Page 79 - Master Handbook of Acoustics

threshold of hearing delineates the lowest level sounds the ear can detect. (B) The threshold of
feeling defines the upper extreme. All of our auditory experiences occur within this area.

Contour A, the threshold of hearing, represents the level at each frequency where sounds are just
barely audible. The contour also shows that human ears are most sensitive around 3 kHz. Another
way to state this is that around 3 kHz a lower-level sound elicits a greater threshold response than

higher or lower frequencies. At this most sensitive region, a sound-pressure level defined as 0 dB can
just barely be heard by a person of average hearing acuity. The reference level of pressure of 20 μPa
was selected to establish this 0-dB level.
Contour B, the threshold of feeling, represents the level at each frequency where a tickling
sensation is felt in the ears. At 3 kHz, this occurs at a sound-pressure level of about 110 dB. Further
increase in level results in an increase in feeling until a sensation of pain is produced. The threshold

tickling is a warning that the sound is becoming dangerously loud and that ear damage is either
imminent or has already taken place.
Between the threshold of hearing and the threshold of feeling is the area of audibility. This area
has two dimensions: the vertical range of sound-pressure level and the horizontal range of

frequencies that the ear can perceive. The sounds that humans experience are of such a frequency and
level as to fall within this auditory area.
The area of audibility for humans is quite different from that of many animals. Bats specialize in
sonar cries that are far above the upper frequency limit of our ears. The hearing of dogs extends
higher than ours, hence the usefulness of ultrasonic dog whistles. Sound in the infrasonic and
ultrasonic regions, as related to the hearing of humans, is no less true sound in the physical sense, but

it does not result in human perception.

Loudness versus Sound-Pressure Level

The phon is a unit of physical loudness level that is referenced to a sound-pressure level at 1 kHz.
This is useful, but it tells us little about human reaction to loudness of sound. Some sort of subjective
unit of loudness is needed. Many experiments conducted with hundreds of subjects and many types of
sound have yielded a consensus that for a 10-dB increase in sound-pressure level, the average person
reports that loudness is doubled. Likewise, for a 10-dB decrease in sound level, subjective loudness
is cut in half. The sone is a unit of subjective loudness. One sone is defined as the loudness (not
loudness level) experienced by a person listening to a 1-kHz tone of 40-phon loudness level. A sound
of 2 sones is twice as loud as 1 sone, and 0.5 sone is half as loud.

Figure 4-9 shows a graph for translating sound-pressure levels to loudness in sones. One point on
the graph is the definition of the sone, the loudness experienced by a person hearing a 1-kHz tone at
40-dB sound-pressure level, or 40 phons. A loudness of 2 sones is then 10 dB higher; a loudness of
0.5 sones is 10 dB lower. A straight line can be drawn through these three points, which can then be

extrapolated for sounds of higher and lower loudness. This graph only applies to 1-kHz tones.