ISO/DTS 1999

FINAL DRAFT
Technical
Specification
ISO/TC 43
Acoustics — Estimation of noise-
Secretariat: DIN
induced hearing loss
Voting begins on:
Acoustique — Estimation de la perte auditive induite par le bruit 2025-04-30
Voting terminates on:
2025-06-25
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Reference number
FINAL DRAFT
Technical
Specification
ISO/TC 43
Acoustics — Estimation of noise-
Secretariat: DIN
induced hearing loss
Voting begins on:
Acoustique — Estimation de la perte auditive induite par le bruit
Voting terminates on:
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Principle . 3
5 Description and measurement of noise exposure . 4
6 Prediction of the effects of noise on hearing threshold . 4
6.1 Statistical distribution of hearing threshold levels of a noise-exposed population .4
6.2 Databases for hearing threshold levels associated with age (HTLA) .4
6.2.1 General .4
6.2.2 Database A .5
6.2.3 Database B.5
6.2.4 Choice of database.5
6.3 Calculation of effect of noise, N .5
7 Assessment of noise-induced hearing loss and impairment .10
7.1 Hearing loss due to noise .10
7.2 Hearing impairment .10
7.3 Risk of hearing impairment due to noise .10
Annex A (informative) Database A, selected values of the statistical distribution of hearing
threshold deviations as a function of age (HTLA) for an otologically normal population
(highly screened) .11
Annex B (informative) Examples for database B .13
Annex C (informative) Examples of calculations of effect of noise, N .23
Annex D (informative) Example of the calculation of risk of hearing impairment due to noise
exposure .25
Annex E (informative) Kurtosis-adjusted noise exposure level normalized to an 8 h working
day, L’ .29
p,A,8 h
Annex F (informative) Derivation of values of effect of noise, N.32
Bibliography .34

iii
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
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www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 43, Acoustics.
This fourth edition cancels and replaces the third edition (ISO 1999:2013), which has been technically
revised.
The main changes are as follows:
— The second and third editions of this standard were based on estimates of effect of noise, N, developed
using the empirical models of two databases (see References [2] and [22]). However, traceability
of the derivation of those estimates has been lost and the Working Group of Technical Committee
43, ISO/TC43/WG1 “Threshold of Hearing,” has been unable to assess its validity. The values of N in
this technical specification were derived from the hearing threshold levels calculated from the same
empirical models as the previous versions (see Annex F).
— In this edition, a recommendation for an adjustment is made to the noise exposure level to improve
prediction of hearing threshold levels for people exposed to noise with substantial impulse/impact
components (see Annex E).
— This revised version introduces a tabular representation of the effect of noise, N, with interpolation
between tabled values to replace the formulae presented by previous editions.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
This document presents, in statistical terms, the relationship between noise exposure, unprotected unless
otherwise stated, and its effect on hearing threshold levels in people of various ages. This document provides
procedures for estimating the hearing loss due to noise exposure in populations by comparison to various
non-noise exposed populations. For any given noise exposure, effect of noise, N, has a range of positive values
reflecting the variability of susceptibility to noise-induced hearing loss among individuals.
Persons regularly exposed to noise can develop hearing loss of varying severity. Due to hearing loss, their
understanding of speech, perception of everyday acoustic signals, or appreciation of music can be impaired.
With the exception of traumatic damage to the ear caused by exposure to blast, high-impulse noise and
extremely high levels of steady-state noise, permanent impairment of the hearing organ takes time and
is progressive over months, years, or decades of exposure. For a single individual, it is not possible to
determine precisely which changes in hearing threshold level are caused by noise and which changes are
caused by other factors. However, for a sufficiently large population exposed to a specific noise, changes in
the statistical distributions of hearing threshold levels can be determined. Predictions from this technical
specification can be used to describe differences in hearing threshold levels between two populations that
are similar in all relevant respects except that one population has had a well-defined (usually occupational)
noise exposure.
Researchers have for many years found that noise with substantial impulse/impact components is more
hazardous to hearing than noise at similar sound pressure levels without substantial impulse/impact
components. In this edition, an adjustment is made to the noise exposure level to improve prediction of
hearing threshold levels for people exposed to noise with substantial impulse/impact components. The
adjustment is based on the distribution of values in the sound pressure waveform using the statistical
measure kurtosis (see Annex E).
