This document discusses key concepts about sound, including: - Sound is caused by fluctuations in air pressure that propagate as waves. Frequency, wavelength, and speed are closely related characteristics of sound waves. - Humans hear different frequencies as different pitches. Higher frequencies are heard as higher pitches like whistles, while lower frequencies have lower pitches like rumbling trucks. - The loudness we perceive depends on both the frequency and amplitude of sound waves. The human ear is most sensitive to frequencies between 300-3,000 Hz, which encompasses most of the frequencies in speech.

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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.

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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