If a CNC machine falls in the forest...
and there’s no one to hear it,
will your wife yell at you anyway? Same here.

What about at work? Most industrial noise falls in the mid-range of human hearing: between 20 and 20,000 hertz. Standard earplugs and earmuffs, on the other hand, are typically tested from 125 to 8,000 hertz. So, can they protect you from the deep rumble of a turbine or the piercing noise of an ultrasonic welder — sounds outside the tested range of those protectors?

Yes, for two reasons. First, Mother Nature has tuned the human ear to hear its master’s voice range; thus we are less sensitive to low- and high-frequency sound. This means a 100 dB sound at 100 hertz will sound much softer than a 100 dB sound at 1,000 hertz. In fact, adult sensitivity to high-frequency noise is so low that a high-frequency device has actually been marketed to break up groups of loitering teenagers by emitting shrill noise that adults can’t hear.

Second, it turns out that earplugs and earmuffs do protect beyond their tested range, particularly against high frequency noises. Low frequency noise is another matter though, mainly because very low frequencies tend to be felt rather than heard. When combined with the ear’s naturally lower sensitivity, the low frequency protection provided by ear plugs or muffs should still be adequate.

The most common complaint workers have about low-frequency noise is that it causes discomfort, headaches and fatigue, again because these vibrations are felt more readily than heard. Among other things, this means that the next time a worker claims a shop noise is rattling his brain, he might be literally right.

To learn more, read the Sound Source Bulletin No. 12
from the Bacou-Dalloz Hearing Safety Group. Visit www.hearingportal.com/hearingconservation/hc-snd-ame.asp.

This article appeared in the October/November 2007 issue of MRO Today magazine. Copyright 2007.