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Sound

Sound. Chapter 15. Sound Waves. Sound wave= a pressure wave that is transmitted through matter Speed of sound in air at room temperature= 343m/s In general, the speed of sound is greater in solids and liquids than in gases

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Sound

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  1. Sound Chapter 15

  2. Sound Waves • Sound wave= a pressure wave that is transmitted through matter • Speed of sound in air at room temperature= 343m/s • In general, the speed of sound is greater in solids and liquids than in gases • Sound CANNOT travel in a vacuum because there are no particles to collide

  3. Detection of Pressure Waves (Sound) • The human ear is a detector that receives pressure waves and converts them to electrical impulses • The ear detects sound waves over a wide range of frequencies and is sensitive to an enormous range of amplitudes • Surprise!

  4. Perceiving Sound • Pitch= dependent on the frequency of vibration • Musical pitches have specific frequencies • Loudness=depends on the amplitude of the pressure wave

  5. Doppler Effect • The change in the frequency of sound caused by the movement of either the source, the detector, or both the detector and the source • As the source moves towards you the frequency increases (the pitch is higher) • As the source moves away from you the frequency decreases (the pitch is lower)

  6. Practice Problem • A trumpet player sounds C above middle C (524Hz) while traveling in a convertible at 24.6 m/s. If the car is coming towards you, what frequency would you hear? • ANS: 564 Hz

  7. Physics of Music

  8. Sources of Sound • Musical Instruments are simply vibrating surfaces that are sources of sound • Examples • Human Voice: vibration of vocal cords • Brass: the lips of the musician vibrate • Woodwinds: the reed vibrates • Stringed Instruments: wires or strings are set into vibration through a variety of methods

  9. Resonances in Air Columns • Resonance increases the amplitude of a vibration by repeatedly applying a small external force at the same natural frequency • Resonating air columns intensify sound • Closed-pipe resonator: resonates when its length is an odd number of quarter wavelengths • Open-pipe resonator: resonates when the length is an even number of quarter wavelengths • Strings: resonate similar to open-pipe resonators

  10. Resonance Frequencies • Closed pipe: • λ=(4/n)L • Fn=nv/4L • ****n can only be an odd number**** • Open Pipe and Strings: • λ=(2/n)L • Fn=nv/2L

  11. Sound Quality -Complex waves are produced by using the principle of superposition to add waves of many frequencies -The shape of the wave depends on the relative amplitudes of these frequencies -The difference between the waves is called timbre, tone color, or tone quality

  12. The Sound Spectrum • Fundamental: the lowest frequency of sound that will resonate • Harmonics: higher frequencies which multiples of the fundamental frequency • Harmonics give certain musical instruments their own unique timbre • Harmonics in closed-pipe resonators are odd numbered multiples ONLY

  13. Sound and Music • Consonance: a pleasant set of pitches • Small ratios of frequencies • Ex: 3:4, 9:8, 3:2 • Dissonance: an unpleasant, jarring set of pitches • Large ratios of frequencies • Ex: 256:243

  14. Sound and Music • Musical Intervals: a step up or down in pitch which is specified by the ratio of the frequencies involved • Two notes with frequencies that differ by the ratio of 1:2 are said to differ by an octave • The fundamental and harmonics are related by octaves • The fundamental and the first harmonic are one octave apart

  15. Beats • Beat: the oscillation of wave amplitude that results from the superposition of two sound waves with almost identical frequencies • Fbeat= fA-fB • Musical instruments are often tuned against one another until the beat disappears

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