*Masking: Masking is the phenomenon by which loud signals prevent
the ear from hearing softer sounds. The greatest masking effect
occurs when the frequency of the sound and the frequency of the
masking noise are close to each other. For example, a 4kHz tone will
mask a softer 3.5kHz tone, but will have little effect on the
audibility of a quiet 1000Hz tone. The masking phenomenon is one of
the main reasons that stereo placement and equalization are so
important in a mixdown. An instrument that sounds fine by itself can
be completely hidden or changed in character by louder instruments
with a similar timbre.
Although one ear is not able to discern the direction from which a
sound originates, two ears can. This ability of two ears to localize
a sound source within an acoustic space is called binaural
localization. This effects results from using three cues that are
received by the ears: interaural intensity differences, interaural
arrive-time differences, and the effects of the pinnae (outer ears).
Middle- to higher-frequency sounds originating from the right side
will reach the right ear at a higher intensity level than the left
ear, causing an interaural intensity difference. This occurs
because the head casts an acoustic block or shadow, allowing only
reflected sound from surrounding surfaces to reach the left ear.
Since the reflected sound travels farther and loses energy at each
reflection, the intensity of sound perceived by the left ear is
reduced, with the resulting signal being perceived as originating
from the right.
This effect is relatively insignificant at lower frequencies,
where wave-lengths are large compared to the diameter of the head and
easily bend around its acoustic shadow. A different method of
localization known as interaural arrive-time differences is
employed at lower frequencies. In our example, time differences occur
because the acoustic path length to the left ear is slightly longer
than that to the right ear. The sound pressure will thus be sensed by
the left ear at a later time than by the right ear. This method of
localization, in combination with interaural intensity differences,
gives us lateral location cues over the entire frequency spectrum.
The intensity and delay cues allow us to perceive the angle from
which a sound originates, but not whether the sound originates from
the front, behind, or below. The pinna, however, makes use of two
ridges that reflect the incident sound into the ear. These ridges
introduce time delays between the direct sound (which reaches the
entrance of the ear canal) and the sound reflected from the ridges
(which varies according to source location).
PITCH The pitch of a sound refers
to whether it is high, like the sound of piccolo or violin, or low,
like the sound of a bass drum or string bass. The physical quantity
that determines pitch is the frequency. The lower the frequency, the
lower the pitch. The human ear responds to frequencies in the range
from about 20Hz to about 20,000Hz. This is called the audible range.
These limits vary somewhat from one individual to another. One
general trend is that as people age, they are less able to hear the
high frequencies, so that the high-frequency limit may be 10,000Hz or
less.
Sound waves whose frequencies are outside the audible range may
reach the ear, but we are not generally aware of them. Frequencies
above 20,000Hz are called ultrasonic. Many animals can hear
ultrasonic frequencies; dogs, for example, can hear sounds as high as
50,000Hz and bats can detect frequencies as high as 100,000Hz.
Sound waves whose frequencies fall below the audible range are
called infrasonic, or occasionally subsonic. Sources of infrasonic
waves are earthquakes, thunder, volcanoes, and waves produced by
vibrating heavy machinery.
The pitch of the sound also factors into the way the ear hears.
The ear has difficulty in associating a point origin to a
low-frequency sound, but is quite accurate in placing the origin of
high-frequencies. This is because high frequencies have wavelengths
shorter than the distance between the ears; sounds above 1000Hz
cannot reach both ears at the same time and at the same intensity, so
one ear is favored and provides the information as to the direction
in the horizontal plane. The ear is less successful in responding to
directions in the vertical plane.
FUNDAMENTALS AND
HARMONICS The initial vibration of a sound sources is called
the fundamental, and thus the initial frequency is known as
the fundamental frequency. The subsequent vibrations, which
are exact multiples of the fundamental frequency, are called the
harmonics. So, a note on a musical instrument with a
fundamental frequency of 100Hz will have a second harmonic at 200Hz,
a third harmonic at 400Hz, et al.
The term octave denotes the difference between any two
frequencies where the ratio between them is 2:1. Thus, an octave
separates the fundamental from the second harmonic in the above
example: 200Hz:100Hz. At the upper end of the frequency spectrum the
same ratio still applies although the frequencies are greater. An
octave still separates 2000Hz from 1000Hz. Two notes separated by an
octave are said to be "in tune." Thus, an octave on the piano
keyboard, separated by eight keys (well, really thirteen), is also an
octave-- frequency-wise.
Whether the harmonics diminish in intensity or retain much of
their energy depends on how the source is initially vibrated and
subsequently damped. It is the strength of the harmonics which
distinguishes the quality (or timbre) of musical instruments and
makes it possible for humans to identify two different instruments
playing the same note. Cool, huh.
INTENSITY Like pitch, loudness
is a sensation in the consciousness of a human being. It, too, is
related to a physically measurable quantity, the intensity of the
wave. Intensity is defined as the energy transported by a wave per
unit time across unit area. Since energy per unit time is power,
intensity has units of power per unit area, or watts/meter2 (W/m2).
The intensity depends on the amplitude of the wave (it is
proportional to the square of the amplitude). [The amplitude of the
wave is the distance between the extremes of the vibration.]
The human ear can detect sounds with an intensity as low as 10-12
W/m2 and as high as 1 W/m2 (and even higher, although above this it
is painful). This is an incredibly wide range of intensity, spanning
a factor of 1012 from lowest to highest. Presumably because of this
wide range, what we perceive as loudness is not directly proportional
to the intensity. Ture, the greater the intensity, the louder the
sound. But to produce a sound that sounds about twice as loud
requires a sound wave that has about ten times the intensity. For
example, a sound wave of intensity 10-9 W/m2 sounds to an average
human being as if it is about twice as loud as one whose intensity is
10-10 W/m2; and an intensity of 10-2 W/m2 sounds about twice as loud
as 10-3 W/m2 and four times as loud as 10-4 W/m2.
Because of this relationship between the subjective sensation of
loudness and the physically measurable quantity intensity, it is
usual to specify sound intensity using a logarithmic scale. The unit
on this scale is the decibel, (dB). The intensity level, b, of any
sound is defined in terms of its intensity, p, as follows:
b(dB)
= 10 log (p1/p0).