Sound

1. Sound Part 2Sound Part 2
2/21/20132/21/2013

2. What is sound?What is sound?
 Any change in air pressureAny change in air pressure
 The molecules in the air exerts a pressure ofThe molecules in the air exerts a pressure of
over 1 ton per square foot on our earsover 1 ton per square foot on our ears
 Must be a rapid change in sound pressure toMust be a rapid change in sound pressure to
be heard, a small rapid change will createbe heard, a small rapid change will create
noisenoise
 Travels as sound wavesTravels as sound waves

3. Tone and NoiseTone and Noise
 Tuning fork – pure tone andTuning fork – pure tone and
related frequenciesrelated frequencies
 We cannot see the tinesWe cannot see the tines
moving back and forthmoving back and forth
because they are moving backbecause they are moving back
and for the too quickly. Two-and for the too quickly. Two-
hundred cycles a second is toohundred cycles a second is too
fast to see.fast to see.
 Noise – random frequenciesNoise – random frequencies

4. Noise travels through a mediumNoise travels through a medium
 A vibrating object creates a disturbanceA vibrating object creates a disturbance
that travels through a mediumthat travels through a medium
 A train’s noise can travel through the steelA train’s noise can travel through the steel
tracks by creating sound wavestracks by creating sound waves
 The vibrations of a speaker creates soundThe vibrations of a speaker creates sound
waveswaves
 Frequency is the number of complete backFrequency is the number of complete back
and forth vibrations per secondand forth vibrations per second

5. Noise travelNoise travel
 Vibrational motion of the medium is set upVibrational motion of the medium is set up
by the object.by the object.
 The vibrations set the molecule of the mediumThe vibrations set the molecule of the medium
into motion.into motion.
 The motion of the molecule in the mediumThe motion of the molecule in the medium
sets the molecule next to it, in motion.sets the molecule next to it, in motion.
 The transfer of energy continues as theThe transfer of energy continues as the
vibration of one molecule sets the nextvibration of one molecule sets the next
molecule into motion.molecule into motion.

6. Sound wave is a pressure waveSound wave is a pressure wave
 Thus an instrument can be used toThus an instrument can be used to
measure the oscillations of high and lowmeasure the oscillations of high and low
pressure variations in the pressure.pressure variations in the pressure.
 These oscillations are shown as theThese oscillations are shown as the
typical sine wave that you may have seentypical sine wave that you may have seen

7. WavelengthWavelength
 Distance which a disturbance travelsDistance which a disturbance travels
along the medium in one complete wavealong the medium in one complete wave
cycle.cycle.
 Measured from one wave trough or crest toMeasured from one wave trough or crest to
the next wave trough or crest, in a transversethe next wave trough or crest, in a transverse
wave. It is from one wave compression to thewave. It is from one wave compression to the
next wave compression in a longitudinal wavenext wave compression in a longitudinal wave
 With a pressure wave it is from one highWith a pressure wave it is from one high
pressure region to the next high pressurepressure region to the next high pressure
regionregion

8. Standing sine wave patterns of air vibrating in aStanding sine wave patterns of air vibrating in a
closed tube. Note the node at the closed end and theclosed tube. Note the node at the closed end and the
antinode at the open end. Only odd multiples of theantinode at the open end. Only odd multiples of the
fundamental are therefore possible.fundamental are therefore possible.

9. Speed of SoundSpeed of Sound
 Sound waves are pressure disturbancesSound waves are pressure disturbances
traveling through a medium by means oftraveling through a medium by means of
particle interactionparticle interaction
 How fast the disturbance is passed fromHow fast the disturbance is passed from
particle to particle determines the speed ofparticle to particle determines the speed of
sound.sound.
 How easily the medium transfers theHow easily the medium transfers the
disturbance determines the speed, which isdisturbance determines the speed, which is
measured in meters per second (m/s)measured in meters per second (m/s)

10. The Speed of SoundThe Speed of Sound
The speed of sound depends on
the material
that the sound is traveling in.
For air, the speed of sound is 343 m/s (767 mph).
Usually, the speed of sound in a liquid is greater
than the speed of sound in a gas.
Usually, the speed of sound in a solid is greater
than the speed of sound in a liquid.