This document was derived from the hearing threshold levels calculated from the same empirical models as
the previous versions (References [2] and [22]; see Annex F). document uses the formula H’ = H + N – (H x N
/120) to calculate expected hearing threshold level (H’) from the combination of components due to noise
(N) and other factors, principally age (H). In order to derive the noise-induced component, this formula was
rearranged to calculate the values of N shown in Tables 1 and 2of this revision: N = (H’ – H)/(1- H/120),
where each value of N was calculated using the unweighted arithmetic mean, or average, of the two values of
H’ and the two values of H in the two databases at each frequency, noise level, duration and percentile value
shown in the table. Interpolation between values of N in Tables 1 and 2 enables calculation of values of N
not included in the table. The formulation was validated by comparison with a broader set of published data
on noise-induced hearing loss, as compiled in Reference [26]. A Hearing Threshold Level Calculator is also
provided with this revision so that hearing threshold levels can be readily determined.
This document can be applied to the calculation of the risk of hearing impairment due to regular occupational
noise exposure or due to any daily repeated noise exposure. In some countries, hearing loss caused by
occupational noise exposure can have legal consequences with respect to responsibility and compensation.
The hearing threshold level at the various frequencies, at which a hearing impairment is deemed to exist
(fence), depends not only on the hearing loss per se, but frequently on legal definitions and interpretations
based on social and economic considerations. In addition, the definition of a hearing impairment depends
on the quality of speech recognition desired, the average level of background noise and with respect to the
relative importance of the various frequencies, perhaps even on the language. Consequently, this document
does not stipulate (in contrast to the first edition of ISO 1999) a specific formula for assessment of the risk of
impairment, but specifies methods for the prediction of hearing threshold levels, which can be used for the
assessment of impairment according to the formula desired or stipulated in a specific country.
The selection of maximum tolerable or maximum permissible noise exposures and protection requirements,
as well as the selection of specific formulae for impairment risk assessment or compensation purposes,
require consideration of ethical, social, economic and political factors not amenable to international
standardization. Individual countries differ in their interpretation of these factors and these factors are
therefore considered outside the scope of this document.

v
For reasons given above, this document, by itself, does not comprise a complete guide for risk assessment
and protection requirements, and for practical use, it has to be complemented by national standards or
codes of practice delineating the factors which are here left open.

vi
FINAL DRAFT Technical Specification ISO/DTS 1999:2025(en)
Acoustics — Estimation of noise-induced hearing loss
1 Scope
This document specifies a method for calculating the expected hearing threshold levels of adult populations
due to various levels and durations of noise exposure; it provides the basis for calculating the risk of hearing
impairment according to various formulae when the hearing threshold levels at commonly measured
audiometric frequencies, or combinations of such frequencies, exceed some certain values.
This document is based on statistical data and therefore cannot be applied to the prediction of the hearing
loss of individual persons.
NOTE 1 This document does not specify frequencies, frequency combinations, nor weighted combinations used
for the evaluation of hearing impairment; nor does it specify a hearing threshold level (fence) which it is necessary
to exceed for hearing impairment to exist. Quantitative selection of these parameters is left to the user. All sound
pressure levels stated in this document do not consider the effect of hearing protectors which would reduce effective
exposure levels and modify the frequency spectrum at the ear. Because there is evidence indicating a transition
between metabolic exhaustion and mechanical damage in the inner ear, application of this technical specification to
exposures involving sound pressures exceeding 200 Pa (140 dB relative to 20 μPa) is not recommended.
The measure of exposure to noise for a population at risk is the equivalent continuous A-weighted sound
pressure level normalized to an 8 h working day, or A-weighted noise exposure level, L , for a number
p,,Ah8
of years of exposure. This document applies to noise at frequencies less than 10 kHz that is steady-state,
intermittent, fluctuating, irregular, or complex with A-weighted noise exposure levels between 75 and
3 3 2
100 dB (daily A-weighted sound exposures between 0,115 × 10 and 115 × 10 Pa s).
