BS EN ISO 389-7:1998 pdf free download.Acoustics Reference zero for the calibration of audiometric equipment. Part 7: Reference threshold of hearing under free-field and diffuse-field listening conditions.
Each part of ISO 389 specifies a specific reference zero for the calibration of audiometric equipment. ISO 389:1991 (to be re-issued as ISO 389-1) and
ISO 389-2 are applicable to audiometric equipment for the transmission of pure tones by means of
supra-aural or insert earphones, respectively. Corresponding audiometric test methods are specified in ISO 8253-1.
In some audiological applications it may, however, be desirable to deliver the test signals by means of loudspeakers, either in a free sound field or in a
diffuse sound field. Corresponding audiometric test methods are specified in ISO 8253-2. This part of ISO 389 specifies the reference zero for the
calibration of audiometric equipment used for sound field audiometry.
In common with other subjective phenomena, the threshold of hearing varies in detail from person to person but, for a group of otologically normal
persons within a restricted age range, values for the central tendency can be determined to characterize the group. This and other parts of ISO 389 specify threshold data applicable to otologically normal
persons in the age range from 18 to 25 years. Compared with ISO 226, a more stringent age criterion has been applied in order to define a population as homogeneous as practicable with respect to the relationship between the threshold of hearing and age.
The data specified in this part of ISO 389 relate to a) pure tones heard under conditions of binaural listening in free progressive plane waves with the subject directly facing the source of sound (frontal incidence), and with the sound pressure level
measured in the free progressive wave at the centre position of the listener’s head with the listener absent;
b) one-third-octave bands of (white or pink) noise heard under conditions of binaural listening in a diffuse sound field with the sound pressure level measured in the sound field at the centre position of the listener’s head with the listener absent.
For frequencies up to 8 kHz, each set of data may be equally applied to any other bands of (white or pink) noise for which the bandwidth is less than the
critical bandwidth.
The data are based on an assessment of technical information provided by laboratories in different countries representing the most reliable data
available at the time. For information, a note on the derivation of the reference values and the origin of the data is given in Annex A and a bibliography is given in Annex B.
This part of ISO 389 specifies a reference threshold of hearing for the calibration of audiometric equipment used under the following conditions.
a) The sound field in the absence of the listener consists of either a free progressive plane wave (free field) or a diffuse sound field. In the case of a free progressive plane wave, the source of sound is directly in front of the listener (frontal incidence).
NOTE 1 Correction values for the threshold of hearing under free-field listening conditions and selected angles of sound incidence deviating from frontal incidence are given in ISO 8253-2 for information.
b) The sound signals are pure (sinusoidal) tones in the case of free-field conditions, and one-third-octave band of (white or pink) noise in the case of diffuse-field conditions.
c) The sound pressure level is measured in the absence of the listener at the position where the centre of the listener’s head would be.
d) Listening is binaural.
e) The sound pressure levels corresponding to the reference threshold of hearing are determined by the median value of the thresholds of an adequately large group of listeners.
1) The listeners are otologically normal persons in the age range from 18 to 25 years inclusive.
NOTE 2 The data given in this part of ISO 389 have been derived from listeners who have hearing threshold levels of 10 dB or less according to ISO 389:1991. This selection criterion is not identical with that used when data for ISO 389:1991 were derived.
g) The threshold of hearing is determined by means of the bracketing or ascending method, as specified in ISO 8253-1.
The data are given in numerical form for the preferred frequencies in the one-third-octave series from 20 Hz to 16 000 Hz inclusive, in accordance with ISO 266 and for intermediate audiometric frequencies.
Figure 1 gives a graphical presentation. It should be emphasized that the threshold data differ from the audiometric zero specified in ISO 389:1991 and ISO 389-2, since the latter refer to monaural listening through earphones with sound pressure levels referred to specified couplers and ear simulators. Direct comparison between the data in ISO 389:1991 or ISO 389-2, respectively, and in this part of ISO 389 is therefore not appropriate.
2 Normative references
The following standards contain provisions which. through reference in this text, constitute provisions of this part of ISO 389. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of ISO 389 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of lEG and ISO maintain registers of currently valid International Standards.
ISO 266:—, Acoustics — Preferred frequencies’. ISO 8253-1:1989, Acoustics — Audiornetric test methods — Part 1: Basic pure tone air and bone conduction threshold audiom etry.
3 Definitions
For the purposes of this part of ISO :389, the following definitions apply.
3.1
threshold of hearing
level of a sound at which, under specified conditions, a person gives 50 % of correct detection responses on repeated trials
3.2
otologically normal person
person in a normal state of health who is free from all signs or symptoms of ear disease and from obstructing wax in the ear canals, and who has no history of undue exposure to noise, exposure to potentially ototoxic drugs, or familial hearing loss
3.3
reference threshold of hearing
at a specified frequency, the sound pressure level of a pure tone or a one-third-octave band of noise
corresponding to the median value of the binaural thresholds of hearing of otologically normal persons within the age limits from 18 to 25 years inclusive
3.4
diffuse sound field
sound field consisting of sound waves arriving at a given location more or less simultaneously from all directions with equal probability and level
4 Specification
The reference thresholds of hearing for the listening conditions specified in clause 1 are given in Table 1. This table also gives the differences between sound pressure levels of one-third-octave bands of noise in a diffuse sound field and the sound pressure levels of pure tones in a frontally incident free progressive wave for equal thresholds of hearing. A graphical
illustration of the reference thresholds of hearing is given in Figure 1.