Chapter 17 - Chapter 17 Waves II Part 1 Acoustic Waves •...

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Unformatted text preview: Chapter 17 Waves II Part 1 Acoustic Waves • Longitudinal waves- the particles oscillate in the direction of the wave propagation. There are regions of compression and expansion. • A medium is required for the waves to propagate (we cannot hear in vacuum, cosmic space) • Wave front : a set of particles that oscillate with the same phase. • Wave rays : geometrical lines perpendicular to the wave fronts. They show the direction of wave propagation. Examples • Audible acoustic waves (sound waves): • Infra sound: f<20Hz • Ultra sound: f>20kHz • Seismic waves (not all seismic waves): small f<20Hz Audible sound Ultrasound Infrasound 20Hz 20kHz f<20Hz f>20kHz Using a Tuning Fork to Produce a Sound Wave • A tuning fork will produce a pure musical note • As the tines vibrate, they disturb the air near them • As the tine swings to the right, it forces the air molecules near it closer together • This produces a high density area in the air • This is an area of compression Using a Tuning Fork, cont. • As the tine moves toward the left, the air molecules to the right of the tine spread out • This produces an area of low density • This area is called a r a r e f a c t i o n Using a Tuning Fork, final • As the tuning fork continues to vibrate, a succession of compressions and rarefactions spread out from the fork • A sinusoidal curve can be used to represent the longitudinal wave • Crests correspond to compressions and troughs to rarefactions What is happening? The speed of sound • The speed of a traveling wave on a string: • The speed of sound • B bulk modulus of the material (rigidity) • ρ- density of material • It takes time for the sound to travel • The speed of sound depends on the properties of the material only! υ = τ μ υ = B ρ Elastic property Elastic property Inertia Inertia Some examples • Gases: • Air (0 o C) 331 m/s • Air (20 o C) 343 m/s • Helium 965 m/s • Hydrogen 1284 m/s • Liquids: • Water (0 o C) 1402 m/s • Water (20 o C) 1482 m/s • Seawater 1522 m/s • Solids: • Aluminum 6420 m/s • Granite 6000 m/s As the properties of the materials change with temperature, so does the speed of sound! Example problem: • Earthquakes generate both transverse waves (S) and longitudinal waves (P). The speed of the S waves is about 4.5 km/s and that of P waves 8 km/s . A seismograph detects the first S wave some 3 min after the first P wave. How far is the Earthquake if the waves travel on a straight line?...
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Chapter 17 - Chapter 17 Waves II Part 1 Acoustic Waves •...

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