ELECTRONICA-Technologies
Rev. January 2017
Series 9000 AUDIOMETERS
Audiometer 9910
Version EN1_H
Page 26
6.
Acoustic scales
In 1937, internationally recognized acoustics standards established the sound pressure
measurement scale as a logarithmic unit expressed in dB (decibell). The 0 dB reference
measure was concomitantly set at 20 µPascal at 1000 Hz, which falls just below the
absolute threshold of human hearing. This scale, called the SPL (Sound Pressure Level),
is used in acoustics and sonometry, and it is also widely popular in North American
audiology centers.
However, one of the specificities of the human ear, which is an extremely sensitive
sensor, is that it is selectively better at pinpointing tones within the 1000 to 3000 Hz range
that at 125 Hz or 8000 Hz.
A statistical study led on a sample population of subjects with “healthy hearing
sensitivity” defined and characterized perception thresholds, as summarized in the table
below:
Frequencies
emitted, in Hz
125
250
500
750
1K
1.5K
2K
3K
4K
6K
8K
Threshold of
hearing, in dB
45
27
13,5
10
7,5
7,5
9
11,5
12
16
15,5
The scale has therefore been adjusted to correct for this factor, using the same units but
with different reference levels created for each frequency. It is called as HL, for Hearing
Level, although it is sometimes termed compensated curve. This is the unit used by the
vast majority of Western-European audiometry centers. It is easier to apply, as the
sensitivity threshold is always indexed to 0 dB whatever the frequency.
