If you don't watch your sound dosage, you might become hard of hearing. Research now shows that exposure to excessive sound, not aging, makes people lose their hearing.
NOTE: See addendum at bottom of page for new discoveries and special cautions on hearing loss from low frequency and subsonic sound.
Selecting the correct weighting
It is important to use the correct weighting for the sound levels being measured. Each weighting is valid only inside the specified range. Attempting to use a weighting outside its range gives erroneous readings. Such false readings cause the readings to be higher or lower than the real SPL levels. Use the correct weighting.
Many otologists think the weighting curves should be adjusted to add sounds which are not perceived as loud, but which do cause hearing damage.
Right now, sounds below 20 Hz are not even represented in the weighting curves, because most ears do not sense them as sounds. But enough acoustical energy in the subsonic area can cause pain, and can cause hearing loss beginning in the 4 KHz area in some individuals.
Many SPL meters do not have a B weighting selection. If a sound falls within the B weighting range, it can be measured with this procedure:
The OSHA Regulation
The OSHA (Occupational Safety and Health Administration) guidelines for sound exposure say that workers may be exposed to the following levels of sound pressure (SPL) for the following time periods (but only ONE of these levels in any 24 hour period). In addition, hearing protection is required to be available at any level above 85 dB SPL. Many otologists feel that the OSHA guidelines do not provide enough protection. (See below for the NIOSH scale.)
A formula for calculating these times (using an ordinary scientific calculator) follows:
OSHA max time (hr):
OSHA permitted exposure times
The NIOSH Recommendation
NIOSH (National Institute for Occupational Safety and Health) and ANSI (American National Standards Institute) have adopted a stricter standard based on equal amounts of energy. Here is their scale of the amount of sound a person may be exposed to in a 24 hour period:
A formula for calculating these times follows:
NIOSH max time (hr):
NIOSH recommended maximum exposure times
The EPA Recommendation
EPA (Environmental Protection Agency) has adopted a yet stricter standard based on equal amounts of energy. But it seems to be based on removing annoyances, rather than hearing protection, since no person in the audience of a concert could hear the music. Here is their scale of the amount of sound a person may be exposed to in a 24 hour period:
A formula for calculating these times follows:
EPA max time (hr):
EPA recommended maximum exposure times
|LEVEL||OSHA MAX TIME||NIOSH MAX TIME||EPA MAX TIME|
|> 115 dB SPL||NONE||NONE||NONE|
|115 dB SPL||15 minutes||28 seconds||NONE|
|112 dB SPL||22 minutes 45 seconds||56 seconds||NONE|
|109 dB SPL||34 minutes 28 seconds||1 minute 52 seconds||NONE|
|106 dB SPL||47 minutes 38 seconds||3 minutes 45 seconds||NONE|
|103 dB SPL||1 hour 20 minutes||7 minutes 30 seconds||NONE|
|100 dB SPL||2 hours||15 minutes||NONE|
|97 dB SPL||3 Hours||30 minutes||3 minutes|
|94 dB SPL||4 hours 36 minutes||1 hour||6 minutes|
|91 dB SPL||7 hours||2 hours||11 minutes 15 seconds|
|88 dB SPL||10 hours 30 minutes||4 hours||22 minutes 30 seconds|
|85 dB SPL||16 hours (protection)||8 hours||45 minutes|
|82 dB SPL||24 hours (continuous)||16 hours||1 hour 30 minutes|
|79 dB SPL||24 hours (continuous)||24 hours (continuous)||3 hours|
|76 dB SPL||24 hours (continuous)||24 hours (continuous)||6 hours|
|73 dB SPL||24 hours (continuous)||24 hours (continuous)||12 hours|
|70 dB SPL||24 hours (continuous)||24 hours (continuous)||24 hours (continuous)|
Comparison between the standards
But how do you figure out your exposure if you are exposed to different levels of these sounds for different periods of time?
Here is one way to calculate your total dose to damaging sound (using the NIOSH levels):
Note: If your equipment gives you the peak level, average level, and minimum level for a given period, calculate the dosage for each figure for the period, and then average the three dosages found. DO NOT average the dB SPL levels.
Note: If you know only the peak level, you must use that. If you know only an average level, add a 10 dB safety margin to it.
Example: You are exposed to:
91 dB SPL - 1 hour
Is your total dose too large?
A spreadsheet is ideal for figuring these doses. Using the same example:
Spreadsheet for figuring NIOSH doses
Manually calculating NIOSH doses:
dose1 = 1 * 2 ^ ((91 - 94) / 3)
dose2 = 3 * 2 ^ ((88 - 94) / 3)
dose3 = 4 * 2 ^ ((85 - 94) / 3)
Sum the doses:
This is larger than 1, so there is a very large chance of damaging hearing.
Recent research has indicated two new facts about hearing loss:
Age related hearing loss is now thought to not exist. Instead, what was thought to be age-related hearing loss has now been found to be acquired hearing loss from cumulative exposures to the loud sounds of steam trains, firearms, and factories.
Extremely low frequency sounds do not follow the tables above, because the weighting curves used for the sound measurements above are based on audibility. But it is now thought that frequencies under 60 Hz, and especially those approaching DC impulse transients, cause hearing damage, even though they are not very audible. This damage begins in the "rock range", starting between 4 KHz and 6 KHz, and expanding in frequency range as exposure continues. So the following precautions should be taken: