CompressionaCompressional Wavesl Waves

Requires a medium for Requires a medium for propagation.propagation.

Compression of molecules Compression of molecules transmit sound. transmit sound.

Compression - molecules of Compression - molecules of medium are crowded medium are crowded together together by energy.by energy.

Rarefaction - molecules of Rarefaction - molecules of medium are far apart.medium are far apart.

- 1 compression + 1 - 1 compression + 1 rarefactionrarefaction

- Compression to Compression to compressioncompression

- Rarefaction to rarefactionRarefaction to rarefaction

Number of Number of CompressionalCompressional waves that pass a point in 1 s.waves that pass a point in 1 s.

v = fv = fλλ || v = d/tv = d/t v = velocityv = velocity |velocity = |velocity = distancedistance f = frequencyf = frequency timetime λλ = wave length = wave length

Amount of compressionAmount of compression Increase in energy Increase in energy

increases compressionincreases compression

High Amplitude

Low Amplitude

Depends on :Depends on : Medium (Ex. air, water)Medium (Ex. air, water)

Wave travels faster in more Wave travels faster in more dense media.dense media.

Temperature of the mediumTemperature of the medium Slower in cold air than warm air.Slower in cold air than warm air. Warm molecules move faster than Warm molecules move faster than cold molecules.cold molecules.

1 million times faster 1 million times faster than soundthan sound

Ex. LightningEx. LightningSee lightning before you See lightning before you hear thunderhear thunder

At 0° C = 332 m/sAt 0° C = 332 m/s

VVair air = 332m/s + ( = 332m/s + (o o C x 0.6 m/s)C x 0.6 m/s)

Find speed of sound in air at 20° C Find speed of sound in air at 20° C

VVair air = 332 m/s + (20 = 332 m/s + (20o o C x 0.6 m/s) = C x 0.6 m/s) = 344m/s344m/s

At 10° C lightning strikes and 8 s later At 10° C lightning strikes and 8 s later you hear thunder. How close is the you hear thunder. How close is the lightning to you ?lightning to you ?

Distance = time x speed of soundDistance = time x speed of sound

d = t x v (at 10° C) d = t x v (at 10° C) d = (8 s) x 332 m/s + (10° C x 0.6m/s)d = (8 s) x 332 m/s + (10° C x 0.6m/s) d = 2704 m d = 2704 m

At 0° C = 1454 m/sAt 0° C = 1454 m/s

V V waterwater = 1454 m/s + = 1454 m/s + ooC (2.3 m/s)C (2.3 m/s)

Find speed of sound in water at 20Find speed of sound in water at 20o o C.C.

V V waterwater = 1454 m/s + (20= 1454 m/s + (20o o C x 2.3 m/s) = 1500 C x 2.3 m/s) = 1500

m/sm/s

A shipA ship’’s sonar pings the bottom of the s sonar pings the bottom of the ocean whose temperature is 30° C . The ocean whose temperature is 30° C . The sound travels down and back in 3.8 s. sound travels down and back in 3.8 s.

d = t x v (at 30° C )d = t x v (at 30° C )

d = (d = (3.8 s3.8 s) x 1454 m/s + (30° C x 2.3m/s) = ) x 1454 m/s + (30° C x 2.3m/s) = 2762.6 m 2762.6 m

22

The "highness" or "lowness" of a sound.

Depends on frequency. Higher Frequency = Higher

Pitch Sound causing glass to resonat

e

A Human ear can hear sound frequencies between 20 Hz and 20,000 Hz

Ultrasonic waves - > 20,000 Hz (heard by bats and dogs)

Infrasonic waves - less than 20 Hz

Used by humans for imaging purposes

Used by many animals for echolocation (sensing space and surroundings through sound)

Porpoises can here up to 160,000 Hz

Can be generated by earthquakes, volcanoes, aurorae, meteors, oceans, and upper atmosphere lightning.

Used by whales to communicate over hundreds of miles.

Can cause unsettling feelings in humans.

Intensity - depends on amount of energy in wave. This is the sound's amplitude.

Measured in decibels (dB)

Threshold of Hearing 0 dBRustling Leaves 10 dBWhisper 20 dBNormal Conversation 60 dBBusy Street Traffic 70 dBLarge Orchestra 98 dBCD Player Max Vol 100 dBVacuum Cleaner 107 dBFront Rows of Rock Concert 110 dB

Threshold of Pain 130 dBMilitary Jet Takeoff 140 dBInstant Perforation of Eardrum 160 dB

Any sound > 120 dB may cause pain and permanent hearing loss.

As you age you cannot hear sounds of higher frequencies.

As an object producing a wave (ie. sound) approaches an observer the waves become compact, increasing the frequency. As the object leaves the waves spread out

doppler effect how it works Doppler effect train

supersonic car

A sonic boom is heard when an object enters supersonic speeds.

Longitudinal waves – compression rarefaction

Transverse Wave – up and down motion

Ocean waves and vibrating strings produce transverse waves.

Sound and produce longitudinal waves (compression waves)

Earthquakes produce both (S waves and P waves)

Cycle = 1 crest + 1 trough or 1 compression + 1 rarefaction

Frequency (f) = cycles/second Measured in Hertz (Hz)

Period (T) = seconds/cycle

standing wave in water

Mechanical waves, such as sound and ocean waves require a medium.

Electromagnetic waves, such as light do not.

Reflection: when a wave bounces off of an object (ie. light off a mirror, an echo.)

Refraction: when a wave bends as it moves from one medium to another (ie. the illusion of a bent straw in water.)

Diffraction: when a wave bends around the edge of an of an object or through a slit.

Interference is when waves from two different sources run into each other.

Destructive interference is when the trough of one wave matches the crest of another. The two waves cancel each other out.

Constructive interference is when the crest of one wave overlaps the crest of another and the two waves add together increasing their amplitude.