A Few Acoustics and
Physics Basics
Sounds are
changes in pressure in an elastic medium
(such as the atmosphere) which can be detected
by the ear. Normal human ears can hear in
the frequency range between approximately
20 and 20,000 Hertz, or
cycles per second.
A sound source
emits sound waves (pressure
disturbances) in all directions.
Of course, a source like the
human voice will emit (radiate)
much more energy in the general
direction the person is facing
than in other directions. The
wave expands, in a spherical
shape, losing intensity as
it travels away from the source.
This energy loss is quite rapid
as the distance from the sound
source increases, since the
total volume through which
the sound is being sent grows
ever larger. More detail is
in the tutorial entitled, Generation & Movement
of Sound.
Loudness
vs. Power/Intensity
Simply put, the amplitude of a sound is its 'strength', or loudness.
More technically, the amplitude is the difference between the air pressure
levels in the sound waves at their strongest and weakest. Remember that the
sound itself is nothing more than a series of compressions and rarefactions in
the atmosphere.
Loudness is
a perceptual quantity which
can only be assessed by an
auditory system, including
the brain. Our perception of
the loudness of a given sound
varies according to pitch,
because the human ear is not
uniformly sensitive to all
frequencies. For instance,
the ear is most sensitive to
pitches in the 1000-3000Hz
range. Lower or higher pitches,
even if sung/produced at the
same volume, will sound softer
by comparison.
Acoustic
power is a measure of
the amount of the total energy
produced (regardless of how
it is perceived by the human
ear) and radiated into the
surrounding air, per second,
measured in watts.
Intensity is
a measure of a sound's power
per unit area, measured in
watts/m2, or in
units called decibels (symbol:
dB). The original unit was
the bel, but that unit proved
to be too large, and so the
decibel, which is a tenth of
a bel, is used instead. Since
the decibel scale is logarithmic,
large differences in the power
of sounds are reflected as
relatively small changes in
decibels. A 10x increase in
sound power is only a 10dB
increase.
Frequency
A sound's frequency is the number of sound waves produced per second by the
sound source. A wave is either a compression or rarefaction.
Pitch
vs. Timbre
Pitch is psychological
impression of the 'highness'
or 'lowness' of a sound based
on the various frequencies
it contains. A good example
of pitch is the sound associated
with the keys on a piano.
Timbre,
by contrast, is the 'quality'
of a sound that we hear, which
is determined by the various
overtones present in the sound,
and their relative strengths.
For instance, we might think
of a given voice or instrument
as sounding 'rich', or 'tinny',
or 'hollow ', or 'fat', or
'thin', or 'dry', or any number
of other perceptions. Voice
qualities are discussed
in detail in the Chapter 10
tutorial.
Tutorials Homepage
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~ National Center for Voice and Speech, www.ncvs.org
