Quantify the real-world attenuation of hearing protector devices (HPDs) and verify the protection of the wearer (no over-protection, no under-protection and in line with legal requirements).
This protocol is applied to the result of Microphone In Real Ear (MIRE)
, an objective acoustic fit-testing method for 2 channel-designed custom earplug. The test is implemented while the earplug is being worn by the user under high noise load (> 85dB(A)).
MIRE is an acoustic fit-test for hearing protector devices where a sound field is generated (by a headset or a loudspeaker). With two microphones the sound pressure levels outside of the ear and underneath the earplug (or earmuff) are measured simultaneously.
The graph visualizes the residual noise level
. being the result of the microphone measuring the sound pressure under the earplug. This should not exceed the red line that represents the health-based safe limit
The MIRE-microphone measuring underneath the earplug registers the ‘residual noise level’. The second MIRE-microphone measures sound outside the ear (ambient sound level)
Furthermore, the difference in sound pressure between the 2 microphones provides the real- world attenuation
The Health-based safe limit is based on the limit values of a Temporary Threshold Shift (TTS). Sounds below this limit, even after a lengthy exposure, do not cause a shift of the hearing treshold.
, presented as single number rating (SNR MIRE)
There are two primary techniques available for measuring the real-world attenuation of HPDs: Real-Ear-at-Threshold (REAT) and Microphone-in-Real-Ear (MIRE). In the former approach, the attenuation provided by an HPD is the difference in the noise level in the ear canal with and without the HPD inserted and is termed Insertion Loss (IL). In the latter approach, the attenuation is the difference between the sound levels measured simultaneously by the internal and external microphones, and is termed noise reduction, (NR).
and per group of related frequencies (High-Mid-Low). Finally, based on the user’s personal daily noise exposure level (Lep,d) the protection function
Single Number Rating MIRE (SNR MIRE) is a specification used to determine the noise reduction by a hearing protector based on a Microphone-In-Real-Ear technology. Expressed as a single number it provides an easy reference to compare the different types of hearing protection and indicates the level noise can be reduced by wearing the hearing protector . Example: If you have a noise level of 110 dBA and you have an SNR MIRE of 30, then the perceived noise level is reduced to 80 dBA.
is checked considering the locally applicable legal requirements, the health-based safe limit and preservation of communication.
The protective function of a hearing protector is an all-encompassing term but primarily links the personal real-world attenuation to the personal noise exposure level that we are protecting against to ensure compliance with regulations (limit value 87) and the health-based safe limit (no under-attenation) ) and to safeguard communication and audibility (no over-attenuation).
Emphasis on laboratory attenuation values and reliance on these numbers for predicting the protection of the wearer is both unwarranted and potentially harmful to the effectiveness of a hearing conservation program. The ‘HPD Compliance’ tool is game changing for professionals when it comes to quantifying the real-world attenuation of custom earplugs and assessing their protective function during exposure.