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Chapter 15

Chapter 15. Sounds. 15.1- Properties and Detection of Sound. Importance of Sound. Place your hand on your throat. Speak to someone next to you for 30 seconds. Sing to someone next to you for 5 seconds. What do you feel?. Movement forward compresses air particles- increases pressure

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Chapter 15

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  1. Chapter 15 Sounds

  2. 15.1- Properties and Detection of Sound Importance of Sound

  3. Place your hand on your throat. • Speak to someone next to you for 30 seconds. • Sing to someone next to you for 5 seconds. • What do you feel?

  4. Movement forward compresses air particles- increases pressure • Movement backwards separates air particles- decreases pressure

  5. Sound waves- longitudinal waves with pressure variation that is transmitted through matter (cannot move in a vacuum) • Speed of sound depends on temperature • 0.6 m/s per 1oC • 343 m/s @ room temperature (20oC) @ sea level • Speeds increase in liquids and solids

  6. Echoes- reflected sounds off hard surfaces

  7. Detection of Pressure Waves • Human ear takes vibrations in the air and transmits them into electrical impulses

  8. Perceiving Sound • Pitch- depends on the frequency of the vibration • Human Ear can hear • 20 Hz-16,000 Hz • 20 Hz-10,000 Hz (older people) • 20 Hz-8,000 Hz (age 70-cannot understand speech)

  9. Find the wavelength in air at 20oC of an 18 Hz sound wave, which is one of the lowest frequencies that is detectable by the human ear.

  10. Loudness- perceived by our sense of hearing, depends primarily on the amplitude of the pressure wave • 1 billionth of an atmosphere or 2x10-5 Pa to 20 Pa (pain) • Sound level-logarithmic scale measured in decibels (dB). • 10 dB increase is about 2x as loud

  11. The Doppler Effect • Doppler Effect- frequency shift

  12. Fd=fs(v-vd/v-vs) • v=velocity of the sound wave • vd=velocity of the detector • vs=velocity of the sound source • fd=frequency received by the detector • fs=waves frequency

  13. Setting up Parameters • + from source to detector • - from detector to source • The velocity of sound is always positive!

  14. You are in an auto traveling at 25.0 m/s toward a pole mounted warning siren. If the siren’s frequency is 365 Hz, what frequency do you hear? Use 343 m/s as the speed of sound.

  15. A sound source plays middle C (262 Hz). How fast would the source have to go to raise the pitch to C sharp (271 Hz)? Use 343 m/s as the speed of sound.

  16. 15.2- The Physics of Music

  17. Resonance in Air Columns • Closed pipe resonator- a resonating tube with one end closed to air • High pressure reflects back on high pressure • Open pipe resonator- resonating tube with both ends open • Low pressure reflects back on high pressure • Increased amplitude from constructive interference causes the sound to get louder

  18. OpenClosed • Flutes • Saxophones • Clarinets • Sea Shells

  19. Resonance on Strings • Each end is clamped and therefore has a node on each end. • Speed of the wave depends on the tension and mass per unit length. • Must attach to a sounding board (which must resonate as many frequencies) to intensify sound

  20. Sound Quality • Tuning fork- uses simple harmonic motion which can be uninteresting • Instruments and Voices- use superposition to blend many frequencies which seems more pleasing to hear • Timbre, tone color, tone quality

  21. The sound spectrum: fundamental and harmonic • Fundamental- lowest frequency (f1) • Closed pipe- f1=λ/4 • Open pipe- f1=v/2L • Harmonics- multiples of the lowest frequency • Closed pipe- odd multiples • Open pipe- even multiples

  22. Consonance and Dissonance • Dissonance- unpleasant set of pitches • Consonance- please set of pitches (pitches with small whole number ratios) • Ex: 1:2, 2:3, 3:4

  23. Musical Intervals • Octave- 2 notes with frequencies related 1:2 • Ex: 440 Hz: 880 Hz • Ex: Fundamental: 1st Harmonic: 2nd Harmonic

  24. Beats • Beat- oscillation of wave amplitude

  25. Sound Reproduction • Stereo system- 20-20,000 Hz frequencies are played with less than 3 dB difference so all notes can be heard • Telephone- 300- 3000 Hz • Noise- mixture of many frequencies (some say has a calming effect)

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