Chapter 13Chapter 13

SoundSound

Section 13.1Section 13.1

Sound WavesSound Waves

The Production of Sound WavesThe Production of Sound Waves Sound is a result of vibrations or Sound is a result of vibrations or oscillationsoscillations.. Ex: As the prong in the tuning fork swings to the Ex: As the prong in the tuning fork swings to the

right, the air molecules in front are forced together. right, the air molecules in front are forced together. As the prong moves to the left, the molecules to the As the prong moves to the left, the molecules to the right spread apart, and the density and air pressure right spread apart, and the density and air pressure in this region became lower than normal.in this region became lower than normal.

A region of lower density and pressure isA region of lower density and pressure is rarefactionrarefaction. .

A region of high molecular density and high air A region of high molecular density and high air pressure is calledpressure is called compressioncompression. .

Sounds waves are Sounds waves are longitudinal waveslongitudinal waves

A longitudinal wave produced by a A longitudinal wave produced by a vibrating object can be represented by a vibrating object can be represented by a sine curvesine curve. .

The The crestscrests in the curve correspond to in the curve correspond to compressions in the sound wave, and the compressions in the sound wave, and the troughstroughs correspond to rarefactions. correspond to rarefactions.

Characteristics of Sound WavesCharacteristics of Sound Waves

Speed of Sound VideoSpeed of Sound Video FrequencyFrequency = the number of cycles per unit of = the number of cycles per unit of

time.time. InfrasonicInfrasonic waves < 20Hz waves < 20Hz UltrasonicUltrasonic waves > 20,000 Hz waves > 20,000 Hz 20 Hz < 20 Hz < audible sound wavesaudible sound waves (humans) < (humans) <

20,000 Hz20,000 Hz

• Dogs can hear ultrasonic waves, and Dogs can hear ultrasonic waves, and

elephants can hear infrasonic waves.elephants can hear infrasonic waves.

Cont.- Frequency determines pitchCont.- Frequency determines pitch

The frequency of an audible sound wave The frequency of an audible sound wave determines how high of low we perceive determines how high of low we perceive the sound to be, known as the sound to be, known as pitchpitch..

As the frequency of a sound wave As the frequency of a sound wave increases, the pitch rises. increases, the pitch rises.

As the frequency As the frequency

decreases, the pitch falls.decreases, the pitch falls.

Ultrasonic waves can produce imagesUltrasonic waves can produce images When wavelength decreases frequency When wavelength decreases frequency

increases. increases. Infrasonic waves have longer wavelengths than Infrasonic waves have longer wavelengths than

audible sound waves and ultrasonic waves have audible sound waves and ultrasonic waves have shorter wavelengths.shorter wavelengths.

AudibleAudible

UltrasonicUltrasonic

InfrasonicInfrasonic

FYIFYI Ultrasonic waves have widespread medical applications Ultrasonic waves have widespread medical applications

like to produce images of objects inside the human body.like to produce images of objects inside the human body. Sound waves are partially reflected when they reach a Sound waves are partially reflected when they reach a

boundary between two materials of different densities. boundary between two materials of different densities. They are clearer and more detailed than other those of a They are clearer and more detailed than other those of a

lower-frequency sound wave because the short lower-frequency sound wave because the short wavelengths of ultrasonic waves are easily reflected off wavelengths of ultrasonic waves are easily reflected off small objects. small objects.

Audible and infrasonic sound Audible and infrasonic sound waves are not as effective waves are not as effective because their longer because their longer wavelengths pass around wavelengths pass around small objects.small objects. A typical wave used in an A typical wave used in an ultrasonic device is about ultrasonic device is about 10 MHz.10 MHz.

Speed of sound depends on the Speed of sound depends on the mediummedium

Sound waves can travel through solids, Sound waves can travel through solids, liquids, and gases.liquids, and gases.

Solid particles respond more quickly to Solid particles respond more quickly to vibrations than gas particles because vibrations than gas particles because those molecules are closer together.those molecules are closer together. Therefore, sound travels faster in solids.Therefore, sound travels faster in solids.

TemperatureTemperature

In a gas, the disturbance can spread faster In a gas, the disturbance can spread faster at higher temperatures than at lower ones at higher temperatures than at lower ones because of an increase in collisions.because of an increase in collisions. The temperature differences between liquids The temperature differences between liquids

and solids is less noticeable due to closer and solids is less noticeable due to closer particles.particles.

Sound waves propagate in Sound waves propagate in three dimensionsthree dimensions

Sound waves travel in all three Sound waves travel in all three dimensions.dimensions.

In class, we work from the premise that all In class, we work from the premise that all sound waves are spherical unless stated sound waves are spherical unless stated otherwise.otherwise.

The circles represent the centers of The circles represent the centers of compressions, compressions, wave frontswave fronts..

The distance between adjacent wave fronts is The distance between adjacent wave fronts is equal to equal to one wavelengthone wavelength..

The radial lines perpendicular to the wave The radial lines perpendicular to the wave fronts are fronts are raysrays. They indicate the direction of . They indicate the direction of the wave motion.the wave motion.

Each wave front crossed by a ray corresponds Each wave front crossed by a ray corresponds to a crest of the sine curve.to a crest of the sine curve.

At great distances from the At great distances from the source, wave fronts appear source, wave fronts appear almost parallel and are almost parallel and are called called plane waves.plane waves.

Plane waves Plane waves can be can be treated like a series of treated like a series of identical linear waves.identical linear waves.

The Doppler EffectThe Doppler Effect

Doppler Effect VideoDoppler Effect Video 2:01 2:01 Pitch is higher as the object moves towards Pitch is higher as the object moves towards

you and lower as it moves away.you and lower as it moves away.

Relative motion creates a Relative motion creates a change in frequencychange in frequency

The frequency of the car horn is constant, but The frequency of the car horn is constant, but because the source is moving towards observer because the source is moving towards observer A, the wave fronts hit Observer A sooner and A, the wave fronts hit Observer A sooner and sooner. As a result, Observer A hears a greater sooner. As a result, Observer A hears a greater frequency sound resulting in frequency sound resulting in

higher pitch.higher pitch.

Remember we are discussing Remember we are discussing frequency and not speed. frequency and not speed. The speed of the sound The speed of the sound waves remains constant.waves remains constant.

Relative motion creates a change Relative motion creates a change in frequencyin frequency

The sound waves don’t reach point B, behind The sound waves don’t reach point B, behind the car as frequently as in front of it. Therefore, the car as frequently as in front of it. Therefore, the frequency heard by the observer is the frequency heard by the observer is lessless than than the source frequency. the source frequency.

The Doppler effectThe Doppler effect

is a shift in frequency is a shift in frequency

due to motion between due to motion between

a wave source and a wave source and

an observer.an observer.

The Doppler effect will occur when the source or The Doppler effect will occur when the source or the observer is moving. the observer is moving.

Even though the it is used mostly with sound Even though the it is used mostly with sound waves, the Doppler effect can be used with waves, the Doppler effect can be used with electromagnetic waves like visible light.electromagnetic waves like visible light.

The Doppler Effect AppletThe Doppler Effect Applet

HomeworkHomework

P 486 #1 - 4, 6P 486 #1 - 4, 6