Formulae are presented to calculate the statistical distributions of hearing threshold levels at a range of
audiometric frequencies due to exposure to noise as a function of level of noise exposure and duration of
exposure (in years). Values of effect of noise, N, do not distinguish between male and female populations. The
database for values of N in Tables 1 and 2 was derived from people who typically started work around age 20
and whose noise exposures continued for many years; thus, this database is most accurate for populations of
workers whose age minus years of exposure is about 20.
NOTE 2 Although the models of hearing loss are based on data assumed to stem primarily from populations
exposed to occupational noise, they can be used, with some caution, for estimating the effects of comparable non-
occupational exposures.
NOTE 3 The prediction method for L presented is based primarily on data collected with essentially
p,,A8h
broadband, steady-state, non-tonal noise. A recommended prediction method for noise environments which include
substantial impulsive/impact noise uses a kurtosis-adjusted noise exposure level normalized to an 8 h working day
(see Annex E).
To calculate hearing threshold levels and also to calculate the risk of acquiring hearing impairment due to
noise exposure, it is necessary to make use of a database for age-associated hearing threshold levels for a
comparable population. This document includes a highly screened otologically normal population (according
to ISO 7029) and examples of relatively unscreened populations of five typical industrialized societies. The
users of this document may choose a comparable population according to their particular requirements.
NOTE 4 All data and procedures presented in this document are based on deliberate simplifications of experimental
data where the daily sound exposure duration did not exceed 12 h. The resulting approximations restrict the validity
to the stated ranges of the variables, percentiles, sound exposure levels and frequency ranges.

2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 9612, Acoustics — Determination of occupational noise exposure —Engineering method
ISO/TR 25417, Acoustics — Definitions of basic quantities and terms
ISO 80000-8, Quantities and units — Part 8: Acoustics
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/TR 25417 and ISO 80000-8 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
NOTE Within this document, all sound pressure levels are relative to 20 μPa. The reference value for hearing
threshold levels depends on audiometric frequency and is defined in ISO 389 series.
3.1
A-weighted noise exposure level normalized to an 8 h working day
L
p,,A8h
T 
e
LL=+10lg dB
pp,,Ah8 ,,AeqT  
e
T
 
where
L is the equivalent continuous A-weighted sound pressure level for T ;
pT,,Aeq e
e
T is the effective duration of the working day in hours; and
e
T is the reference duration (T = 8 h).
0 0
Note 1 to entry: The quantity “noise exposure level normalized to an 8 h working day” may also be called “daily noise
exposure level.”
Note 2 to entry: Noise exposure level is usually calculated on a daily basis, but there may be circumstances where
a weekly noise exposure period is considered to be more appropriate. In such circumstances, noise exposure is
normalized to an 8 h working day. A working day depends on work patterns and may be a synthesized working day,
including fragments of different real days of a week. The objective is for the working day to be representative of the
overall work cycle. The same principle may be applied to the recommended kurtosis-adjusted noise exposure level
presented in Annex E.
Note 3 to entry: If noise exposure representing a work pattern of more than one day, normalized over an 8 h working
day, is desired, the following formula may be used:
1 X 01, ⋅L
 
p,,Ah8 ,i
L =10lg 10 dB
p,A,8h ∑
 
i=1
X 
where L is the daily noise exposure level for day i and X is the total number of days.
p,,Ah8 ,i
For example, X = 5 leads to a daily noise exposure level normalized to a week of five 8 h working days. X should not
normally be greater than 5.
Note 4 to entry: In some cases, L specified by ISO 9612 may be used instead of L .
EX,8 h p,A,8h
3.2
hearing loss
increase in the threshold of hearing in decibels from normal
3.3
hearing impairment
hearing loss (3.2) sufficient to adversely affect the activities of daily living
[31]
Note 1 to entry: This is sometimes called “activity limitation ”.
