Sound Waves Physics Chapter 13 Section 1

I. Production of sound waves Produced by an object vibrating Produced by an object vibrating -ex. Tuning fork Prongs move back and forth pushing the air molecules together or spreading them apart Prongs move back and forth pushing the air molecules together or spreading them apart

Compression  medium molecules move together Compression  medium molecules move together  higher density and pressure  higher density and pressure Rarefaction  medium molecules spread apart Rarefaction  medium molecules spread apart  lower density and pressure  lower density and pressure Online Tuning Forks Online Tuning Forks

As the tuning fork vibrates it sends a series of compressions and rarefactions that expand out in all directions. As the tuning fork vibrates it sends a series of compressions and rarefactions that expand out in all directions. *like pond ripples *air molecules vibrate Sound waves are longitudinal waves (particles move parallel to wave direction) Sound waves are longitudinal waves (particles move parallel to wave direction) Compresional waves video Compresional waves video

II. Characteristics of sound waves Frequency – number of cycles per unit of time Frequency – number of cycles per unit of time audible sound waves (humans can hear) audible sound waves (humans can hear)  frequency range 20 Hz to 20,000 Hz frequencies < 20 Hz  infrasonic (very long wavelength) sound waves. frequencies < 20 Hz  infrasonic (very long wavelength) sound waves. frequencies > 20,000 Hz  ultrasonic (short wavelength) sound waves. frequencies > 20,000 Hz  ultrasonic (short wavelength) sound waves.

audible depends on our ability to detect audible depends on our ability to detect factors: age, ear damage due to excessive loud noises factors: age, ear damage due to excessive loud noises

as frequency increases, wavelength decreases as frequency increases, wavelength decreases frequency determines pitch frequency determines pitch pitch: how high or low we perceive a sound to be pitch: how high or low we perceive a sound to be depending on the frequency of a sound wave. depending on the frequency of a sound wave. low pitch = low frequency low pitch = low frequency high pitch = high frequency high pitch = high frequency * a perceived measurement* * a perceived measurement*

ultrasonic waves produce images of objects ultrasonic waves produce images of objects  short wavelength  provides medical uses - images of internal body structures - images of internal body structures - waves reflect off small objects of varying - waves reflect off small objects of varying densities densities

ex. Ultrasounds of fetuses ex. Ultrasounds of fetuses Ultrasonic pulses emitted and received as they reflect off of fetal tissues Ultrasonic pulses emitted and received as they reflect off of fetal tissues Ultrasound Pictures Ultrasound PicturesUltrasound PicturesUltrasound Pictures ex. Echolocation of dolphins and bats ex. Echolocation of dolphins and bats

Speed of sound depends on the medium Speed of sound depends on the medium  sound waves can pass through solids,  sound waves can pass through solids, liquids, and/or gases (mechanical waves) liquids, and/or gases (mechanical waves)

Depends on how quickly particles of the medium can transfer the vibration Depends on how quickly particles of the medium can transfer the vibration More dense (particles are closer together) = faster transfer of energy More dense (particles are closer together) = faster transfer of energy  solids generally conduct sound the fastest  gases generally conduct sound the slowest

Also depends on the temperature of the medium Also depends on the temperature of the medium higher temperature = particles of the medium colliding more frequently higher temperature = particles of the medium colliding more frequently  faster sound waves  faster sound waves

Each medium has its own set of values for speed of sound (table 13-1 page 482) Each medium has its own set of values for speed of sound (table 13-1 page 482)  In normal air (25 o C) the speed of sound equals 346 m/s equals 346 m/s

Sound waves spread out (propagate) in 3 dimensions in approximately spherical patterns Sound waves spread out (propagate) in 3 dimensions in approximately spherical patterns Represented on paper Represented on paper in 2-D as concentric in 2-D as concentric circles circles

Distance between each adjacent wave front = Distance between each adjacent wave front = 1 wavelength ( ) 1 wavelength ( ) Rays indicate direction of travel of the wave fronts Rays indicate direction of travel of the wave fronts Plane wave – a segment of a wave front very far from the source that appears to be a straight line and parallel to the adjacent wave fronts Plane wave – a segment of a wave front very far from the source that appears to be a straight line and parallel to the adjacent wave fronts

Doppler Effect – frequency shift that is the result of relative motion between the source of waves and an observer Doppler Effect – frequency shift that is the result of relative motion between the source of waves and an observer (page 485 figure 13-6) Doppler effect Doppler effect link

Example: #1  A car (train, ambulance) moving toward or away from an observer while blowing the horn or siren Example: #1  A car (train, ambulance) moving toward or away from an observer while blowing the horn or siren  Pitch appears to change, but the frequency is not changing  Frequency of the source remains constant  Wave fronts will reach the observer in front of the moving source more often (with higher frequency) due to source moving toward the frequency) due to source moving toward the observer observer  Wave fronts “pile up” on each other  Wave fronts “pile up” on each other  Pitch of sound gets higher

Example #2  Source moving away from observer Example #2  Source moving away from observer  Wave fronts will reach observer behind a moving source less often (lower frequency) moving source less often (lower frequency) due to source moving away from observer due to source moving away from observer  Wave fronts “spread out”  Pitch sounds lower

** Speed of the sound waves does not change Same will happen with a stationary source and a moving observer Same will happen with a stationary source and a moving observer Doppler effect occurs whenever there is relative motion between an observer and a source of wave fronts Doppler effect occurs whenever there is relative motion between an observer and a source of wave fronts ** Most common to sound waves, but happens with all waves - Moving faster than the speed of sound Moving faster than the speed of soundMoving faster than the speed of sound