For this blog entry we will discuss how we can know if a worker with hearing impairment can hear alarms.
I have an employee that has moderately-severe hearing loss bilaterally in the high frequencies, moderate low frequency loss in the right ear, and mild low frequency loss in the left. I have requested information about the sound levels of alarms in the area he will be working in, but I was curious as to how to calculate if he will be able to hear the alarms once he is wearing hearing protection. My instinct is the level of the alarm, minus the NRR rating, and if it is greater than his loss levels, he should be able to hear it?
There are a lot of variables in this question. So let’s fill in the blanks for one scenario.
1. Let’s look at the sound frequency of the alarm system. Frequencies below 1,000 Hz are masked easier by low frequency noise; frequencies greater than 2,000 Hz need louder volume levels to be heard due to the employee’s hearing loss.
2. We don’t know the type signal the alarm is using. It could be pulsed, like horn beeps or a continuous steady state tone. Intermittent sounds are more recognizable than steady state tones.
3. We don’t know the background noise level in the work area. A comfortable listening level would be 6 dBA above background noise levels. Since this is an alarm, I would think it would need to be 10-12 dBA or greater to be recognized as such. But the signal should not be so loud as to elicit a startle effect, thereby increasing heart rate, blood pressure, and stress in general.
4. We do know the employee has a hearing loss.
So let’s go at it this way. Since the most comfortable listening level is 6 dBA above background noise, and we suspect that 10 dBA would be sufficiently loud enough to be a good warning alarm, then the attenuation of the hearing protector (HP) would lower both the warning alarm and background noise about the same. So one method of calculating the attenuation of the hearing protector is to subtract 7 from the NRR, and then divide by two (in case of loose fit). To apply, say the work area is 90 dBA and the hearing protector has a NRR of 31. 90 dBA less HP attenuation (31-7=24/2=12), and (90-12=78). With HP inserted 78 dBA would be the loudness level of the background noise at the eardrum. The warning signal would be presented at 88 dBA if the signal to noise ratio was 10, or the alarm was 10 dB greater than the background noise (90-12+10=88). Since we suspect the employee will be able to hear an 88 dBA signal level, it should work.
Of course a cheap shot is to put the hearing protection on the employee and ask them if they can hear the alarm system and if it is sufficient to alert them to danger.
Courtesy of George Cook, Au.D., Occupational Audiologist