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26.1 - SOUND

26.1 - SOUND. Longitudinal wave of material objects because of vibrations Pitch = frequency High pitch = high freq. Range: 20 Hz – 20,000 Hz < 20 Hz = infrasonic > 20,000 Hz = ultrasonic. 26.2 – SOUND IN AIR.

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26.1 - SOUND

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  1. 26.1 - SOUND • Longitudinal wave of material objects because of vibrations • Pitch = frequency • High pitch = high freq. • Range: 20 Hz – 20,000 Hz • < 20 Hz = infrasonic • > 20,000 Hz = ultrasonic

  2. 26.2 – SOUND IN AIR • The vibrating source “pushes” and “pulls” on the air causing alternating locations of compression & rarefaction • Compression – air molecules are squeezed together • Higher pressure • Rarefaction – molecules are spread apart • Lower pressure

  3. 26.3 – MEDIA THAT TRANSMIT SOUND • Sound travels through all materials • Speed of sound: solid > liquid > gas • Liquid is a better conductor of sound

  4. 26.4 – SPEED OF SOUND • Depends on temperature & mass of particles & elasticity • ↑ temperature = ↑ speed ~ 0.6 m/s per 1°C • Lighter particles move faster • The faster the material can change back to original shape (its elasticity) – the faster sound travels

  5. 26.5 - LOUDNESS • Sound intensity α A2 • Threshold of hearing = 10-12 W/m2 • Threshold of pain = 1 W/m2 • This is a huge range • How loud something is ≠ intensity • Doubling loudness, ↑ intensity by 10 • Use log (base 10) scale • Loudness has range: 0 – 12 bel (B) • A finer scale is used: the decibel (dB) • Range: 0 – 120 dB

  6. 26.6 – NATURAL FREQUENCY • All objects when disturbed will generate their own characteristic vibration • Depends upon: material (elasticity) & shape • This is the frequency to which the least amount of energy is required to continue the vibrations

  7. 26.7 – FORCED VIBRATIONS • An object is made to vibrate because of vibrations of another object • Sounding boards in musical instruments make sound much louder

  8. 26.8 - RESONANCE • The frequency of vibration matches objects natural frequency • Causes an increase in amplitude • Swings, breaking step across bridges

  9. 26.9 - INTERFERENCE • Sound waves can have constructive/destructive interference • Produces louder/quieter sound • In phase = loud, out of phase = quiet • Used in noise cancelling technology

  10. 26.10 - BEATS • Periodic variation in loudness due to two tunes with small difference in frequency as they interfere • Beat frequency = difference between the two frequencies • Used to tune instruments

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