Sound Waves Sources of Sound Sound begins with
Sound Waves
![Sources of Sound ] Sound begins with a vibration. • • • Pencils on](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-2.jpg "Sources of Sound ] Sound begins with a vibration. • • • Pencils on")
Sources of Sound ] Sound begins with a vibration. • • • Pencils on glass Speakers Vocal chords ] Air is pushed by the vibration – this is sound. ] The sound reaches our ear and vibrates our eardrum.
![Pressure Wave ] ] Sound waves are longitudinal waves. Pressure varies in the air](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-3.jpg "Pressure Wave ] ] Sound waves are longitudinal waves. Pressure varies in the air")
Pressure Wave ] ] Sound waves are longitudinal waves. Pressure varies in the air as sound energy is transmitted. • Pressure is easier to measure than displacement 1 wavelength Dx x – propagation direction Real displacements in the air represent less than 1% of the wavelength.
![Speed of Sound ] ] The speed of a wave on a string is](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-4.jpg "Speed of Sound ] ] The speed of a wave on a string is")
Speed of Sound ] ] The speed of a wave on a string is related to tension and density. ] Look at them in general ] • restoring force: tension FT • Inertia: density m = m/L In liquids and solids: • Restoring force: bulk modulus B • Inertia: density r In gases: • Restoring force: pressure P • Inertia: density r • Factor for gas molecule g
![Hot Air ] Temperature affects the speed of sound in gases. • • •](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-5.jpg "Hot Air ] Temperature affects the speed of sound in gases. • • •")
Hot Air ] Temperature affects the speed of sound in gases. • • • ] At 0 C the speed in air is about 331 m/s Each degree adds 0. 60 m/s in air Less affect in liquids or solids Combine facts to get an equation for speed in air as a function of temperature T ( C). • About 340 m/s at 20 C
![Thunder ] ] A common rule of thumb is that thunder follows lightning by](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-6.jpg "Thunder ] ] A common rule of thumb is that thunder follows lightning by")
Thunder ] ] A common rule of thumb is that thunder follows lightning by 5 s for every mile. ] Use 340 m/s as the speed in air. ] Convert to mi/s. Is this a good rule to use? • 340 m/s = 0. 34 km/s • (0. 34 km/s)(1 mi/1. 6 km) = 0. 21 mi/s ] Find the distance • (0. 21 mi/s)(5 s) = 1. 06 mi
![Helium Voice ] When you inhale helium form a balloon your voice gets higher.](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-7.jpg "Helium Voice ] When you inhale helium form a balloon your voice gets higher.")
Helium Voice ] When you inhale helium form a balloon your voice gets higher. Air density is 1. 20 kg/m 3 and helium density is 0. 167 kg/m 3. ] A person makes a sound with wavelength 64 cm and frequency 536 Hz. What frequency is that same wavelength in helium (ignore g)? ] Frequency is related to the speed of sound. ] The frequency will change by the square root of the ratio of densities.
![On Track ] The speed of sound varies in different materials. ] Steel has](http://slidetodoc.com/presentation_image_h/10430ffd61ba76b58994773ded20a9ab/image-8.jpg "On Track ] The speed of sound varies in different materials. ] Steel has")
On Track ] The speed of sound varies in different materials. ] Steel has a very high bulk modulus, B = 90 GPa. ] ] The sound of a train 10 km away on a railroad track would take 30 s. ] If you listen to the sound in the rail directly it gets there faster. This results in a high velocity of sound in steel (or iron), v = 5000 m/s. • 5000 m/s = 5. 0 km/s • (10 km)/(5. 0 km/s) = 2 s ] The sound arrives in 2 s!
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