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Sound in air

Working group 1.61

Determination of reference data for human ear characteristics

Testperson during a subjective audiometry

Audiometry serves the metrological determination of human audition. Pure-tone and speech audiometry are amongst the basic procedures. These procedures therefore use, on the one hand, pure sinusoidal tones to test the audition near the threshold of hearing and, on the other hand, listed numerals, monosyllabic words, rimes or sentences for the measurement of the intelligibility of speech. Therefore, the patient is brought in a still room and tones or speech are played e.g. via a headset. The patient must indicate whether he has heard the tones or how much of the speech he could understand. The pure-tone threshold of hearing or the intelligibility limit of speech is as-sessed by varying the sound pressure level.

For the assessment of the measured audiograms, the otologist needs reference data, i.e. mean hearing threshold curves or mean curves of speech intelligibility that enable a statement as to whether or how much the audiograms of a patient deviate from those of an average, normally hearing person. For each sound transducer (headset or bone-conduction receiver), these data must be compared. Speech audiometers additionally require the mean frequency response of the sound transducer used on the natural ear.

These characteristics are determined using groups of normally hearing adults at an age between 18 and 25 years (so-called otologically normal persons). The selection of the test persons is carried out according to standardised criteria. Groups of 25 or more persons are examined and the individual characteristics are then averaged.

The next step consists in measuring the characteristics gathered on the otologically normal test group on "artificial ears" or "artificial mastoids" - these are technical replica of the external ear or of the bone behind the auricle. It is only after this operation that sound pressure levels valid for all kinds of sound transducers of a similar type are available.

Worldwide comparison measurements have shown that the average auditive capacity of normally hearing humans from all five continents is the same within the scope of this measurement accu-racy. Thus, this enables an internationally unified determination of the characteristics which is being carried out by the ISO standardisation group (TC43/WG1).

Scheme for the determination of the equivalent reference sound pressure level (RETSPL).
(1.) Determination of the stimulus amplitudes at the hearing threshold for otologically normal test persons.
(2.) Measurement of the sound pressure level on an artificial ear.
Experimental set-up for the determination of the sound pressure level. The sound transducer is therefore placed on the artificial ear.
Scheme for the determination of the equivalent reference sound pressure level (RETSPL).

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