Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 106

Fig. 4.18 Example sound levels (in decibels). a [140] Rock band (amplified) at close range;
b [120] Loud thunder or fireworks; c [100] Jet plane at 500 ft; d [100] Subway train at 20 ft; [90]
(not shown) Potential for permanent hearing loss; e [80] Busy street corner; f [60] Normal
conversation; g [40] Typical room; h [20] Whisper; [0] (not shown) Threshold of hearing
The intensities of various common sounds are shown in Fig. 4.18 . The sensi-
tivity to both frequency and loudness varies from person to person. Generally,
though, the ear is insensitive to frequency changes below about 20 dB (i.e., below
a whisper). Once levels get above 90 dB (as shown by the line in the table),
prolonged exposure can lead to permanent hearing loss. This level is often sur-
passed by some industrial jobs, and by the iPods and MP3 players of those people
who have the volume control turned up excessively high.
This scale does not describe perceived loudness well. Increasing an auditory
stimulus by equal ratios does not produce equal increments in sensation. It is
obvious that the difference in loudness between a whisper and a normal conver-
sation is less than the difference between a normal conversation and a subway
train. What is said, the context, and background noise also influence perceived
loudness. However, the ratio of the energy in a whisper to that in a conversation is
about the same as the ratio in conversation to that of the noise of a subway train.
This explains why a 100-W stereo is much louder than a 1-W pocket radio, and a
200-W stereo is not much louder than a 100-W stereo. The corollary of this is that
when measuring loudness, asking people to directly estimate apparent intensity of