Sound
Pitch
Loudness
Beats
Doppler Effect
Physics
Ms. Shaver
Sound
Sound is a longitudinal mechanical wave
 aka a pressure or compression wave

Compressions(High Pressure)
 Rarefactions(Low Pressure)

Sound
Tiny hairs inside the inner ear
(cochlea) translate air pressure
into electrical impulses that can
be read by the brain
Graphing Sound
Speed of Sound
 Depends
on the medium.
 The
more elastic the medium the
faster sound will travel through it.
 Speed
in metals>speed in
water>speed in air
 Sound
can’t travel through vacuum.
Speed of Sound in Air
v = 331 + 0.6 T
( in meters/sec)
T is the temperature in 0C.
 In higher humidity, sound will travel faster.

For 15°C, v = 331 + 0.6*15 = 340 m/s
 Use 340 m/s as an average speed of sound
in air.

Faster than the speed of Sound
Supersonic – motion faster than speed of
sound
 Sonic boom – caused by an object
breaking the sound barrier (supersonic
planes, bullets)

Plane reaches speed above 770 mph – Explosion of sound waves
caused by air crashing behind the plane
The air can condense or sweep up vapor from engine – causing
this picture
Pitch: the frequency of a
sound wave.

Musical notes have a given pitch.

The note C has a frequency of 327Hz.

When two notes differ by a ratio of 2:1
they are one octave apart.

What would be the next higher C?
(Ans:654Hz)
Pitch – frequency of sound wave
Humans can hear frequencies between
20 Hz and 20,000 Hz
 Infrasonic frequencies: Below 20 Hz

◦ used by elephants and submarines to
communicate over long distances

Ultrasonic frequencies
◦ Dog whistles
Loudness = Amplitude
Loudness is measured in decibels (dB)
 A +10 dB change we hear as twice as
loud
 A -10 dB change we hear as half as loud

Decibels – Loudness measure

EXAMPLE: If a sound is 20 dB loud,
answer how many dB these would be:
1)
A sound twice as loud: 30 dB
A sound half as loud: 10 dB
A sound three times as loud: 35 dB
2)
3)
Decibels – Loudness measure
Examples of Sound Intensity Levels










jet plane taking off
air raid siren
threshold of pain
loud rock music
ear damage starts
busy traffic
normal conversation
quiet library
soft whisper
threshold of human hearing
140 dB
125 dB
120 dB
115 dB
85 dB
70 dB
60 dB
40 dB
20 dB
0 dB
Timbre – tone quality
What makes a particular musical sound
different from another, even when they
have the same pitch and loudness
 EX: difference between a guitar and a
piano playing the same note at the same
loudness
 Sounds can be described in terms of
“coloration,” e.g. bright, dark, warm, harsh,
etc.

Timbre – tone quality
Color of the note is due to the presence
of different harmonics.
 The “oo” has mostly low harmonics, while
the “ee” has mostly high harmonics

Harmonics - Review
v = λ f - The velocity of the wave stays
the same, even if considering different
harmonics….it’s still the same wave!!!
Harmonics - Review
Properties of Sound
 Reflection
(Echo)
 Refraction
 Interference
 Diffraction
Echoes

Echoes are sounds that are reflected back
by a hard boundary.
Echoes
Echolocation – used by many animals like
bats to see through dark water or at
night
 Sonar – sound through water
 Radar – light waves through air
 Sonograms – sound waves through the
human body

Beats
Interference effect
 When two waves of close frequencies
interact causing alternating constructive
and destructive interference

Beats

The number of beats = difference of two
frequencies (absolute value)

EX: f1 = 345 Hz; f2 = 342 Hz
◦ Number of beats = 345 – 342 = 3
◦ There will be three beats per second
http://www.school-forchampions.com/science/sound_beat_frequencies.htm#.U2gpwletJW8
Bow (Shock) Waves
When the speed of a moving sound
source is greater than the speed of the
wave, a pressure ridge builds similar to
the wave created by the bow of a ship.
physlet animation
Sonic Boom

When the pressure ridge of a bow wave
of a jet passes over an observer on the
ground, the observer experiences a sonic
boom.
Doppler Effect

The change in a wave's perceived frequency
due to the motion of either the sound source or
the observer.

It is applicable to any type of wave.

Austrian physicist Christian Doppler (1803-1853).
train sound clip
Simulations
 physlet animation
 http://www.walter-fendt.de/ph14e/dopplereff.htm
The Doppler Effect
http://www.physicsclassroom.com/Class/sound/u11l3b.cfm
The Doppler Effect
In front of the source the sound waves
are compressed (shorter wavelength λ)
and this raises the frequency (pitch)
 Behind the source the sound waves are
stretched (longer wavelength λ) and this
drops the frequency (pitch)
 Anything moving at the same speed as
the source will experience no change in
frequency
