2. Sound is really tiny fluctuations of air pressure
– units of pressure: N/m2 or psi (lbs/square-inch)
Carried through air at 343 m/s (770 m.p.h) as
compressions and rarefactions in air pressure
3. Wavelength () is measured from crest-to-crest
– or trough-to-trough, or upswing to upswing, etc.
For traveling waves (sound, light, water), there is a speed (c)
Frequency (f) refers to how many cycles pass by per second
– measured in Hertz, or Hz: cycles per second
– associated with this is period: T = 1/f
These three are closely related:
f = c
or T
pressure
4. We hear frequencies of sound as
having different pitch.
A low frequency sound has a low
pitch, like the rumble of a big
truck.
A high-frequency sound has a
high pitch, like a whistle or siren.
In speech, women have higher
fundamental frequencies than
men.
5. When we hear complex sounds, the nerves in the ear
respond separately to each different frequency. The brain
interprets the signals from the ear and creates a “sonic
image” from the frequencies. The meaning in different
sounds is derived from the patterns in how the different
frequencies get louder and softer.
6. How we hear the loudness
of sound is affected by the
frequency of the sound as
well as by the amplitude.
The human ear is most
sensitive to sounds between
300 and 3,000 Hz.
The ear is less sensitive to
sounds outside this range.
Most of the frequencies that
make up speech are between
300 and 3,000 Hz.
7. 1. A common way to record sound starts with a
microphone. A microphone transforms a sound
wave into an electrical signal with the same
pattern of oscillation.
8. 2. In modern digital recording, a sensitive circuit
converts analog sounds to digital values between 0
and 65,536.
9. 3. Numbers correspond to the amplitude of the signal
and are recorded as data. One second of compact-
disk-quality sound is a list of 44,100 numbers.
10. 4. To play the sound back, the string of numbers is
read by a laser and converted into electrical signals
again by a second circuit which reverses the
process of the previous circuit.
11. 5. The electrical signal is amplified until it is powerful
enough to move the coil in a speaker and reproduce
the sound.
12. A sound wave is a wave of alternating high-pressure
and low-pressure regions of air.
Sound is a longitudinal wave, meaning that the motion
of particles is along the direction of propagation
13. Waves in air can’t really be transverse, because the
atoms/molecules are not bound to each other
– can’t pull a (momentarily) neighboring molecule sideways
– only if a “rubber band” connected the molecules would this work
– fancy way of saying this: gases can’t support shear loads
Air molecules can really only bump into one another
14. The pitch of a sound depends on the frequency of
the tone that the ear receives. High notes are
produced by an object that is vibrating a greater
number of times per second than for a low note
The intensity of a sound is the amount of energy
crossing a unit area in unit time or the power
flowing through the unit area. The SI unit is watts
per square meter.
The loudness of the sound depends upon the
subjective effect of intensity of sound waves on the
human ear.
15. The shift in frequency caused by motion is called the
Doppler effect.
It occurs when a sound source is moving at speeds less
than the speed of sound.
16. Any waveform can be
analyzed as the sum
of a set of sine waves,
each with a particular
amplitude, frequency,
and phase.
17. The speed of sound in air is 343 meters per second
(660 miles per hour) at one atmosphere of pressure
and room temperature (21°C).
An object is subsonic when it is moving slower than
sound.
18. We use the term supersonic to describe motion at
speeds faster than the speed of sound.
A shock wave forms where the wave fronts pile up.
The pressure change across the shock wave is what
causes a very loud sound known as a sonic boom.
19. A complex wave is really a sum of component frequencies.
A frequency spectrum is a graph that shows the amplitude
of each component frequency in a complex wave.
20. A single frequency by itself does not have much meaning.
The meaning comes from patterns in many frequencies
together.
A sonogram is a special
kind of graph that shows
how loud sound is at
different frequencies.
Every person’s sonogram
is different, even when
saying the same word.
21. The eardrum vibrates
in response to sound
waves in the ear canal.
The three delicate
bones of the inner ear
transmit the vibration
of the eardrum to the
side of the cochlea.
The fluid in the spiral
of the cochlea vibrates
and creates waves that
travel up the spiral.
22. The nerves near the
beginning see a
relatively large channel
and respond to longer
wavelength, low
frequency sound.
The nerves at the small
end of the channel
respond to shorter
wavelength, higher-
frequency sound.
23. The pitch of a sound is how high or low we hear its
frequency. Though pitch and frequency usually mean
the same thing, the way we hear a pitch can be
affected by the sounds we heard before and after.
Rhythm is a regular time pattern in a sound.
Music is a combination of sound and rhythm that we
find pleasant.
Most of the music you listen to is created from a
pattern of frequencies called a musical scale.
24. Harmony is the study of how sounds work together to
create effects desired by the composer.
When we hear more than one frequency of sound and the
combination sounds good, we call it consonance.
When the combination sounds bad or unsettling, we call it
dissonance.
25. Consonance and dissonance are related to beats.
When frequencies are far enough apart that there
are no beats, we get consonance.
When frequencies are too close together, we hear
beats that are the cause of dissonance.
Beats occur when two frequencies are close, but
not exactly the same.
Beats are created by the interference of two waves with different frequencies.
26. A listener will hear the alternating loudness, known
as beats.
The number of beats per second, called the beat
frequency, equals the difference between the
frequencies of the two individual waves.
To tune an instrument accurately, a musician
listens carefully and adjusts her instrument to
eliminate beats between the instrument and a given
pitch.
27.
28. Echolocation is the method
of detecting objects by
emitting a sound, receiving
the echo and correctly
identifying the location, size
and structure of the object.
These sound waves are very
high-pitched, and most
humans are unable to hear
them.
29. The same note sounds different when played on different
instruments because the sound from an instrument is not
a single pure frequency.
The variation comes from the harmonics, multiples of
the fundamental note.
30. The End
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