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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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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

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


26 3 media that transmit sound

26.3 – MEDIA THAT TRANSMIT SOUND

  • Sound travels through all materials

  • Speed of sound: solid > liquid > gas

  • Liquid is a better conductor of sound


26 4 speed of sound

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


26 5 loudness

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


26 6 natural frequency

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


26 7 forced vibrations

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


26 8 resonance

26.8 - RESONANCE

  • The frequency of vibration matches objects natural frequency

    • Causes an increase in amplitude

    • Swings, breaking step across bridges


26 9 interference

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


26 10 beats

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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