Determination of Noise Immission From Sound Sources Close to the Ears

When a sound exposure stems from a sound source that is close to the ear of the exposed person, the noise is described in terms of the free-field related or diffuse-field related sound pressure level, i.e. the level of a free or a diffuse-sound field that would result in the same exposure of the person's ear as that stemming from the close sound source. The at-ear sound exposure level is measured either by MIRE technique (microphones in real ears) or by a manikin, and the free-field related or diffuse-field related sound pressure levels are obtained by subtracting the free-field-front or the diffuse-field head-related transfer function (HRTF) expressed in dB. The use of either, the eardrum, the open entrance, or the blocked entrance as measurement point for the MIRE-technique is evaluated. The results are the same, wherever in the ear canal the measurements are made. There is good agreement between human HRTFs measured at different laboratories, and for eardrum, open-entrance and blocked entrance, standard HRTF data have been derived, which may be used instead of HRTFs measured for each subject. The resulting statistical uncertainty depends on the choice of measurement point, and whether individual or standard HRTF data are used. Generally, measurements at the blocked entrance are practical and produce results with low statistical uncertainty. The results from manikin-measurements do not agree well with results from humans (MIRE), when HRTFs from the actual manikin or from the manikin standards (IEC 60959 and ITU-T P.58) are used. A better agreement is obtained with HRTFs constructed by multiplying human blocked-entrance data with the transfer function of the standardized coupler for manikins. This method is therefore described (and data tabled) in ISO 11904-2. A comparison between humans, manikins, and the manikin standards suggest, that standards do not specify an average human and should be revised.