Page 79 - The Master Handbook Of Acoustics

54 CHAPTER THREE

beginning to be painful (curve B). These two curves represent the
extremes of our perception of loudness.
Curve A of Fig. 3-8, the threshold of hearing, tells us 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 of 0 dB can just barely be heard by a person of
average hearing acuity. Is it fortuitous that this threshold is at a nice,
round, 0-dB level? No, the reference level of pressure of 20 mPa (20
micropascals) was selected for this reason. It is both instructive and
comforting to know that a sound-pressure level of 60 dB turns out to
be approximately 60 dB above our threshold of hearing.
Curve B of Fig. 3-8 represents the level at each frequency at which
a tickling sensation is felt in the ears. This occurs at a sound-pressure
level of about 120 or 130 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.
In between the threshold of hearing (curve A of Fig. 3-8) and the
threshold of feeling (curve B) is the area of audibility. This is an area
with two dimensions: the vertical dimension of sound-pressure level
and the horizontal range of frequencies that the ear can perceive. All
the sounds that humans experience must be of such a frequency and
level as to fall within this auditory area. Chapter 5 details more specif-
ically how much of this area is used for common music and speech
sounds.
The area of audibility for humans is quite different from that of
many animals. The bat specializes 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 vs. Sound-Pressure Level

The phon is the unit of loudness level that is tied to sound-pressure
level at 1,000 Hz as we have seen in Figs. 3-6, 3-7, and 3-8. This is