A new method, bone conduction loudness balance (BCLB), for measuring the degree of hearing protector noise attenuation under actual use conditions was developed. The method was designed for use in industrial environments where the background noise levels usually precluded using the standard real attenuation at threshold (REAT) technique. The BCLB procedure used pulsed sounds covering the frequency range of 125 to 8,000 hertz (Hz) that were alternately delivered by a bone conduction vibrator onto the forehead of the test subject and from a loudspeaker located outside the hearing protector. The bone conduction sound level was maintained at a constant level. The sounds from the loudspeaker were varied in 0.33 octave levels until the test subject reported that the sounds from the bone conductor and loudspeaker appeared to be equally loud. The procedure was performed with the subject wearing and not wearing the hearing protector. The procedure was evaluated with eight volunteers, four males, 18 to 46 years old, wearing or not wearing EAR Classic slow recovery foam earplugs while seated in an acoustic chamber. Background noises in the chamber varied from 21 to 35 and 53 decibels-A (dBA). The degree of attenuation measured for the protector was compared with that measured by the standard REAT procedure. The authors conclude that the BCLB procedure appears to a valid, accurate technique for measuring the attenuation of earplugs in industrial environments from the highest to the lowest frequencies normally tested.
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