BS 3383:1988 download.Specification for Normal equal-loudness level contours for pure tones under free-field listening conditions.
0 Introduction
Curves defining relations between frequencies of pure tones or narrow bands of noise and their sound pressure levels for the condition of constant loudness level (equal-loudness level contours) express a fundamental property of the human auditory system, and are of basic importance in the field of psychological acoustics.
In common with other subjective phenomena, loudness relations vary in detail from person to person but, for a group of persons within a restricted age band and free from hearing impairments, a value for the central tendency can be determined to characterize the group. BS 3383 gives curves applicable to otologically normal persons in the age group from 18 to 30 years.
The data specified in BS 3383 primarily relate to continuous 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 hut with the listener absent. For other conditions of listening or other configurations of sound field, different relations exist between loudness level and sound pressure level. One example of another sound field configuration is a diffuse field. The relation between frontal incidence and diffuse field together with the resulting equal-loudness level function for narrow bands of noise in a diffuse sound field are specified in Annex C to this International Standard.
NOTE The relation between frontal incidence and diffuse field was given in ISo 454: with the inclusion of this relation in this edition of ISO 226, ISO 454 has become superfluous and has been withdrawn (see the “Foreword’).
The hearing threshold may be considered as a special case of equal-loudness level, and the corresponding threshold sound pressure levels [minimum audible field (MAF)] are included in BS 3383. It should be emphasized that the minimum audible field differs from the audiometric zero specified in ISO 389, since the latter refers to monaural listening through earphones with sound pressure levels referred to specified couplers and artificial ears. Direct comparison between the data in ISO 389 and in this International Standard is therefore not appropriate.
1 Scope and field of application
BS 3383 specifies the relations existing, for the condition of equal-loudness level, between the sound pressure levels and frequencies of pure (sinusoidal) continuous tones in the following conditions:
a) the source of sound is directly in front of the listener (frontal incidence);
b) the sound field in the absence of the listener consists of a free progressive plane wave;
c) the sound pressure level is measured in the free progressive plane wave in the absence of the listener;
d) the listening is binaural:
e) the conditions of equal-loudness level are determined by the modal value of the judgements of an adequately large group of listeners;
the listeners are otologically normal persons in the age group from 18 to 30 years inclusive.
The relations are expressed by means of an equation in bilinear form, with the sound pressure level as the independent variable and the loudness level as the dependent variable, for the preferred frequencies in the one-third octave series from 20 to 12 500 Hz inclusive.
NOTE I In accordance with convention, the reference dependent variable is taken to be the sound pressure level of a 1 000 Hz tone, i.e. the loudness level expressed in phons. However, the form of the equation remains unchanged (but with transformed coefficients) if the variables are interchanged or if a tone of another frequency in the preferred one-third octave series is taken as the reference.
NOTE 2 The data in BS 3383 are approximately equal to equal-loudness level relations between narrow bands of random noise, not exceeding the auditory critical bandwidth.
NOTE 3 Without extrapolation of the experimental data, the upper limit of the frequency range could not be extended to the next preferred frequency in the one-third octave series. For further information regarding the range from 12 500 to 15 000 Hz, see [1].
Graphical and short tabular presentations of the relations are given in Annex A and Annex B. Annex C specifies the correction to be applied to obtain the equal-loudness level relations for narrow bands of random noise in a diffuse sound field and the resulting equal-loudness level function.
2 References
Iso 131, Acoustics — Expression of physical and subjective magnitudes of sound or noise in air. ISO 266, Acoustics — Preferred frequencies for measurements.
ISO 389, Acoustics — Standard reference zero for the calibration of pure tone air conduction audiometers.
3 Definitions
For the purposes of BS 3383, the following definitions are used, the conditions of applicability stated in clause 1 being understood. where appropriate.
loudness
that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from soft to loud
3.2
loudness level
of a given sound. the sound pressure level of a reference sound, consisting of a sinusoidal plane progressive wave of frequency 1 000 Hz coming from directly in front of the listener, which is judged by otologically normal persons to be equally loud to the given sound. Loudness level is expressed in phons
3.3
equal-loudness level function
for a pure tone of given frequency or for a narrow band of noise of a given frequency band, the relation between loudness level, expressed in phons, and sound pressure level, expressed in decibels
3.4
equal-loudness level contour
a curve in the sound pressure levellfrequency plane, connecting points the co-ordinates of which represent pure tones or narrow bands of noise judged equally loud
3.5
normal equal-loudness level function
the equal-loudness level function corresponding to the modal judgement of otologically normal persons within the age limits from 18 to 30 years inclusive
3.6
normal equal-loudness level contour
the equal-loudness level contour corresponding to the modal judgement of otologically normal persons within the age limits from 18 to 30 years inclusive
3.7
hearing threshold
the level of a sound at which, under specified
conditions, a person gives 50 % of correct detection responses on repeated trials
3.8
otologically normal person
a person in a normal state of health who is free from all signs or symptoms of ear disease and from excess wax in the ear canals, and who has no history of
undue exposure to noise
3.9
minimum audible field (MAF)
at a specified frequency the sound pressure level of a pure tone or a narrow band of noise corresponding to the modal value of the binaural hearing threshold of otologically normal persons within the age limits from 18 to 30 years inclusive
3.10
critical bandwidth
the widest frequency band within which the
loudness of a band of continuously distributed
random noise of constant band sound pressure level is independent of its bandwidth
4 Specification
The loudness level, LN, in phons, of a pure tone of frequency f, in hertz, is given by the equation.