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Unformatted text preview: Chapter 14: Sound I) Sound waves have their source in a vibrating object A) Tuning fork: common device for producing pure musical notes 1) Two metal prongs (tines) that vibrate when struck 2) Vibration disturbs the air near them 3) Compression: When a tine swings to the right the molecules in an element of air in front of its movement are forced closer together than normal a) Creates region of high molecular density and high air pressure b) Moves away from fork like ripple in pond 4) Rarefraction: when tine swings to the let, the molecules in an element of air to the right of the tine spread apart and the density and air pressure in this region is lower than normal a) Molecules to the right of the rarefraction move to the left b) Rarefraction itself moves to the right B) Sinusoidal curve is used to represent a sound wave 1) Crests: compressions 2) Troughs: rarefractions C) Sound waves in gases travel at about the molecular rms speed II) The motion of the elements of the medium in a longitudinal sound wave is back and forth along the direction in which the wave travels III) Sound waves fall into three categories: A) Audible waves: 2020000Hz B) Infrasonic waves: frequencies bellow the audible range (i.e. earthquakes) C) Ultrasonic waves: frequencies above the audible range for humans, but some animals can hear them 1) Frequencies greater than 20kHz 2) Piezoelectric effect: method of transforming electrical energy into mechanical energy a) When electrical contacts are made to opposite faces of a crystal b) If alternating voltage of high frequency is applied to these contacts, the crystal vibrates at the same frequency as the applied voltage and emits a beam of ultrasonic waves c) Reversible if some external surface causes the crystal to vibrate an alternating voltage is produced d) A single crystal can be used to generate and receive ultrasonic waves 3) Ultrasound imaging is possible b/c a sound wave is partially reflected whenever it is incident on a boundary b/t two materials having different densities a) Percentage of the incident sound wave intensity reflected (PR): PR=(( i- t )/( i + t )) 2 x 100 b) Assumes that the direction of the incident sound wave is perpendicular to the boundary and that the speed of sound is approximately the same in the two materials IV) Speed of sound A) Speed of sound in a fluid depends on the fluids compressibility and inertia 1) v=(B/ ) (B is the bulk modulus) 2) Also holds true for a gas B) Speed of a longitudinal wave in a solid rod:...
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- Fall '08