs. And finally, the use of amplitude sensitive devices may be advantageous for use on firing ranges where they have been shown to provide adequate protection for limited exposures.
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Vinay Pathak is Deputy General Manager at 3M India