Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
Figure 4.10 Critical bands graphed from Table 4.4 Amplitude Perception
In the early 1930s at Bell Laboratories, groundbreaking experiments by Fletcher and Munson
clarified the extent to which our perception of loudness varies with frequency (Fletcher and
Munson 1933). Their results, refined by later researchers (Robinson and Dadson, 1956) and
adopted as International Standard ISO 226, are illustrated in a graph of equal-loudness contours
shown in Figure 4.11. In general, the graph shows how much you have to “turn up” or “turn
down” a single frequency tone to make it sound equally loud to a 1000 Hz tone. Each curve on
the graph represents an n-phon contour. One phon is defined as a 1000 Hz sound wave at a
loudness of 1 dBSPL. An n-phon contour is created as follows:
Frequency is on the horizontal axis and loudness in decibels is on the vertical axis
n curves are drawn.
Each curve, from 1 to n, represents the intensity levels necessary in order to make each
frequency, across the audible spectrum, sound equal in loudness to a 1000 Hz wave at n
Let’s consider, for example, the 10-phon contour. This contour was creating by playing a
1000 Hz pure tone at a loudness level of 10 dBSPL, and then asking groups of listeners to say
when they thought pure tones at other frequencies matched the loudness of the 1000 Hz tone.
Notice that low-frequency tones had to be increased by 60 or 75 dB to sound equally loud. Some
of the higher-frequency tones in the vicinity of 3000 Hz actually had to be turned down in
volume to sound equally loud to the 10 dBSPL 1000 Hz tone. Also notice that the louder the
1000 Hz tone is, the less lower-frequency tones have to be turned up to sound equal in loudness.
For example, the 90-phon contour goes up only about 30 dB to make the lowest frequencies
sound equal in loudness to 1000 Hz at 90 dBSPL, whereas the 10-phon contour has to be turned
up about 75 dB.
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