3.5
fence
F
hearing threshold level in decibels for a specified combination of audiometric frequencies above which
hearing impairment (3.3) is deemed to exist
3.6
risk of hearing impairment due to noise
percentage of a noise-exposed population with a hearing impairment (3.3) minus the percentage of a
population not exposed to noise with a hearing impairment (3.3) but otherwise equivalent to the noise-
exposed population
3.7
hearing threshold level associated with age
HTLA
H
for a specified fraction of a population, the hearing threshold level in decibels observed as a function of age
and sex without any exposure to occupational noise
3.8
effect of noise
N
for a specified fraction of a population, the effect in decibels on hearing threshold level estimated to be
caused solely by exposure to noise, in the absence of other causes
Note 1 to entry: In ISO 1999:1990 and in ISO 1999:2013, the terms N and NIPTS (noise-induced permanent threshold
shift) were defined to be the same thing. Many studies have calculated NIPTS as H’ – H, but this is not the same as N.
From Formula (1), H’ – H = N – (H x N /120). To avoid confusion, NIPTS has not been used in this revision. See NOTE in
Annex F.
3.9
hearing threshold level associated with age and noise
HTLAN
H'
for a specified fraction of a population, the permanent hearing threshold level in decibels for a given age, sex
and noise exposure
Note 1 to entry: Hearing threshold levels (HTL), as defined in ISO 8253-2, are expressed in decibels.
Note 2 to entry: The value HTLAN is a combination of the components associated with age (HTLA) and noise (N), as
defined in 6.1.
4 Principle
This document provides a method for calculation of the statistical distribution of hearing threshold levels
of a noise-exposed population using values of effect of noise, N, and hearing threshold levels associated with
age, H. The values of H and N are combined to yield hearing threshold levels expected in the noise-exposed
population using the formula given in 6.1.

The effect of noise depends on the A-weighted noise exposure level normalized to an 8 h working day and
the number of years of exposure. It is calculated separately for each audiometric frequency and according to
the required percentile of the population using methods described in 6.3.
The A-weighted noise exposure level may be adjusted according to the kurtosis of the noise, measured using
the method described in Annex E. This reflects the finding that noise with substantial impact or impulse
components tends to have a greater adverse effect on hearing than noise having the same sound pressure
level without those components. This adjustment has not been included in previous versions of the standard.
Hearing threshold levels associated with age, H, may be obtained using databases tabulated in Annex A
or Annex B. Annex A gives the procedure for calculating the statistical distribution of hearing threshold
levels as a function of age for highly screened otologically normal males and females in accordance with
ISO 7029. Annex B gives five examples representing the statistical distributions of hearing threshold levels
as a function of age for unscreened males and females of five typical industrialized societies.
This document is accompanied by a calculator that allows computation of expected hearing threshold levels
at standard audiometric frequencies for a given percentile of the statistical distribution. Calculations are
performed for a given age, sex, noise exposure level and exposure duration.
5 Description and measurement of noise exposure
Methods for the determination of occupational noise exposure are specified in ISO 9612.
6 Prediction of the effects of noise on hearing threshold
6.1 Statistical distribution of hearing threshold levels of a noise-exposed population
The hearing threshold level associated with age and noise (HTLAN), in decibels, H', of a noise-exposed
population is calculated, for the purposes of this document, by using Formula (1):
H’ = H + N – (H × N/120) (1)
where
H is the hearing threshold level associated with age (HTLA) expressed in decibels;
N is the predicted effect of noise expressed in decibels.
This formula is applicable only to corresponding (same percentile) values of H', H and N.
NOTE The above relationship is an approximation to the biological processes occurring in the ear.
Values of H’ may be found using the Annex A or Annex B tables discussed in 6.2 to find values of H. Values of N
may be derived from Tables 1 and 2, as discussed in 6.3. Alternatively, the hearing threshold level calculator
may be used.
6.2 Databases for hearing threshold levels associated with age (HTLA)
6.2.1 General
The hearing of a non-noise-exposed population as a function of age depends on the degree to which other
factors are inadvertently included: diseases, history of exposure to ototoxic substances and unknown
noise exposure of occupational or non-occupational origin may modify the HTLA. Different approaches
to screening such data have been used and the selection of the most appropriate database depends on the
purpose of the application (see 6.2.4). This document includes two databases (databases A and B) to be

used for HTLA in 6.1. Database A is fully specified, whereas database B is at the discretion of the user. Five
examples of database B are presented.
