In most cases, the sound level from firing a weapon is sufficient to require the use of hearing protection, even if the weapon is fired only one time. Recent NIOSH studies of sound levels from weapons fires have shown that they may range from a low of 144 dB SPL for small caliber weapons such as a 0.22 caliber rifle to as high as a 172 dB SPL for a 0.357 caliber revolver.
Consequently, NIOSH recommends that hunters and shooters use double hearing protection each
and every time a weapon is fired. Double protection involves wearing both earplugs and earmuffs. The best combination is a deeply inserted foam earplug and
a well-seated earmuff.
NIOSH examined the performance of several types of hearing protectors with a variety of weapons. Earplugs were able to reduce the peak sound pressure level by 10 to 30 decibels and earmuffs yielded 20 to 38 decibels of peak reduction. Active level-dependent earmuffs were found to react sufficiently fast to provide the same protection level as when they were turned off [Murphy and Little (2002) J. Acoust. Soc. Am. 111:2336; Franks and Murphy, (2002). J. Acoust. Soc. Am. 112:2294]. The effect of volume gain setting was minimal for active level-dependent devices (see Figure 1). Each protector was tested with the volume set for unity gain, maximum gain or turned off. At unity gain the sound under the protector is as loud as when no protector is worn. The peak reduction was mostly unaffected by the change in the volume setting.
The formula for determining the maximum number of shots is:
As shown in Figure 2, the amount of reduction for 12-gauge shotgun using the David Clark Model 27 earmuff is 31 dB, reducing the peak level from 161(red line) to 130 dB SPL( blue line). So, with the earmuff alone, the number of shots recommended by NIOSH would be ten per 24-hour period. The addition of a deeply inserted foam earplug reduces another 21 dB, increasing the allowed number of shots to more than 1200 per 24-hour period. A smaller caliber weapon with a lower peak level, such as 0.22 caliber rifles with peak levels of 144 dB SPL, could be fired as many as 63000 rounds per 24-hour period if the shooter were to wear an active level-dependent earmuff along with a deeply inserted foam earplug. The combination of an earmuff and a deeply-inserted foam earplug can provide as much as 50 dB of peak reduction, which is adequate in most cases.
Double hearing protection can severely compromise the ability to communicate when both devices are passive, linear protectors. The use of an active level-dependent earmuff with a deeply-inserted foam earplug can compensate for the loss of communication ability when double protected. In some cases, the earmuff may provide sufficient gain to counter the attenuation of the earplug.
For shooters, a key indication that whatever protection is being used is inadequate is ringing of the ears or a feeling of fullness in the ears after an episode of shooting.
There are also active level-dependent hearing protectors that are built into pre-molded earplugs and custom earplugs as well on the market. There are no data on their effectiveness. At present, there are no data on the use of an active level-dependent earplug in combination with a passive earmuff. Given the attenuation chain, it is doubtful that the pairing would be as effective as the active level-dependent earmuff with foam earplug.