Here are some useful hearing conservation sites:

National Hearing Conservation Association: http://www.hearingconservation.org/

National Institute for Occupational Safety and Health:

http://www.cdc.gov/niosh/topics/noise/default.html

And an article from the Sargent College Compendium:

I've heard loud sound can damage my hearing, but how do I know when a sound is dangerously loud?

This question is tricky to answer because loudness is a subjective experience. In a survey of musicians who were asked to describe their feelings about very loud music, one respondent said "(It's) all a matter of perspective; my loud may be your soft." Some people tolerate and even enjoy dangerously loud sounds while others would prefer to wear earplugs even for sounds that do not pose a risk. Also, the loudness experience can vary for one person depending on his or her mood or other factors; a particular sound may be enjoyable at one time of day and annoying another time. So we cannot rely on our subjective judgment of loudness to recognize dangerously loud sounds.

Hearing conservation guidelines are based on two objective measurements: sound pressure level (which we perceive as loudness) in decibels, and the duration of exposure time. You can find guidelines for recommended exposure limits on the National Institute of Occupational Safety and Health (NIOSH) website. According to these guidelines, exposure to a sound of 85 decibels is acceptable for up to 8 hours a day. This time limit is halved for every three decibels added to the intensity.

How loud is 85 decibels? I recently measured the sound level produced by my vacuum cleaner, and found it was about 85 decibels (dBA SPL) at the position of my ears when I was vacuuming a carpeted floor. Very loud restaurants or noisy parties can get to levels of about 100 decibels (at this level, NIOSH recommends no more than 15 minutes without a quiet break). And rock concerts typically attain levels of 120 decibels in the audience.

Another way to assess sound level is to analyze your ability to communicate when you are in a noisy environment. Can you have a conversation with another person at a comfortable distance? Do you have to raise your voice? How much effort is required for a conversation? If conversation is effortful, you are probably in an environment with a sound level greater than 85 decibels. We are currently conducting research aimed at providing more precise descriptions of how conversation is affected by background noise (thanks to the Boston University Hearing Research Center, which funded the pilot study).

When you are exposed to sound beyond the acceptable sound level and time limits, the hair cells in the cochlea become over-stimulated and lose their ability to transmit signals to the brain. When this happens, you may experience a numb or even painful sensation in your ears, and you may perceive sound as muffled. This may be accompanied by tinnitus, which is usually described as a ringing, rushing sound, or high-pitched tone. All these sensations may go away after a period of a few minutes, hours, or even days. But in some cases the symptoms never disappear, and the person suffers permanent hearing loss and tinnitus. There is significant individual difference in susceptibility to hearing loss from loud sound, so it pays to be cautious.

Here are a few tips. If you are going to a rock concert or car race, if you use power tools or other loud equipment, or if you are riding a motorcycle or jet skis, wear earplugs. The foam plugs available at most drugstores are sufficient protection for most of these activities, as long as you insert them properly. Follow the directions on the package. If you are a hunter or shooter, earmuffs or a combination of earplugs and earmuffs is recommended. If you wear headphones to listen to music, set the volume when you are in a quiet place and don't turn it up to compensate for increased background noise you may encounter.

I welcome your impressions and thoughts about the loud sounds in your life. Please visit my website and fill out the Loud Sounds survey.

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