NOTE 1 A decrease in hearing ability is not necessarily caused by ageing itself, but by many injurious influences
during a lifetime, which are not known in detail.
NOTE 2 The databases presented in Annexes A and B are from populations of European, North American and Asian
countries. These populations are not necessarily representative for the populations of other geographical areas.
Even if there are no differences between different ethnic populations, differences in lifestyle, non-occupational noise
exposure, incidence of disease and exposure to ototoxic substances are nevertheless liable to occur.
6.2.2 Database A
Database A derives from otologically normal persons: in a normal state of health who are free from all signs
or symptoms of ear disease, with no obstructing wax in the ear canals, with no history of undue exposure to
noise, with no exposure to potentially ototoxic substances and with no familial hearing loss. The statistical
distributions of the threshold deviations of such highly screened populations have been standardized
in ISO 7029 separately for male and female populations. Selected values of the statistical distribution of
hearing threshold levels (in decibels) using database A can be obtained from Annex A.
6.2.3 Database B
For database B, a set of data usually collected on a control population not occupationally exposed to noise
of the country under consideration, but exposed to other risk factors to a degree similar to occupationally
exposed populations, is recommended. When such an ideal control population is unavailable and in countries
where risk factors such as non-occupational noise are more prevalent in occupationally exposed populations
than in the general population, a totally unscreened control population can be preferable.
A separate HTLA database for both men and women is required unless it can be shown that there are no
substantial differences. It is essential that the sample size be large enough to allow calculations of a valid
statistical distribution.
Therefore, the user should apply appropriate selection criteria to compile a database of hearing threshold
levels to compare to the levels given in Annex B of this document. For instance, the HTLA for data based on
the average of left and right ears, the better or more sensitive of left and right ears, using both left and right
ears or the ear showing the greatest hearing loss, can be taken as the basis for database B.
It is emphasized that for practical situations the accuracy of the prediction of the hearing threshold level of a
noise-exposed population will largely be a function of the accuracy of the selected database for HTLA. Since
audiometric measurement techniques affect threshold measurements, the same measurement technique
should be used in the establishment of a specific HTLA database as might be used to obtain or verify the
threshold of hearing of the noise-exposed population.
6.2.4 Choice of database
The choice of database from Annex A or Annex B should aim to match the database to the target population
of noise-exposed people. Whether database A or B is the more appropriate (or whether the numerical
examples for database B in Annex B are suitable) depends on what question is to be answered. When the
occupationally exposed population is not highly screened, unscreened populations will form the more
appropriate databases.
6.3 Calculation of effect of noise, N
Effect of noise, N, to be used in 6.1 is a function of the A-weighted noise exposure level, exposure duration,
percentage of the population and audiometric frequency. Values of N are found for values of L, D and Q:

is the A-weighted noise exposure level, L , normalized to an 8-hour working day (see
L
p,A,8h
3.2), expressed in decibels;
D is the exposure duration, expressed in years (typically 8 h/day, 5 days/week);
N
is the value of N, expressed in decibels, for L, D and Q; and
LD,,Q
is the percentage of the population exceeding N
Q
LD,,Q
Tables 1 and 2 include values of N for L = 75, 80, 85, 90, 95 and 100 dB; D = 10, 20, 30 and 40 years; Q =
L,D,Q
5, 10, 25, 50, 75, 90 and 95 %; and audiometric frequencies 0,5, 1, 2, 3, 4 and 6 kHz. Tails of the statistical
distributions for 0 % < Q < 5 % and for 95 % < Q < 100 % are unreliable and should not be estimated since
few experimental data exist to validate these ranges. If L, D and Q are given in Tables 1 and 2, then N can
L,D,Q
be read from Tables 1 and 2.
