Sound WavesSound Waves

Sound...Sound... ...a ...a longitudinallongitudinal wave in air caused by a wave in air caused by a

vibrating object.vibrating object.

Produced by tiny fluctuations of air Produced by tiny fluctuations of air pressurepressure

Carried through air at 345 m/s (770 m.p.h) Carried through air at 345 m/s (770 m.p.h) as as compressionscompressions and and rarefactionsrarefactions..

wavelengthcompressed gas

rarefied gas

Waves in air can’t be transverse, because Waves in air can’t be transverse, because the molecules are not bound to each other.the molecules are not bound to each other.

Air molecules can only bump into one Air molecules can only bump into one another.another.

Why is Sound Longitudinal?Why is Sound Longitudinal?

Origin of SoundOrigin of Sound infrasonicinfrasonic

– frequencies < 20 Hzfrequencies < 20 Hz

ultrasonicultrasonic

– frequencies > 20,000 Hzfrequencies > 20,000 Hz

human hearing rangehuman hearing range

– frequencies between 20 Hz and 20,000 Hzfrequencies between 20 Hz and 20,000 Hz

The frequency of an audible sound The frequency of an audible sound determines how high or low we perceive the determines how high or low we perceive the sound to be. sound to be. (pitch)(pitch)

greater frequency = greater the pitch.greater frequency = greater the pitch.

Origin of Sound (Cont.)Origin of Sound (Cont.)

Nature of Sound in Air and SolidsNature of Sound in Air and Solids

Speed of sound in air is related to the Speed of sound in air is related to the frantic motions of molecules as they jostle frantic motions of molecules as they jostle and collide.and collide.

Solids have faster sound speeds due to close Solids have faster sound speeds due to close molecule proximity & molecular bonding.molecule proximity & molecular bonding.– don’t have to rely on atoms to traverse gapdon’t have to rely on atoms to traverse gap

– spring compression can (and does) travel faster than actual spring compression can (and does) travel faster than actual atom motion.atom motion.

MediumMedium sound speed (m/s)sound speed (m/s)

air (25air (25C)C) 345345

waterwater 14901490

goldgold 32403240

brickbrick 36503650

woodwood 3800–46003800–4600

glassglass 51005100

steelsteel 57905790

aluminumaluminum 51005100

Example Sound SpeedsExample Sound Speeds

What is the approximate distance of a What is the approximate distance of a thunderstorm when you note a 3 second thunderstorm when you note a 3 second delay between the flash of the lightning and delay between the flash of the lightning and the sound of the thunder?the sound of the thunder?

Answer: 3 seconds Answer: 3 seconds 340 meters/second 340 meters/second

= 1020 meters= 1020 meters

Sample ProblemSample Problem

DOPPLER EFFECTDOPPLER EFFECT ……a frequency shift that is the result of a frequency shift that is the result of

relative motion between the source of relative motion between the source of the waves and the observer.the waves and the observer.

Doppler with SoundDoppler with Sound

Some applications of the Doppler effectSome applications of the Doppler effect

Doppler flow meter measures speed of blood flow in blood vessels. A transmitter generates high frequency (~5MHz) “sounds” that move through the body and bounce off red blood cells. The motion of the blood cells cause a Doppler shift in the sound waves (in the hundreds of Hz range), that can be measured by a receiver.

Ultrasound, sonar, echolocation and seismic explorationUltrasound, sonar, echolocation and seismic exploration are all ways in which are all ways in which machines or animals can image the surroundings using sound waves. A sound machines or animals can image the surroundings using sound waves. A sound wave is emitted, bounces off reflective surfaces in the environment, and the wave is emitted, bounces off reflective surfaces in the environment, and the reflected signals are observed. The time delay between transmission and reflected signals are observed. The time delay between transmission and reception of the wave is related to the distance to the reflecting surface, reception of the wave is related to the distance to the reflecting surface, transmission and reflection properties of the media, etc., and this information can transmission and reflection properties of the media, etc., and this information can be used to reconstruct an image of the surroundings. be used to reconstruct an image of the surroundings.

ultrasound image of a fetus sonar image of a shipwreck in Lake Ontario

Some applications of Doppler effectSome applications of Doppler effect

Doppler radar used in weather forecasting to detect local velocities in a storm system: same principle as with the Doppler effect in sound waves, only here the frequency shift is in radio waves bouncing off moving water droplets in a storm.

Some applications of Doppler effectSome applications of Doppler effect

Radar

True Velocity

Tangential Velocity

Radial Velocity

Some applications of Doppler effectSome applications of Doppler effect