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Chapter 13 - Sound

Chapter 13 - Sound. 13.1 Sound Waves. The Production of Sound Waves. The Production of Sound Waves. Compression: the region of a longitudinal wave in which the density and pressure are greater than normal

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Chapter 13 - Sound

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  1. Chapter 13 - Sound 13.1 Sound Waves

  2. The Production of Sound Waves

  3. The Production of Sound Waves • Compression: the region of a longitudinal wave in which the density and pressure are greater than normal • Rarefaction: the region of a longitudinal wave in which the density and pressure are less than normal • These compressions and rarefactions expand and spread out in all directions (like ripples in water)

  4. The Production of Sound Waves

  5. Characteristics of Sound Waves • The average human ear can hear frequencies between 20 and 20,000 Hz. • Below 20Hz are called infrasonic waves • Above 20,000 Hz are called ultrasonic waves • Can produce images (i.e. ultrasound) • f = 10 Mhz, v = 1500m/s, wavelength=v/f = 1.5mm • Reflected sound waves are converted into an electric signal, which forms an image on a fluorescent screen.

  6. Characteristics of Sound Waves • Frequency determines pitch - the perceived highness or lowness of a sound.

  7. Speed of Sound • Depends on medium • Travels faster through solids, than through gasses. • Depends on the transfer of motion from particle to another particle. • In Solids, molecules are closer together • Also depends on temperature • At higher temperatures, gas particles collide more frequently • In liquids and solids, particles are close enough together that change in speed due to temperature is less noticeable

  8. Speed of Sound

  9. Propagation of Sound Waves • Sound waves spread out in all directions (in all 3 dimensions) • Such sound waves are approximately spherical

  10. Propagation of Sound Waves

  11. The Doppler Effect • When an ambulance passes with sirens on, the pitch will be higher as it approaches you and lower as it moves away • The frequency is staying the same, but the pitch is changing

  12. The Doppler Effect The wave fronts reach observer A more often thanobserver B because of the relative motion of the car The frequency heard by observer A is higher thanthe frequency heard by observer B

  13. HW Assignment • Section 13-1: Concept Review

  14. Chapter 13 - Sound 13.2 - Sound intensity and resonance

  15. Sound Intensity • When you play the piano • Hammer strikes wire • Wire vibrates • Causes soundboard to vibrate • Causes a force on the air molecules • Kinetic energy is converted to sound waves

  16. Sound Intensity • Sound intensity is the rate at which energy flows through a unit area of the plane wave • Power is the rate of energy transfer • Intensity can be described in terms of power • SI unit: W/m2

  17. Sound Intensity • Intensity decreases as the distance from the source (r) increases • Same amount of energy spread over a larger area

  18. Intensity and Frequency Human Hearing depends both on frequency and intensity

  19. Relative Intensity • Intensity determines loudness (volume) • Volume is not directly proportional to intensity • Sensation of loudness is approximately logarithmic • The decibel level is a more direct indication of loudness as perceived by the human ear • Relative intensity, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing

  20. Relative Intensity • When intensity is multiplied by 10, 10dB are added to the decibel level • 10dB increase equates to sound being twice as loud

  21. Forced Vibrations • Vibrating strings cause bridge to vibrate • Bridge causes the guitar’s body to vibrate • These forced vibrations are called sympathetic vibrations • Guitar body cause the vibration to be transferred to the air more quickly • Larger surface area

  22. Resonance • In Figure 13.11, if a blue pendulum is set into motion, the others will also move • However, the other blue pendulum will oscillate with a much larger amplitude than the red and green • Because the natural frequency matches the frequency of the first blue pendulum • Every guitar string will vibrate at a certain frequency • If a sound is produced with the same frequency as one of the strings, that string will also vibrate

  23. The Human Ear The basilar membrane has different natural Frequencies at different positions

  24. Chapter 13 - Sound 13.3 - Harmonics

  25. Standing Waves on a Vibrating String • Musical instruments, usually consist of many standing waves together, with different wavelengths and frequencies even though you hear a single pitch • Ends of the string will always be the nodes • In the simplest vibration, the center of the string experiences the most displacement • This frequency of this vibration is called the fundamental frequency

  26. The Harmonic Series Fundamental frequency or first harmonic Wavelength is equal to twice the string length Second harmonic Wavelength is equal to the string length

  27. Standing Waves on a Vibrating String • When a guitar player presses down on a string at any point, that point becomes a node

  28. Standing Waves in an Air Column • Harmonic series in an organ pipe depends on whether the reflecting end of the pipe is open or closed. • If open - that end becomes and antinode • If closed - that end becomes a node

  29. Standing waves in an Air Column The Fundamental frequency can be changed by changing the vibrating air column

  30. Standing Waves in an Air Column Only odd harmonics will be present

  31. Standing Waves in an Air Column • Trumpets, saxophones and clarinets are similar to a pipe closed at one end • Trumpets: Player’s mouth closes one end • Saxophones and clarinets: reed closes one end • Fundamental frequency formula does not directly apply to these instruments • Deviations from the cylindrical shape of a pipe affect the harmonic series

  32. Harmonics account for sound quality, or timbre • Each instrument has its own characteristic mixture of harmonics at varying intensities • Tuning fork vibrates only at its fundamental, resulting in a sine wave • Other instruments are more complex because they consist of many harmonics at different intensities

  33. Harmonics account for sound quality, or timbre

  34. Harmonics account for sound quality, or timbre • The mixture of harmonics produces the characteristic sound of an instrument : timbre • Fuller sound than a tuning fork

  35. Fundamental Frequency determines pitch • In musical instruments, the fundamental frequency determines pitch • Other harmonics are sometimes referred to as overtones • An frequency of the thirteenth note is twice the frequency of the first note

  36. Fundamental Frequency determines pitch

  37. Beats • When two waves differ slightly in frequency, they interfere and the pattern that results is an alternation between loudness and softness - Beat • Out of phase: complete destructive interference • In Phase - complete constructive interference

  38. Beats

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