11. IntensityIntensity
 Inverse square relationshipInverse square relationship
 The mathematical relationship of intensity andThe mathematical relationship of intensity and
the distance from the sourcethe distance from the source
 As you move away from the source (largerAs you move away from the source (larger
distance) the area gets larger and thedistance) the area gets larger and the
intensity will decrease.intensity will decrease.
 If the distance from a source doubles the intensityIf the distance from a source doubles the intensity
will decrease by a factor of 4.will decrease by a factor of 4.

12. LoudnessLoudness
 Loudness of a noise is a more subjectiveLoudness of a noise is a more subjective
response. Factors that affect theresponse. Factors that affect the
perception of loudness includes age andperception of loudness includes age and
frequencyfrequency

13. LoudnessLoudness
loudness: a subjective evaluation of a sound. Most closely
related to the pressure amplitude of a sound.
pressure amplitude: the change in pressure from a
condensation to a rarefaction.
A typical pressure amplitude for human speech is 0.03 Pa.
Typical air pressure is 101,000 Pa. The eardrum is a very
sensitive instrument.
A sound with a big pressure amplitude is interpreted as a
loud sound.

14. Sound RangesSound Ranges
A typical young human hears sounds in the range
from
20 Hz to 20,000 Hz (20 kHz)
infrasonic: sounds below the range of human
hearing
(with frequencies less than 20 Hz)
ultrasonic: sounds above the range of human
hearing
(with frequencies more than 20 kHz)

15. Speech frequenciesSpeech frequencies
 Speech frequencies: generally regardedSpeech frequencies: generally regarded
to be 500 to 3000 hertzto be 500 to 3000 hertz
 Frequency range of perceivable sound:Frequency range of perceivable sound:
20 Hz to 15,000 to 20,000 Hertz.20 Hz to 15,000 to 20,000 Hertz.
 Tuning forksTuning forks

16. FrequencyFrequency && PitchPitch
Just as the amplitude of a sound wave relates to its
loudness, the frequency of the wave relates to its pitch.
The higher the pitch, the higher the frequency. The
frequency you hear is just the number of wavefronts that
hit your eardrums in a unit of time. Wavelength doesn’t
necessarily correspond to pitch because, even if wavefronts
are very close together, if the wave is slow moving, not
many wavefronts will hit you each second.
Frequency ↔ Pitch
Amplitude ↔ Loudness

17. PitchPitch
pitch: a subjective evaluation of a sound. Most
closely related to the frequency of the sound.
A sound with a high frequency will sound like a high
pitch.

18. Changing PitchChanging Pitch
 Lungs: Air From theLungs: Air From the
lungs rushes up thelungs rushes up the
tracheatrachea
 Vocal Cords: which areVocal Cords: which are
located in your voicelocated in your voice
box, orbox, or larynxlarynx vibratevibrate
as air rushes pass themas air rushes pass them
 Sound: Sound wavesSound: Sound waves
produced by theproduced by the
vibrating vocal cordsvibrating vocal cords
come out through thecome out through the
mouthmouth

19. Changing PitchChanging Pitch
 Pitch is an important property of musicPitch is an important property of music
 To change pitch, you use the muscles inTo change pitch, you use the muscles in
your throat to stretch and relax your vocalyour throat to stretch and relax your vocal
cordscords
 When your vocal cords stretch they vibrateWhen your vocal cords stretch they vibrate
more quickly, which creates higher-frequencymore quickly, which creates higher-frequency
sound waves with a higher pitchsound waves with a higher pitch
 When you vocal cords relax they vibrateWhen you vocal cords relax they vibrate
slower, which creates lower-frequency soundslower, which creates lower-frequency sound
waves with a lower pitchwaves with a lower pitch
 Musical instruments can also change pitch!Musical instruments can also change pitch!

20. Thunder and LightningThunder and Lightning
If you see a flash of lightning,
and count the number of seconds until you hear the thunder,
you can figure out how far away the lightning is.
How?

21. The Human EarThe Human Ear
The exterior part of the ear (the auricle, or pinna) is made of cartilage
and helps funnel sound waves into the auditory canal, which has wax
fibers to protect the ear from dirt. At the end of the auditory canal lies
the eardrum (tympanic membrane), which vibrates with the incoming
sound waves and transmits these vibrations along three tiny bones
(ossicles) called the hammer, anvil, and stirrup (malleus, incus, and
stapes). The little stapes bone is attached to the oval window, a
membrane of the cochlea.
The cochlea is a coil that converts the vibrations it receives into
electrical impulses and sends them to the brain via the auditory nerve.
Delicate hairs (stereocilia) in the cochlea are responsible for this signal
conversion. These hairs are easily damaged by loud noises, a major
cause of hearing loss!
The semicircular canals help maintain balance, but do not aid hearing.

