Chapter 26
Sound & Music
Sound …
...a longitudinal wave in air
caused by a vibrating object
Origin of Sound
Infrasonic
frequencies 20,000 Hz
Human hearing range
frequencies between 20 Hz and 20,000 Hz
Audible Frequencies
Sound waves with frequencies between 20 Hz
and 20 kHz
200 Hz
500 Hz
1000 Hz
2000 Hz
5000 Hz
wiki Sounds
Ultrasound
Nature of Sound in Air
Sound requires a medium.
• solid, liquid or gas
Sound waves have compression and
rarefaction regions.
Speed of Sound in Air
331 meters/second
760 miles/hour
Mach 1
What is the approximate distance of a
thunderstorm when you note a 3
second delay between the flash of the
lightning and the sound of the
thunder?
Answer: 3 seconds 331 m/s
= 993 meters
Acoustics...
...the study of sound properties.
When a sound wave strikes a surface it
can be.…
(a) reflected.
(b) transmitted.
(c) absorbed.
(d) all of these.
Reflection of Sound
e.g. an echo
Reverberation - re-echoed sound, multiple
reflections of sound waves from walls
Compare reflections from a hard wall with
that from a carpet wall.
Refraction of Sound
Refraction - the bending of a wave
Sound waves bend toward cooler air.
Forced Vibrations...
…the setting up of vibrations in an object by a
vibrating force.
Examples of Forced Vibration:
• A tuning fork touching a wood surface
• Sounding boards for stringed instruments
• Matching tuning fork boxes
During forced vibration sound is
intensified because a larger
surface area is available to
vibrate air molecules.
Natural Frequency...
…the frequency at which an elastic
object naturally tends to vibrate.
At this frequency, a minimum energy is
required to produce a forced vibration.
The natural frequency of a body
depends on its elasticity and its shape.
Natural Frequency Examples
Dropping Aluminum Rods
Ringing Small and Large Bells
Xylophone
Rubbing a Wine Glass
Mass on a Spring
Resonance...
…is the result of forced vibrations in a body
when the applied frequency matches the
natural frequency of the body.
The resulting vibration has a high amplitude
and can destroy the body that is vibrating.
Examples of Resonance
• Breaking a wine glass using sound
• Mass on a spring at resonance
• A singing rod caused by forced
vibration
• A tuning fork exciting a guitar string
• In 1940, the Tacoma Narrows Bridge
was destroyed by wind-generated
resonance.
Resonance allows energy
to be transferred to a
vibrating object efficiently
if the energy is delivered at
the natural frequency of
vibration.
Sound Interference
Overlapping crests of a wave will
result in an increased amplitude.
Overlapping a crest and a trough
results in a decrease in amplitude.
Beats - the periodic variation in loudness of
two sounds played together
The beat frequency is equal to the difference in
the frequency of the two sounds.
What is the beat frequency when a 262 Hz and
a 266 Hz tuning fork are sounded together?
400 Hz and 403 Hz
400 Hz and 410 Hz
Radio Broadcasts
• AM - Amplitude Modulation
• 535 kHz to 1605 kHz
• FM - Frequency Modulation
• 88 MHz to 108 MHz
• Modulation - an impression of the sound
wave on a higher frequency radio wave
Noise Versus Music
What is the difference between noise and
music?
Pitch...
… is the "highness" or "lowness" of a
tone.
Pitch corresponds to frequency.
Concert A on the Musical Scale has a
frequency of 440 Hertz.
Major Scale
Letter Frequency Frequency
Note Name (Hz) ratio Interval
do C 264
9/8 Whole
re D 297
10/9 Whole
mi E 330
16/15 Half
fa F 352
9/8 Whole
sol G 396
10/9 Whole
la A 440
9/8 Whole
ti B 495
16/15 Half
do C 528
Sound Intensity and Loudness
Intensity of Sound refers to
the amplitude of the
pressure variations in the
sound wave
Loudness
•The physiological sensation
directly related to the sound
intensity
•Measured in bels
•(10 bels = 1 decibels)
Loudness
• A sound of 10 decibels is or 101 or 10
times as intense as 0 decibels.
• 20 decibels is 102 or 100 times the
intensity 0 decibels.
• How much more intense is sound at 100
dB than sound at 50 dB?
• Answer: 100000 times
Source of Sound Loudness (db)
Threshold of Hearing 0
Conversation 60
Ear Damage Begins 85
Amplified Music 110
Jet Airplane at 30 meters 140
Common Sound Intensities
2
Source of Sound Intensity (W/m ) Sound Level (db)
2
Jet 30 m away 10 140
Air-raid siren, nearby 1 120
-1
Disco music, amplified 10 110
-3
Riveter 10 90
-5
Busy street traffic 10 70
-6
Conversation in home 10 60
-8
Quiet radio in home 10 40
-10
Whisper 10 20
-11
Rustle of leaves 10 10
-12
Threshold of hearing 10 0
Quality...
…is the characteristic sound that
allows us to distinguish between two
musical instruments.
Partial Tones - one of the many
frequencies present in a complex tone
Fundamental Frequency
• the lowest frequency of vibration
• a.k.a. the first harmonic
Harmonic
• a partial tone that is an integer
multiple of the fundamental
frequency
Same Note - Different Instrument
Harmonics
• Harmonics on a Guitar String
• Harmonics in an Organ Pipe
• Open on one end, close on the other
• Open on both ends
Fourier Analysis...
…is a mathematical method that
will resolve any periodic wave
form into a series of simple sine
waves.
http://www.phys.unsw.edu.au/~jw/strings.html
COMPACT DISCS
Phonograph players give
analog signals.
Digital signal is in binary code.
CD has flats and pits and is sampled
44,100 times per second.
1. The quality of a musical note
can be changed by simply
playing the note on a different
instrument.
(a) True
(b) False
2. The three characteristics of a
musical tone are loudness,
quality, and timbre.
(a) True
(b) False
3. Frequencies of sound that are
too high for the human ear to
hear are called...
(a) faster than the speed of sound
(b) supersonic
(c) infrasonic
(d) ultrasonic
(e) subsonic
4. For the same temperature for
air, does sound travel faster in
humid Galveston or in dry El
Paso?
(a) Galveston
(b) El Paso
(c) same speed in either city
5. The bending of sound through
air of uneven temperature is
called
(a) reflection
(b) refraction
(c) interference
(d) reverberation
(e) resonance
6. Lightning is seen, then ten
seconds later thunder is heard.
Approximately, how far away in
meters is the thunder cloud?
(a) 10,500 m
(b) 5280 m
(c) 1050 m
(d) 3400 m
(e) 680 m
7. A 250 Hz tuning fork and a
260 Hz tuning fork are vibrating
near each other. How many beats
per second are heard?
(a) 255
(b) 510
(c) 10
(d) 250
(e) 260