Chapter 5Wave Motions and Sound

Vibrations

• Repetitive, back-and-forth motion– Periodic or oscillation

• Cycle – one complete vibration

• Period – time for one cycle

• Frequency – number of cycles per second

(units = hertz, Hz)• Period and frequency

inversely related.

• Repetitive, back-and-forth motion– Periodic or oscillation

• Cycle – one complete vibration

• Period – time for one cycle

• Frequency – number of cycles per second

(units = hertz, Hz)• Period and frequency

inversely related.

Fig 5.4 (Simple Harmonic Motion)

Waves

• Disturbance that moves thru a medium • Causes

– Periodic motion– Pulse

• Two major considerations about a wave?– Traveling Disturbance– Transports Energy

• Mechanical waves– Require medium for propagation– Waves move through medium– Medium remains in place.

• Disturbance that moves thru a medium • Causes

– Periodic motion– Pulse

• Two major considerations about a wave?– Traveling Disturbance– Transports Energy

• Mechanical waves– Require medium for propagation– Waves move through medium– Medium remains in place.

Kinds of Waves

Longitudinal waves• Vibration direction parallel

to wave propagation direction

• Particles in medium move closer together/farther apart– Example: sound waves

• Gases and liquids – Support only longitudinal

waves.

Longitudinal waves• Vibration direction parallel

to wave propagation direction

• Particles in medium move closer together/farther apart– Example: sound waves

• Gases and liquids – Support only longitudinal

waves.

Kinds of Waves (cont’d)

Transverse waves• Vibration direction

perpendicular to wave propagation direction– Example: plucked string

• Solids – Support both longitudinal

and transverse waves.

Transverse waves• Vibration direction

perpendicular to wave propagation direction– Example: plucked string

• Solids – Support both longitudinal

and transverse waves.

Waves in Air

• Longitudinal waves only• Large scale - swinging door

creates macroscopic currents• Small scale - tuning fork

creates sound waves• Series of:

– Condensations (overpressures) and

– Rarefactions (underpressures).

• Longitudinal waves only• Large scale - swinging door

creates macroscopic currents• Small scale - tuning fork

creates sound waves• Series of:

– Condensations (overpressures) and

– Rarefactions (underpressures).

Describing Waves

Graphical representation

• Pure harmonic waves = sines or cosines

• Wave terminology• Wave propagation speed.

Graphical representation

• Pure harmonic waves = sines or cosines

• Wave terminology• Wave propagation speed.

Sound Waves

• Require medium for transmission

• Speed varies with– Inertia of molecules– Interaction strength

• Various speeds of sound.

• Require medium for transmission

• Speed varies with– Inertia of molecules– Interaction strength

• Various speeds of sound.

Velocity of Sound in Air

• Varies with temperature• Greater kinetic energy thus sound impulse transmitted faster• Increase factor (units!):

0.6 m/s per °C 2.0 ft/s per °C.

Eq 4.2

HW BONUS QuestionHW BONUS QuestionIf the air temperature is 86ºF during a thunderstorm andIf the air temperature is 86ºF during a thunderstorm and

the thunder took 2.3 seconds after the lightning was seen.the thunder took 2.3 seconds after the lightning was seen. How far away was the lightning strike? How far away was the lightning strike?

Determine the answer in feet and then convert to miles. Determine the answer in feet and then convert to miles.

HW BONUS QuestionHW BONUS QuestionIf the air temperature is 86ºF during a thunderstorm andIf the air temperature is 86ºF during a thunderstorm and

the thunder took 2.3 seconds after the lightning was seen.the thunder took 2.3 seconds after the lightning was seen. How far away was the lightning strike? How far away was the lightning strike?

Determine the answer in feet and then convert to miles. Determine the answer in feet and then convert to miles.

Then what?

“Visualization” of Sound Waves

• Boundary effects– Refraction – Reflection– Absorption.

• Boundary effects– Refraction – Reflection– Absorption.

Refraction

Bending of wave fronts uponencountering a boundary.

Bending of wave fronts uponencountering a boundary.

Reflection

Wave rebounding off boundary surface.Wave rebounding off boundary surface.

Absorption

Wave energy dissipated.Wave energy dissipated.

Sounds from Moving Sources

• Doppler effect– Wave pattern changed by

motion of source or observer– Moving Towards - shifted to

higher frequency (blue) – Moving Away - shifted to

lower frequency (red).

• Doppler effect– Wave pattern changed by

motion of source or observer– Moving Towards - shifted to

higher frequency (blue) – Moving Away - shifted to

lower frequency (red).

Doppler Homepage

Sounds from Moving Sources

• Supersonic speed – Shock wave and sonic

boom produced.

• Supersonic speed – Shock wave and sonic

boom produced.

http://www.wilk4.com/misc/soundbreak.htm

Bell X-1

Next Time: Quiz 2 (Ch 4 and 5)then Electricity