ISO 28961:2012

INTERNATIONAL ISO
STANDARD 28961
First edition
2012-01-15
Acoustics — Statistical distribution of
hearing thresholds of otologically normal
persons in the age range from 18 years
to 25 years under free-field listening
conditions
Acoustique — Répartition statistique des niveaux liminaires d’audition
de personnes otologiquement normales âgées de 18 à 25 ans dans des
conditions d’écoute en champ libre
Reference number
©
ISO 2012
ISO 28961:2012(E)
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ii    © ISO 2012 – All rights reserved

ISO 28961:2012(E)
Contents  Page
Foreword .iv
Introduction .v
1  Scope .1
2  Normative references .1
3  Calculation of percentiles of hearing threshold distribution .2
3.1  General .2
3.2  Threshold distribution at frequencies lower than 10 000 Hz .2
3.3  Threshold distribution at 10 000 Hz and higher frequencies .4
Annex A (informative) Numerical examples to illustrate the procedure .5
Annex B (informative) Selected values of the statistical distribution of hearing thresholds of
otologically normal persons in the age range 18 years to 25 years inclusive .6
Annex C (informative) Selected values of �� .9
�
Annex D (informative) Notes on the derivation of the statistical distribution of normal hearing
thresholds.10
Bibliography .11
ISO 28961:2012(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 28961 was prepared by Technical Committee ISO/TC 43, Acoustics.
iv    © ISO 2012 – All rights reserved

ISO 28961:2012(E)
Introduction
The threshold of hearing in a free sound field is specified in ISO 226 and ISO 389-7. Threshold data in these
documents were obtained from otologically normal persons in the age range 18 years to 25 years inclusive.
As described in ISO 389-7, the threshold of hearing varies among people; the documents merely present
median values of hearing thresholds. However, other values on the threshold distribution of individuals are
necessary to evaluate the hearing ability of a person in relation to that of the population. Those values have
been used also in noise evaluation for estimating the ratio of young people with normal hearing in the population
who might be able to detect a sound of concern, e.g. an unwanted sound emitted from a machine.
This International Standard provides a method for calculating percentiles of the hearing threshold distribution
for one-third-octave-band and other audiometric frequencies from 20 Hz to 16 000 Hz. The mean value of
distribution is set to be the threshold of hearing specified in ISO 226 and ISO 389-7. Furthermore, the method
has been developed using many of the hearing threshold data on which those documents were based.
NOTE Percentiles of the hearing threshold distribution can also be determined for bands of noise. However, only
percentiles for pure tones are specified in this International Standard because insufficient data for bands of noise are
available. Nevertheless, it is possible that this International Standard is applicable to one-third-octave bands of noise.
INTERNATIONAL STANDARD  ISO 28961:2012(E)
Acoustics — Statistical distribution of hearing thresholds of
otologically normal persons in the age range from 18 years to
25 years under free-field listening conditions
1  Scope
This International Standard provides descriptive statistics, percentiles, of the hearing threshold distribution
whose mean is the reference threshold value specified in ISO 226 and ISO 389-7. The mean and percentile
thresholds are specified under the following conditions:
a) the sound field in the absence of the listener consists of a free progressive plane wave (free field);
b) the sound source is directly in front of the listener (frontal incidence);
c) the sound signals are pure (sinusoidal) tones;
d) 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;
e) listening is binaural;
f) the listeners are otologically normal persons within the age range 18 years to 25 years inclusive.
NOTE 1 The �th percentile threshold is the value of threshold below which �% �of the individual thresholds of population
fall. The threshold distributions in this International Standard have been derived from the results of comprehensive
statistical analyses. See Annex D.
NOTE 2 The applicability of the percentiles and the values of parameters given in this International Standard to diffuse-
field listening conditions has not been examined. They are expected to be applicable to the conditions for the frequencies
from 20 Hz to 250 Hz where the threshold difference between free-field and diffuse-field listening conditions is zero as
specified in ISO 389-7:2005, Table 1.
The percentiles 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 some intermediate audiometric frequencies.
The percentiles are applicable to the assessment of an individual’s hearing in relation to the distribution of
hearing thresholds under the above conditions. The percentiles can also be used to evaluate the audibility of
low-level noise around hearing threshold.
NOTE 3 An application example of hearing threshold distribution to noise evaluation can be found in ISO 7779:2010,
Annex D.
2  Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 226:2003, Acoustics — Normal equal-loudness-level contours
ISO 266, Acoustics — Preferred frequencies
ISO 389-7:2005, Acoustics — Reference zero for the calibration of audiometric equipment — Part 7: Reference
threshold of hearing under free-field and diffuse-field listening conditions
ISO 28961:2012(E)
3  Calculation of percentiles of hearing threshold distribution
3.1  General
The threshold distribution for the frequencies � below 10 000 Hz shall be approximated by a Gaussian distribution,
which is determined using a mean value � and a standard deviation �. Using ��and �, any percentile on the
� � � �
distribution is calculable as specified in 3.2.
Percentiles at 10 000 Hz and higher frequencies shall be calculated using a similar procedure, as specified
in 3.3. However, a numerical transformation shall be performed first so that the threshold distribution can be
approximated using a Gaussian distribution in the transformed domain.
For both frequency ranges, the mean value of the distribution shall be set to the reference threshold in free field
� in decibels given in ISO 226 and ISO 389-7.
�
Examples of calculation are shown in Annex A. Selected values of the statistical threshold distribution which
are calculated following those procedures are presented in tabular and graphical forms in Annex B.
3.2  Threshold distribution at frequencies lower than 10 000 Hz
The �th percentile P �of threshold distribution (reference value 20 �Pa) in decibels at frequencies lower than
�,�
10 000 Hz shall be calculated using Equation (1):
P ����+ �� (1)
��������
where � is a �-score of the proportion of Gaussian distribution which corresponds to the �th percentile.
�
Selected values of � are presented in Annex C. The values of standard deviation � for the threshold distribution
� �
at frequencies lower than 10 000 Hz are presented in Table 1. The values are the outcome of comprehensive
statistical analyses. The accuracy provided is for calculation only.
NOTE As � approaches 0 or 100, P becomes liable to a larger uncertainty of threshold estimation because of
�,�
uncertainty in the experimental data on which this International Standard is based.
2    © ISO 2012 – All rights reserved

ISO 28961:2012(E)
Table 1 — Standard deviation of threshold distribution, �, at frequencies � lower than 10 000 Hz
�
� �
�
in Hz in dB
20 6,14
25 6,38
31,5 6,48
40 6,37
50 6,04
63 5,40
80 4,58
100 4,03
125 3,79
160 3,78
200 3,84
250 3,84
315 3,75
400 3,61
500 3,54
630 3,63
750 3,81
800 3,91
1 000 4,29
1 250 4,69
1 500 4,94
1 600 5,00
2 000 5,09
2 500 5,02
3 000 4,91
3 150 4,89
4 000 4,86
5 000 5,03
6 000 5,30
6 300 5,38
8 000 5,75
9 000 5,95
ISO 28961:2012(E)
3.3  Threshold distribution at 10 000 Hz and higher frequencies
The threshold distribution at 10 000 Hz and higher frequencies shall be approximated using a Gaussian
distribution after individual thresholds are operated mathematically using the following power-transformation
equation:
β
f

uu=−α (2)
()
f
where
� is an individual threshold in the distribution in decibels (reference value 20 �Pa);
� is the parameter that determines the origin of power transformation;
�
� is the exponent of power transformation.
�
Parameters � and � are
...