Natural Frequency, Interference, and the Doppler

Effect

Natural Frequency, Interference, and the Doppler

EffectSec 13.7 – 13.10Sec 13.7 – 13.10

Think back to the Sound Wave Lab

Strike an unmounted tuning fork – faint soundTouch the tuning fork to a styrofoam cup and it gets louder

Vibration of the cup is a forced vibration

Other examplesFactory floor due to heavy machinerySounding boards of a stringed instrument

Natural Frequency

A wrench and a bat do not make the same sound when dropped on the floor.

They each vibrate at their own special frequency called natural frequency.

An object’s natural frequency depends on:ElasticityShape

Actually, if you could sing at the natural frequency of a glass loud enough, you could shatter it!!

ResonanceResonance is when the frequency of forced vibrations imposed on an object matchesResonance means to sound againDemonstration

Hold two tuning forks set to vibrate at the same frequencyStrike one forkThe other is then set into vibration (without touching)

Matching FrequenciesIf the tuning forks are not at matching frequencies, resonance does not occurTuning your Radio

When you change the station on the radio, you are changing the natural frequency of the electronics to match the frequency of a radio stationThis way you hear one radio station and not all of them.

InterferenceConsider Transverse Waves…Interference happens when the crest of one wave overlaps with the crest of another wave

Constructive interference

It also happens when the crest of one wave overlaps with the trough of another wave

Destructive interference

Constructive Interference

Destructive Interference

BeatsWhen two tones of slightly different frequencies are sounded together, a fluctuation in loudness is heard.

Loud, then faint, then loud, then faint, etc.This periodic variation in loudness is called beats

When frequencies are identical, the beats go away.

Standing Waves

Imagine having a rope attached to a wall.You move it up and down to make a wave.It reflects off the wall, interfering with waves you are creating.This causes a standing wave, where the nodes are stationary.The positions on a standing wave that have the largest displacements are known as antinodes.

Doppler EffectImagine a bug jiggling in a puddle of water.As the bug moves, so does the wave.

When this happens, the frequency changes.Depending on where an observer is standing, the frequencies could be different.

Doppler Effect

The change in frequency due to motion is called the Doppler Effect.

Sound waves exhibit the Doppler Effect as well.

Moving ambulanceThe pitch of the siren changes as it approaches and then passes you.Approach – pitch higher, more frequentMoving away – pitch lower, less frequent

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