SoundSound13-113-1

A “physical phenomenon that stimulates the

sense of hearing.”

What do you think?What do you think?• What is sound?What is sound?

• What do all of the sounds that you hear have in What do all of the sounds that you hear have in common?common?

• How do they differ?How do they differ?• Can sounds travel through solids? Liquids? Can sounds travel through solids? Liquids?

Gases?Gases?• Is one type of material better for transmitting Is one type of material better for transmitting

sound waves?sound waves?• When race cars or emergency vehicles pass When race cars or emergency vehicles pass

you, the sound changes. In what way, and you, the sound changes. In what way, and why?why?

What is Sound?What is Sound? Sound is a wave that is Sound is a wave that is

produced by vibrating produced by vibrating material objects.material objects. Tuning forks, guitar strings, Tuning forks, guitar strings,

drum head, vocal cords, drum head, vocal cords, speakersspeakers

The vibrating object pushes The vibrating object pushes the air molecules together, the air molecules together, forming a forming a compressioncompression. .

It then spreads them apart, It then spreads them apart, forming a forming a rarefactionrarefaction..

Graphing Sound WavesGraphing Sound Waves The diagram shows The diagram shows

compressions (dark) and compressions (dark) and rarefactions(white). If rarefactions(white). If you measured the you measured the pressure or density of pressure or density of the air and plotted these the air and plotted these against position, how against position, how would the graph appear?would the graph appear?

Sound is a Sound is a mechanical wavemechanical wave requires a medium.requires a medium.

Cannot travel Cannot travel through a vacuumthrough a vacuum

““Nobody can hear Nobody can hear you scream in you scream in space”space”

In air, sound is a In air, sound is a longitudinal longitudinal (pressure) wave(pressure) wave

Sound TransmissionSound Transmission

Demo: Sound in a Demo: Sound in a vacuumvacuum

Click below to watch the Visual Concept.

Visual Concept

Sound WavesSound Waves

Characteristics of SoundCharacteristics of Sound

FrequencyFrequency is the number of waves per second. is the number of waves per second. You have heard of You have heard of ultrasoundultrasound. What is it?. What is it? Frequencies audible to humans are between Frequencies audible to humans are between

20 Hz and 20 000 Hz.20 Hz and 20 000 Hz. Middle C on a piano is 262 Hz.Middle C on a piano is 262 Hz. The emergency broadcast signal is 1 000 Hz.The emergency broadcast signal is 1 000 Hz.

InfrasoundInfrasound frequencies are lower than 20 Hz. frequencies are lower than 20 Hz. UltrasoundUltrasound frequencies are greater than frequencies are greater than

20 000 Hz. 20 000 Hz.

Click below to watch the Visual Concept.

Visual Concept

Comparing Infrasonic and Comparing Infrasonic and Ultrasonic SoundsUltrasonic Sounds

PitchPitch Pitch is the human perception of how Pitch is the human perception of how

high or low a sound appears to be.high or low a sound appears to be. Pitch is primarily determined by Pitch is primarily determined by

frequency.frequency. Pitch also depends slightly on other Pitch also depends slightly on other

factors.factors. Higher frequencies appear to have a higher Higher frequencies appear to have a higher

pitch when played loudly, even though the pitch when played loudly, even though the frequency does not change.frequency does not change.

Speed of SoundSpeed of Sound

Sound waves travel though solids, Sound waves travel though solids, liquids and gases.liquids and gases. In which would the speed generally be In which would the speed generally be

greatest? Why?greatest? Why? Solids. Because the molecules are more Solids. Because the molecules are more

closely packed, the particles respond more closely packed, the particles respond more rapidly to compressions.rapidly to compressions.

Demo: Sound Demo: Sound Propagation Propagation

Sound travels better Sound travels better through elastic through elastic liquids and solids, liquids and solids, such as water and such as water and rocks, than through rocks, than through air.air.

This is due to the This is due to the close proximity of close proximity of the atoms as they the atoms as they vibrate.vibrate.

How might the temperature of air affect the speed of How might the temperature of air affect the speed of sound waves? Why?sound waves? Why?

Higher temperature increases the speed of the waves because the Higher temperature increases the speed of the waves because the particles are moving faster and colliding more often.particles are moving faster and colliding more often.

Where Where v v is the speed of sound in air measured in m/s is the speed of sound in air measured in m/s and and T T is the temperature of the air measured in is the temperature of the air measured in degrees Celsius (degrees Celsius (ooC).C).

Speed of Sound in AirSpeed of Sound in Air

331 0.6soundv T

32

1.8

o

oF

C

TT

CCheck heck YYourselfourself

Find the speed of sound in air at Find the speed of sound in air at room temperature (70room temperature (70ooF).F).

Speed of SoundSpeed of Sound

Spherical WavesSpherical Waves In which directions does In which directions does

sound propagate?sound propagate? The diagram shows:The diagram shows:

Crests or wave fronts (blue Crests or wave fronts (blue circles)circles)

Wavelength (Wavelength ()) Rays (red arrows)Rays (red arrows)

Rays indicate the direction Rays indicate the direction of propagation.of propagation.

How would these wave How would these wave fronts appear different if fronts appear different if they were much farther they were much farther from the source?from the source?

Spherical WavesSpherical Waves

Wave fronts and rays Wave fronts and rays become more nearly become more nearly parallel at great parallel at great distances.distances.

Plane waves are simply Plane waves are simply very small segments of very small segments of a spherical wave a long a spherical wave a long distance from the distance from the source.source.

Reflection of Sound Reflection of Sound WavesWaves

Sound waves can reflect off of surfaces.Sound waves can reflect off of surfaces. An An EchoEcho is a reflection of sound emitted is a reflection of sound emitted

from a source.from a source. The human ear can only distinguish The human ear can only distinguish

sounds separated by about 1/10 of a sounds separated by about 1/10 of a second (0.1s).second (0.1s). i.e. For you to hear an echo, the reflected i.e. For you to hear an echo, the reflected

sound must arrive at least 0.1s after it was sound must arrive at least 0.1s after it was emitted.emitted.

SSample Problem:ample Problem:

What is the minimum distance away What is the minimum distance away a reflective surface must be in order a reflective surface must be in order for you to hear an echo on a day for you to hear an echo on a day when it is 20when it is 20ooC.C.

What do you think?What do you think?• On a blank sheet of paper reflect on the following:On a blank sheet of paper reflect on the following:• What is sound?What is sound?

• What do all of the sounds that you hear have in What do all of the sounds that you hear have in common?common?

• How do they differ?How do they differ?• Can sounds travel through solids? Liquids? Gases?Can sounds travel through solids? Liquids? Gases?

• Is one type of material better for transmitting Is one type of material better for transmitting sound waves?sound waves?

• When race cars or emergency vehicles pass you, When race cars or emergency vehicles pass you, the sound changes. In what way, and why?the sound changes. In what way, and why?