Natural Frequency, Interference, and the Doppler Effect Sec 13.7 – 13.10

Think back to the Sound Wave Lab Strike an unmounted tuning fork – faint sound Touch the tuning fork to a styrofoam cup and it gets louder Vibration of the cup is a forced vibration Other examples Factory floor due to heavy machinery Sounding 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: Elasticity Shape Actually, if you could sing at the natural frequency of a glass loud enough, you could shatter it!!

Resonance Resonance is when the frequency of forced vibrations imposed on an object matches Resonance means to sound again Demonstration Hold two tuning forks set to vibrate at the same frequency Strike one fork The other is then set into vibration (without touching)

Matching Frequencies If the tuning forks are not at matching frequencies, resonance does not occur Tuning 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 station This way you hear one radio station and not all of them.

Interference Consider 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

Beats When 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 Effect Imagine 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 ambulance The pitch of the siren changes as it approaches and then passes you. Approach – pitch higher, more frequent Moving away – pitch lower, less frequent

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