If necessary, additional values of N may be calculated using Formula (2) and by linear interpolation using
Formulae (3), (4) and (5), where L and L are exposure levels ()LL< from Tables 1 and 2 below and
1 2 12
above the required value of L, D and D are durations ()DD< from Tables 1 and 2 below and above the
1 2 12
required value of D, and Q and Q are percentiles ()QQ> from Tables 1 and 2 above and below the
1 2 12
required value of Q(percentiles go from high to low):
If D<10, then N is tapered towards zero using N in Table 1, and
LD,,Q LQ,,10
ND=+lg()11/lg()1 ×N (2)
LD,,QL,,10Q
If L is between L and L , N is calculated using N . and N in Tables 1 and 2, and
1 2 LD,,Q LD,Q LD,Q
1, 2,
NN=+ LL− / LL− ×−NN (3)
() ()
()
LD,,QL ,,DQ 12 1 LD,,QL ,,DQ
12 1
If D is between D and D , then N is calculated using N and N in Tables 1 and 2, and
1 2 LD,,Q LD,,Q LD,,Q
1 2
NN=+ DD− / DD− ×−NN (4)
() ()
()
LD,,QL,,DQ 12 1 LD,,QL,,DQ
12 1
If Q is between Q and Q , then Nis calculated using N and N in Tables 1 and 2, and
1 2 LD,,Q LD,Q LD,Q
, 1 , 2
NN=+ QQ− / QQ− ×−NN (5)
() ()
()
LD,,QL,,DQ 11 2 LD,,QL,,DQ
12 1
If D≥10 and interpolation is required on L, D or Q, repeated application of Formulae (3), (4) or (5) can be
required. For example, if interpolation is required on L and D and Q, the following procedure may be used.
First, apply Formula (3) four times to interpolate on L between L and L using values of N at DQ , DQ ,
1 2 11 21
DQ and DQ in Tables 1 and 2. That will yield four intermediate interpolated values, N , N ,
12 22 LD,,Q LD,,Q
11 21
NN, . Second, apply Formula (4) twice to interpolate on D between N and N and
LD,,QL,,DQ LD,,Q LD,,Q
12 22 11 21
between N and N to yield two further intermediate interpolated values, N and N .
LD,,Q LD,,Q LD,,Q LD,,Q
12 22 1 2
Finally, apply Formula (5) to interpolate on Q to yield the required value of N .
LD,,Q
If D<10 and interpolation is required on L or Q, repeated application of Formula (3) or (5) can be required.
For example, if interpolation is required on L and Q, the following procedure may be used. First, using D=10,
apply Formula (3) twice to interpolate on L between L and L using values of N at Q and Q in Table 1.
1 2 1 2
That will yield two intermediate interpolated values, N and N . Second, apply Formula (5) to
LQ,,10 LQ,,10
1 2
interpolate on Q to yield N . Finally, apply Formula (2) to yield the required value of N .
LQ,,10 LD,,Q
For the purposes of this document, hearing threshold levels associated with age shall be obtained from the
values tabulated in Annex A or Annex B. For combinations of age and percentile not listed in the tables,
linear interpolation between adjacent values shall be used.

The hearing threshold level calculator provided with this document (https:// standards .iso .org/ iso/ ts/ 1999/
ed -4/ en) uses the above procedures. Example calculations of N are shown in Annex C.
LD,,Q
NOTE 1 Calculation of N by linear interpolations between values in Tables 1 and 2 does not depend on the order
L,D,Q
of the interpolations made.
NOTE 2 Exposure durations of more than 40 years represent an extrapolation from the source data that has not
been validated.
NOTE 3 Near the tails of the statistical distributions and for longer durations, the reliability of the estimates of N is
reduced.
NOTE 4 Formula (3) and Formula (4) have the same structure, because N increases with increasing L or increasing D.
Formula (5) has a slightly different structure because N decreases with increasing Q.