22. The Human EarThe Human Ear

24. Range of Human HearingRange of Human Hearing
The maximum range of frequencies for most people is from about
20 to 20 thousand hertz. This means if the number of high pressure
fronts (wavefronts) hitting our eardrums each second is from 20 to
20 000, then the sound may be detectable. If you listen to loud
music often, you’ll probably find that your range (bandwidth) will
be diminished.
Some animals, like dogs and some fish, can hear frequencies that are
higher than what humans can hear (ultrasound). Bats and dolphins
use ultrasound to locate prey (echolocation). Doctors make use of
ultrasound for imaging fetuses and breaking up kidney stones.
Elephants and some whales can communicate over vast distances
with sound waves too low in pitch for us to hear (infrasound).

25. EchoesEchoes && ReverberationReverberation
An echo is simply a reflected sound wave. Echoes are more
noticeable if you are out in the open except for a distant, large
object. If went out to the dessert and yelled, you might hear a
distant canyon yell back at you. The time between your yell
and hearing your echo depends on the speed of sound and on
the distance to the to the canyon. In fact, if you know the
speed of sound, you can easily calculate the distance just by
timing the delay of your echo.
Reverberation is the repeated reflection of sound at close
quarters. If you were to yell while inside a narrow tunnel, your
reflected sound waves would bounce back to your ears so
quickly that your brain wouldn’t be able to distinguish between
the original yell and its reflection. It would sound like a single
yell of slightly longer duration.

26. SonarSonar
SOund NAvigation and Ranging
In addition to locating prey, bats and dolphins use sound waves
for navigational purposes. Submarines do this too. The
principle is to send out sound waves and listen for echoes. The
longer it takes an echo to return, the farther away the object that
reflected those waves. Sonar is used in commercial fishing boats
to find schools of fish. Scientists use it to map the ocean floor.
Special glasses that make use of sonar can help blind people by
producing sounds of different pitches depending on how close an
obstacle is.
If radio (low frequency light) waves are used instead of sound
in an instrument, we call it radar (radio detection and ranging).

27. Sonic BoomsSonic Booms
When a source of sound is moving at the
speed of sound, the wavefronts pile up on
top of each other. This makes their
combined amplitude very large, resulting in
a shock wave and a sonic boom. At
supersonic speeds a “Mach cone” is formed.
The faster the source compared to sound, the
smaller the shock wave angle will be.

28. Resonance
the frequency of sound waves
exactly matches the natural frequency
of an object.

29. The Doppler EffectThe Doppler Effect
The Doppler Effect is the change in frequency (pitch) of the
sound detected by an observer because the sound source
and the observer have different velocities with respect to the
medium of sound propagation.
The Doppler Effect describes why the pitch of a siren that is
approaching you sounds higher than the pitch of a siren that
is moving away from you.

30. The Doppler EffectThe Doppler Effect
Moving SOURCE
Approaching observer
If the source of the sound (the fire truck, for example) is
moving towards an observer, the sound waves “bunch up” in
front of the source, causing the wavelength observed by a
stationary person to shorten.
(Smaller wavelengths, bigger frequency.)
Receding from observer
Behind the source, the sound waves “stretch out”, and the
wavelength observed by a stationary person lengthens.
(Bigger wavelength, smaller frequency.)

31. Doppler EffectDoppler Effect
A tone is not always heard at the same frequency at which it is
emitted. When a train sounds its horn as it passes by, the pitch of
the horn changes from high to low. Any time there is relative
motion between the source of a sound and the receiver of it, there is
a difference between the actual frequency and the observed
frequency. This is called the Doppler effect. Click to hear effect:
The Doppler effect applied to electomagnetic waves helps
meteorologists to predict weather, allows astronomers to estimate
distances to remote galaxies, and aids police officers catch you
speeding.
The Doppler effect applied to ultrasound is used by doctors to
measure the speed of blood in blood vessels, just like a cop’s radar
gun. The faster the blood cell are moving toward the doc, the greater
the reflected frequency.