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Sound Propagation

Sound Propagation. Reflection, refraction, diffraction. What do we know about sound propagation?. Speed of sound in air Speed of sound in solids/liquids/gases. Sequences of compression and rarefaction ( click me ). Classification. Infra sound. Earthquakes, heavy traffic. 20 Hertz.

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Sound Propagation

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  1. Sound Propagation Reflection, refraction, diffraction

  2. What do we know about sound propagation? • Speed of sound in air • Speed of sound in solids/liquids/gases • Sequences of compression and rarefaction (click me)

  3. Classification Infra sound Earthquakes, heavy traffic 20 Hertz 3000 Hz best hearing Audible range 20 000 Hertz=20kHz Medical imaging, dog whistles, bats Ultra sound Growing Wave length? Growing frequency

  4. Wave lengths of the audible range • 20 kHz: • 20 Hz: Width of a finger Width of a house Objects in our environment are of the same order of size as the wave length of sound waves.

  5. How can we represent sound in space? Direction of propagation Rays Surfaces of equal compression = planes Wave fronts Plane waves

  6. How can we represent sound in space? How many rays are there? Direction of propagation Where do you measure the wavelength? Surfaces of equal compression = spheres Rays Spherical waves

  7. Comparison • spherical waves • Plane waves Parallel rays Radial rays Wave fronts are planes Wave fronts are spheres Amplitude is constant Amplitude diminishes with distance from source Rays are perpendicular to wave fronts Rays straight lines, except …

  8. Waves encountering a surface Will all these rays be reflected back to the person? Can the blue person hear the echo? Reflection

  9. Waves encountering a surface Specular Reflection

  10. Waves encountering a surface Diffuse Reflection

  11. How can the two people hear each other? Each of the rays traveled different lengths of way Each of the rays used a different amount of time

  12. Variable speed of sound Rays are bent away From surface! Cool air = low speed Refraction Wave front Plane waves at constant speed Warm air = high speed Wave fronts if the air is warmer at the ground

  13. Variable speed of sound Rays are bent toward surface! Warm air = high speed inversion Refraction Wave fronts if the air is cooler at the ground Cool air = low speed

  14. wind wind Resulting motion of sound waves sound sound Resulting motion of sound waves Variable speed of sound The rays are bent upward upwind of the source. The rays are bent downward downwind of the source. Refraction

  15. Reflection and refraction • Reflection: • At solid obstacles, a sharp change of direction • Echo, ringing effect in halls • Refraction: • Inhomogeneous speed, gradual change of direction • Air layers with different temperatures, wind Reflection is of more importance to us than refraction.

  16. Why can we hear around a corner? Reflection ?

  17. Sound waves at obstacles Top view: Diffraction

  18. Diffraction at small opening Small: d<<λ Wavelength large compared to opening Diffraction bends the wave fronts heavily at the edges of a small Obstacle. Shadow zone Shadow zone

  19. Diffraction at large opening Small: d>>λ Wavelength small compared to opening Shadow zone Diffraction bends the wave fronts only lightly at the edges of a large obstacle. Shadow zone

  20. Where can we observe diffraction? • Speak to a person, but not directly: • Mouth: 5 cm<< wave length 10..30 cm • Speakers: • Bass sounds spread in all directions • Treble more clearly in a narrow cone • Your left ear hears sound coming from right: • Bass more clearly • Treble not that clearly

  21. Outdoor Music • Problems: • Waves propagate away from source (no wall reflections) • Deadness (no overlap from secondary reflections) • Uneven distribution over a large audience

  22. Possible solutions • “Shell” behind the performers Audience

  23. Possible Solutions • Shell behind performers: top view Ideal position for performer: a little closer than half the radius of curvature

  24. Possible Solutions • Roofed outdoor pavilions: Traps sound Not good for outside audience

  25. Possible Solutions • Roofed outdoor pavilions: Helps audience Band members can hardly hear each other

  26. Possible Solutions • Roofed outdoor pavilions: compromise Okay for band and audience

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