Table 1 — Values of effect of noise, N , by A-weighted noise exposure level (L), 75 dB, 80 dB,
LD,,Q
85 dB, 90 dB, 95 dB and 100 dB, exposure duration (D), 10 and 20 years and percentage of the
population (Q %) exceeding N
LD,,Q
Level L Duration D
10 20
dB years
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 0 0 0 0 0 0 0 0 0 0 0
90 % 0 0 0 0 0 0 0 0 0 0 0 0
75 % 0 0 0 0 0 0 0 0 0 0 1 0
75 50 % 0 0 0 1 1 0 0 0 0 1 1 1
25 % 0 0 0 1 1 1 0 0 1 1 2 1
10 % 0 0 1 2 2 1 0 0 1 2 3 2
5 % 0 0 1 2 3 2 0 1 2 3 4 3
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 0 0 0 0 0 0 0 0 2 0 2
90 % 0 0 0 0 0 0 0 0 0 2 1 2
75 % 0 0 0 1 1 1 0 0 0 3 2 2
80 50 % 0 0 0 3 4 2 0 0 1 3 5 3
25 % 0 1 1 4 7 4 0 1 1 4 8 4
10 % 1 1 2 6 11 6 1 1 2 6 11 6
5 % 1 1 3 7 13 7 1 1 3 8 14 7
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 0 0 1 0 0 0 0 1 3 3 2
90 % 0 0 0 2 2 1 0 0 1 4 4 3
75 % 0 0 0 3 4 3 0 0 1 5 6 4
85 50 % 0 0 1 5 7 5 0 1 2 6 9 6
25 % 1 1 3 7 11 8 1 1 4 8 13 9
10 % 2 2 6 10 16 11 2 3 6 11 17 12
5 % 2 3 8 12 19 14 2 3 8 13 21 15
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
TTabablele 1 1 ((ccoonnttiinnueuedd))
Level L Duration D
10 20
dB years
95 % 0 0 0 3 5 0 0 0 1 5 7 3
90 % 0 0 0 4 6 2 0 0 2 7 8 4
75 % 0 0 1 6 9 5 0 1 3 9 11 7
90 50 % 0 1 3 10 13 9 1 1 4 12 15 11
25 % 1 2 6 15 18 14 2 2 8 17 20 16
10 % 3 3 9 20 23 19 3 4 11 23 26 21
5 % 4 5 12 24 27 22 4 5 14 27 30 25
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 1 0 6 11 2 0 2 3 10 14 5
90 % 0 1 0 8 13 4 0 2 4 11 16 7
75 % 0 2 3 12 17 9 1 3 6 15 20 11
95 50 % 1 3 6 17 21 14 1 4 9 21 25 17
25 % 2 4 11 24 27 21 3 6 15 28 31 24
10 % 4 7 16 31 33 28 5 8 20 35 37 31
5 % 5 8 20 36 37 32 6 10 24 40 41 36
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 1 2 0 13 19 6 2 4 4 17 23 10
90 % 2 3 1 15 21 9 3 5 6 20 25 13
75 % 3 4 5 20 26 14 4 6 10 25 30 19
100 50 % 4 6 9 26 31 21 5 8 15 32 36 26
25 % 6 9 16 34 38 29 8 11 23 40 42 35
10 % 8 12 24 42 44 37 10 15 31 48 48 43
5 % 10 14 29 47 48 42 12 18 36 52 51 48
Table 2 — Values of effect of noise, N , by A-weighted noise exposure level (L), 75 dB, 80 dB,
LD,,Q
85 dB, 90 dB, 95 dB and 100 dB, exposure duration (D), 30 and 40 years, and percentage of the
population (Q %) exceeding N
LD,,Q
Level L Duration D
10 20
dB years
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 0 0 0 0 0 0 0 0 0 0 0
90 % 0 0 0 0 0 0 0 0 0 0 1 0
75 % 0 0 0 1 1 1 0 0 0 1 1 1
75 50 % 0 0 1 1 1 1 0 0 1 1 2 1
25 % 0 0 1 2 3 2 0 0 1 2 3 2
10 % 0 1 1 3 4 3 0 1 2 4 5 4
5 % 1 1 2 4 6 4 1 1 3 5 7 6
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
TTabablele 2 2 ((ccoonnttiinnueuedd))
Level L Duration D
10 20
dB years
95 % 0 0 1 4 2 4 1 1 2 6 4 6
90 % 0 0 1 4 2 4 1 1 2 6 4 6
75 % 0 0 1 4 4 4 1 1 2 5 5 5
80 50 % 0 0 1 4 6 4 0 0 1 5 7 5
25 % 0 0 1 5 8 4 0 0 1 5 9 5
10 % 0 1 2 6 12 6 0 1 1 6 13 6
5 % 0 1 2 8 15 7 0 1 1 7 16 7
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 1 2 6 5 4 1 1 3 8 7 7
90 % 0 1 2 6 6 5 1 1 3 8 8 7
75 % 0 1 2 6 7 6 1 1 3 8 9 8
85 50 % 0 1 2 8 10 8 1 1 3 9 12 9
25 % 1 2 4 10 14 10 1 2 4 11 16 12
10 % 1 3 7 12 19 14 1 3 7 13 21 15
5 % 2 3 9 14 23 16 2 4 8 16 25 18
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 1 1 3 8 10 5 1 1 5 11 13 7
