• Sound – The vibrational motions of particles in a medium caused by a vibrating object.
  • Human frequency response is 20 Hz to 20 000 Hz
• Infrasonic Sound – Sound with frequencies < 20 Hz
• Ultrasonic Sound – Sound with frequencies > 20 000 Hz

• Sound travels at 332 m/s in air at 0^o C and 1 atmosphere and increases in speed by 0.6 m/s for each 1^o C increase in temperature.

• The speed of sound was first measured in 1738 with cannons on hills 29 km apart. The difference between flash and sound was used to estimate the speed of sound
• Sound needs a material medium for it to transmit.
• Otto von Guericke showed that sound does not travel in a vacuum.
• The speed of sound varies from substance to substance.
  • Generally:
    • Sound is fastest in solids and slowest in gases
    • There are some exception.

• Mach Number – The ratio of the speed of an object to the speed of sound.

• Used to specify speed for supersonic aircraft, etc.
• Speed of sound varies with air pressure, and therefore, with altitude.
  • At sea level and 0^o C the speed of sound is 332 m/s (1200 km/h). At 10 km above sea level and 0^o C the speed of sound is 294 m/s (1060 km/h).
• Example:
  • What is the Mach number for an airplane traveling at 10 km altitude and a speed of 1800 km/h if the air temperature is 0^o C?

Mach # = (Speed of object)/(Speed of Sound)

= (1800 km/h)/(1060 km/h) = 1.7

• Sound is a longitudinal wave.

• As the tines move out, they compress the air outside of the tuning fork and cause molecules to collide with each other (a compression forms). Between the tines, air molecules are pulled apart (a rarefaction forms).
• The opposite happens when the times move inwards (a compression forms between the tines and a rarefaction forms outside of the tuning fork).
• The air molecules vibrate about some average position but do not move from source to observer.
• The variation of air pressure versus position can be plotted to produce a graph that looks like a transverse wave.

• Because of this, we can see that all the terminology used with transverse waves can be used with longitudinal waves.
  • Wavelength is the distance between two successive in phase points.
  • We can talk about a phase of 60^o from the graph (90 degrees on the first crest on the graph).
• Amplitude of a sound describes how the air pressure changes with respect to normal air pressure.
  • Loud sounds produce greater densities of particles in compressions than soft sounds.
• Pitch – The term that is sometimes substituted for frequency of sound.
  • High pitch means high frequency.
  • Low pitch means low frequency.

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November 12, 2013