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Abstract
This work was carried out to investigate the acoustic attenuation characteristics and the speech intelligibility (SI) for three newly designed hearing protection devices (HPDs), namely, active hearing protector, aviation headset fitted with ANR system and helicopter helmet fitted with ANR system.
The data measurements are carried out at in the reverberation room- National Institute for Standard (NIS) in which the diffusivity of this room was improved by hanging diffusers as specified in ISO/R 354:1963. Speech intelligibility (SI) for headset and helmet were conducted at the reverberation room of Housing & Building Research Center (HBRC). For the reverberation rooms at NIS and HBRC, the acoustic environments met the specification of the diffuse field described in ANSI S3.19-1974.
Twelve subjects (volunteers) with normal hearing throughout the frequency rang from 125-8000 Hz in accordance with ANSI S3.6 2004 have participated in the experiments. Pink noise as an external noise not exceeding 90 dB SPL (for the subjective methods) was used as a sound source to provide a reasonable balance between energy at the frequency range used.
An artificial head [acoustic test fixture (ATF)] is constructed with an ear simulator to fulfill the requirements of IEC 60711 and ANSI S3.25 was used for the measurements of the active hearing protector (AHP).
The general performance characteristics of the active hearing protector (AHP) with respect to cut-off level (the noise level at which protector starts to attenuate), sound attenuation, and speech intelligibility were studied. Test results of the transfer function and insertion loss (IL) measurements for the active mode of AHP in combination with the results of IL of passive mode have been used to estimate the effective A-weighted SPLs as recommended at EN ISO 4869-4: 2000. It was found that the active hearing protector (AHP) has better cut off characteristics at a level above which the damage risk began to start (~85 dB). As expected, the passive attenuation obtained on the ATF with IL measurements was better (46.4 dB) than that obtained on the human ear measurements (43.4dB). The passive attenuation obtained on the real ear by subjective method (REAT) agreed with that of the manufacturer on the octave bands where both were measured on the real ear.