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Ref. No.: ISO/R 226-1961 (E)
IS0
I N T E RN AT I O N A L ORGAN I2 AT I ON FOR STA N DARD I Z AT1 O N
IS0 RECOMMENDATION
R 226
NORMAL EQUAL-LOUDNESS CONTOURS FOR PURE TONES
AND NORMAL THRESHOLD OF HEARING
UNDER FREE FIELD LISTENING CONDITIONS
1st E D I T I O N
December 1961
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 226, Normal Equal-Loudness Contours for Pure Tones
and Normal Threshold of Hearing Under Free Field Listening Conditions, was drawn up
by Technical Committee ISO/TC43, Acoustics, the Secretariat of which is held by the
British Standards Institution (B.S.I.).
Work on this question by the Technical Committee began in 1957 and led,in 1959,
to the adoption of a Draft IS0 Recommendation.
In December 1959, this Draft IS0 Recommendation (No. 352) was circulated to aii
the IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of
an editorial nature, by the following Member Bodies :
Austria India Romania
Belgium Ireland Spain
Burma Italy Sweden
Chile Mexico Switzerland
Colombia Netherlands United Kingdom
New Zealand
Czechoslovakia U.S.A.
Denmark Norway U.S.S.R.
France Poland Yugoslavia
Hungary Portugal
One Member Body opposed the approval of the Draft: Germany.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in December 1961, to accept it as an IS0 RECOMMENDATION.
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iSO/ R 226-1961 (E)
IS0 Recommendation R 226 Decem ber 1961
NORMAL EQUAL-LOUDNESS CONTOURS FOR PURE TONES
AND NORMAL THRESHOLD OF HEARING
UNDER FREE FIELD LISTENING CONDITIONS
FOREWORD
Curves defining relations between frequencies of sounds and their sound pressure levels for
the condition of constant loudness, i.e. “ equal-loudness contours ”, are involved in many aspects
of subjective acoustics. They are fundamental to a proper understanding of the function of the
human ear in the perception of loudness. They also have important practical applications, for
example in the development of methods of computing loudness from the physical properties of
sounds.
The following IS0 Recommendation specifies the equal-loudness contours and threshold
of hearing for continuous sinusoidal tones for subjects of normal hearing under the conditions
of binaural listening in free progressive plane waves, with the subject directly facing the source
of sound.
It is emphasized that, since the contours here recommended relate only to pure tones pre-
sented in a specified manner in free field, they are not directly applicable to other types of sounds
(e.g. bands of noise) or to other types of stimulus presentation (e.g. by earphone or with random
incidence).
For these other types of sounds and other methods of presentation, contours appropriate to
the circumstances will be needed, and may be issued as further data become available.
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IS0 / R 226-1961 (E)
1. SCOPE
This IS0 Recomniendatioii specifies, for the frequency range 20 to 15 000 Hz (c/s) * and
for the conditions stated below:
1.1 The normal relations existing between sound pressure level and frequency for pure tones
of equal loudness.
1.2 Values for the normal threshold of hearing (normal binaural minimum audible field
or MAF).
Conditions of applicability :
(a) The source of sound is directly ahead of the listener;
(b) The sound reaches the listener in the form of a free progressive plane wave;
(c) The sound pressure level is measured in the absence of the listener;
(d) The listening is binaural;
(e) The listeners are otologically normal persons in the age group 18 to 25 years inclusive.
NOTE. Appendix B presents correction formulae to be applied to the above data in respect of otologically
normal persons of other ages up to 60 years.
2. EXPLANATION OF TERMS
For the purpose of this IS0 Recommendation, the following terms are einployed, the pure
tone, and the conditions of applicability stated in clause 1.2 being understood where appropriate.
2.1
Equal-loudness relation. A curve or function expressing, for a pure tone of given frequency,
the relation between its loudness level, expressed in phons, and its sound pressure level,
expressed in decibels.
2.2
Equal-loudness contour. A curve in the sound pressure level/frequency plane, connecting
points whose co-ordinates represent pure tones judged equally loud.
2.3 Normul equal-loudness relation (contour). The equal-loudness relation (contour) corre-
sponding to the judgments of otologically normal subjects within the age limits 18 to
25 years inclusive.
2.4
Threshold of hearing (binaural minimum audible field). For a given person at a specified
frequency, the minimum value of the sound pressure level which excites the sensation of
hearing.
2.5
Normal threshold of hearing (normal binaural minimum audible field or MAF). At a specified
frequency, the value of the threshold of hearing corresponding to the judgments of otologic-
ally normal subjects within the age limits 18 to 25 years inclusive.
NOTE. An ‘‘ otologically normal subject ” is understood to be a person in a normal state of health who is
free from all signs or symptoms of ear disease and from wax in the ear canal.
Hz (Hertz): International use
cls (cycles per second): Used in United Kingdom and U.S.A.
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IS0 / R 226-1961 (E) E
3. SPECIFICATION
The recommended normal equal-loudness contours and the recommended normal threshold
of hearing (normal binaural minimum audible field or MAF) are presented in graphical form
in Figure 1.
NOTE. Further information concerning the method of derivation of the recommended equal-loudness con-
tours, and correction formulae for the effects of the age of listeners, are given, for information only, in the
Appendices to this IS0 Recommendation. Appendix A, Table A-1 , also includes a tabular version of the
normal threshold contoiu.
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FIG. 1. - Normal equal-loudness contours for pure tones (Binaural free-field listening)
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IS0 I R 226 - 1961 (E)
APPENDIX A
(for information only)
Method of derivation of recommended normal equal-loudness contours
In the
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