90 % 1 1 4 9 11 6 1 1 6 12 14 9
75 % 1 1 5 11 14 9 1 1 6 14 16 11
90 50 % 1 1 6 15 18 13 1 2 8 17 20 15
25 % 2 3 9 20 23 18 2 3 11 23 26 21
10 % 3 4 13 25 29 24 3 5 15 28 33 27
5 % 4 6 16 29 33 28 4 6 18 33 37 32
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 0 3 6 13 18 7 1 4 10 17 21 10
90 % 1 3 7 15 19 9 1 4 11 19 23 13
75 % 1 4 10 19 23 14 1 5 13 23 27 17
95 50 % 2 5 13 24 28 20 2 6 17 28 33 24
25 % 3 7 19 32 35 28 4 8 23 36 39 32
10 % 5 10 25 40 41 35 6 11 29 45 46 41
5 % 7 12 29 45 45 40 7 13 33 50 50 46
Frequency
0,5 1 2 3 4 6 0,5 1 2 3 4 6
kHz
95 % 3 5 10 22 27 15 4 8 15 27 31 20
90 % 4 6 12 25 29 18 5 8 17 30 34 23
75 % 5 8 16 30 34 24 6 10 22 35 39 30
100 50 % 6 10 21 37 40 32 8 12 28 42 45 38
25 % 9 14 29 45 46 41 10 16 36 51 52 48
10 % 12 18 37 53 52 49 13 20 44 60 58 58
5 % 14 21 42 58 56 55 16 23 49 65 62 64

7 Assessment of noise-induced hearing loss and impairment
7.1 Hearing loss due to noise
Potential hearing loss due to occupational exposure to noise is assessed by calculating hearing threshold
level, H’, in accordance with 6.1 for the exposure conditions and populations under consideration from
which a hearing threshold level associated with age, H, is subtracted to give the noise-induced hearing loss.
This loss can be
a) considered separately for each frequency of interest,
b) added for a certain number of frequencies to result in a “total” hearing loss, and
c) averaged for a number of selected frequencies usually representing the main speech intelligibility
frequency range (see 7.2).
7.2 Hearing impairment
For the calculation of hearing impairment, a combination of hearing threshold levels at specified frequencies
shall be used. The hearing threshold levels for the populations shall be calculated in accordance with
Clause 6.
Users should consult national guidelines or criteria to determine how to assess hearing impairment.
7.3 Risk of hearing impairment due to noise
The risk of hearing impairment due to noise exposure is a frequently used measure of the harmful effects
of noise exposure on a population. A fence may be selected, consisting of a specified hearing threshold
level for a combination of audiometric frequencies, above which a hearing impairment is deemed to exist.
The percentage of the population having hearing threshold levels equal to or exceeding this fence is the
percentage of the population considered to have a hearing impairment. The risk of hearing impairment
due to noise is calculated as the percentage of a noise-exposed population exceeding the fence minus the
percentage of a population not exposed to noise exceeding the fence but otherwise equivalent to the noise-
exposed population. An example of the calculation of risk of hearing impairment due to occupational noise
exposure is shown in Annex D.
The risk of hearing impairment due to noise should not be regarded as an unambiguous descriptor. It varies
with choice of frequency combination, choice of fence and choice of HTLA. When quoting the risk of hearing
impairment due to noise, details of these parameters should also be provided.
NOTE 1 The risk of hearing impairment due to noise gives the
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