User Guide

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obtain that amount of noise reduction for an 85-dB SPL exposure and at least that much or more noise

reduction for a 150-dB SPL exposure. These devices are sometimes referred to as “conventional” or

“non-level-dependent” hearing protectors.

Passive Level-Dependent Hearing Protectors

HPDs that incorporate specialized acoustic filters are often referred to as “level-dependent” or

“nonlinear.” Unlike traditional passive hearing protectors, these devices create a relatively transparent

barrier at low sound levels, using a tiny orifice or a thin diaphragm. The intention is for the amount of

noise reduction to increase in proportion to the sound level to which the wearer is exposed.

At low sound levels, below 110 dB SPL for example, these devices provide little or no attenuation,

allowing the wearer to maintain better hearing ability*. However, when the wearer is exposed to very

high level, short-duration impulse noises, the acoustic filters instantaneously restrict the transmission of

sound into the ear by a greater amount that increases as sound level increases, to help boost the

attenuation of the peak sound pressure wave, L

, and reduce the exposure of the wearer.

Some level-dependent HPDs, such as the 3M™ Combat Arms™ Earplugs, allow the wearer to switch

from the impulse noise protection mode to a continuous noise protection mode by sealing the acoustic

filter, thus causing the device to function as a traditional hearing protector. This can be useful when the

wearer is exposed to impulse noise at certain times and continuous noise at other times.

Electronic Level-Dependent Hearing Protectors

These HPDs use electronic technology to maintain, and in some cases enhance, hearing ability when

sound levels are low. They are often referred to as “active” hearing protectors. Environmental

microphones (also referred to as tactical, surround, or situational-awareness microphones) on the

device pick up the low-level (non-hazardous) sounds in the area around the wearer and reproduce them

inside the hearing protector. Typically the wearer can control the loudness using a volume control on

the device. The amount of sound that is electronically reproduced inside the HPD decreases

proportionally as the sound level outside the device increases. Electronic compression is used to limit

the level of the reproduced sound inside the headset to non-hazardous levels.

Since the maximum attenuation provided (in the absence of electronic reproduction) is based on the

physical properties of the earmuff cups or earplugs that create the acoustic seal around the ear or in the

ear, these devices provide protection against both impulse and continuous noises in the same way as

traditional HPDs. The big difference is the ability of electronic level-dependent HPDs to allow the

wearer to hear more effectively during periods of low noise without the need to remove the device.

All 3M™ PELTOR™ electronic level-dependent hearing protectors limit the sound reproduced from the

environmental microphones to 82 dB SPL. In the absence of the reproduced signal (even if the active

circuitry is powered off), some sound will continue to be transmitted into the ear since the barrier

created by the earmuff cups or earplugs themselves has the same limitations as does a traditional HPD.

In other words, even electronic level-dependent hearing protectors eventually depend on the non-

electronic components to help protect the wearer’s hearing.

*Hearing ability is a general term to describe various factors related to auditory situational awareness such as

sound detection, recognition, identification, localization and communication.