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Goal: To understand sound

Goal: To understand sound. Objectives: To learn about Sound waves To understand the Speed of sound To learn about Doppler Shifts To learn about Resonance. Sound waves. What type of wave is a sound wave?. Compression.

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Goal: To understand sound

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  1. Goal: To understand sound Objectives: To learn about Sound waves To understand the Speed of sound To learn about Doppler Shifts To learn about Resonance

  2. Sound waves • What type of wave is a sound wave?

  3. Compression • Since sound is the compression of a material, sound is a Longitudinal wave (compression wave). • You compress the material (in our case air) at some rate (so you compress the air some # of times per second), and this comes out as a pitch. • This is usually created from the in and out vibrations of an object which causes the compression of air. • The pitch is how often the object vibrates.

  4. Speed of Sound • In air at room temperature sound travels about 340 m/s. • So, if the lightning strike is about 1 mile away it will take about 4.5 seconds for you to hear the “ka-boom”. • However, the speed of sound depends on temperature. At 32 F the speed is 330 m/s. • It also depends on the medium. In water the speed of sound can be 1500 m/s.

  5. Quick sample • A rock hits the bottom of a cliff that is 50 m high. • If the speed of sound is 340 m/s how long will it take for you to hear the impact? • HW note, you will have to find the time it takes the rock to fall to the bottom also.

  6. Energy • Sound waves have energy. • A large enough “boom” can be felt, and not just heard. • The energy of sound is given as a decibel level – which is an exponential scale. • If the sound has enough energy, it can harm your eardrums. • If even stronger it can break windows.

  7. Decibel Scale • I = power / area = P / (4π r2) • Β = 10 dB * log10(I / Io) • Io = 1 * 10-12 W/m2 • Example: • A 20 W speaker outputs its max power of sound. • From a distance of 4 m away find: • A) The intensity of sound • B) The decibel level of the sound

  8. Quick math note: • 10(B / 10 dB) = I / Io • So, if you have to solve for I for some reason (to then solve for a distance), you will have to use this trick

  9. Doppler Shift (day 2) • When a source of waves moves you have what is called a Doppler shift. • Think to when you are creating a wave. • The front part of the wave will move away from you at some constant wave speed. • However, the position of the end of the wave depends on where you are when you FINISH the wave. • So, if you move forward, the wave will be shortened. • If you move backwards the wave will be lengthened. • The amount of the change of the wave will just depend on how far you move in the time that it takes to make the wave.

  10. Doppler equation • So, the equations are: λ Obs = λ Emit * [(Vwave – Vsource)/ (Vwave - Vobserver) ] fo = fs * [(Vwave - Vobserver) / (Vwave – Vsource)] NOTE: You do have to watch direction!!! If the sound is traveling opposite to the direction of one of the 2 then you have to make that velocity negative.

  11. Train/Indy Car • Imagine a train or an Indy car moving towards you. • While it moves towards you any noise it makes is “blue-shifted” – which means it goes to higher frequency (shorter wavelength means higher frequency). • Once it passes you it is moving away from you, so the noise is then “red-shifted” – which means it goes to lower frequency.

  12. Example • A police officer chases a speeder. • The siren from the officer at rest would have a frequency of 250 Hz. • The cop is traveling at 50 m/s forward • The speeder is traveling 45 m/s forward • Find the frequency that the speeder observes.

  13. Doppler shift and reflection • Here you will get a double Doppler shift • First you find the frequency the reflected object gets. • That will be the reflected frequency • The reflected frequency will be Doppler shifted again. • However this time the source and the observer switch places and velocities switch sign as the wave is traveling in the opposite direction.

  14. Example if time permits • A fruit bat flies at 17 m/s towards a tree. • If the bat emits a 22,000 Hz pulse towards the tree then find (assuming speed of sound is 340 m/s): • A) The frequency the tree gets • B) The frequency that the tree “emits” from the reflection • C) The frequency the bat gets back

  15. Resonance • If a waves moves an integer number of wavelengths moving down and back this will create resonance. • The waves going out will add to the ones coming in. • Since the reflected and incoming waves will be in phase they will add to become a bigger wave.

  16. Conclusion • Sound is a compression wave • Like properties of waves, sound obeys all of those properties, and does NOT travel in space. • We have learned how to use the Doppler shift equations • We have examined resonance

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