Sound Waves.

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Presentation transcript:

Sound Waves

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 as sound energy is transmitted. Pressure is easier to measure than displacement 1 wavelength Real displacements in the air represent less than 1% of the wavelength. Dx x – propagation direction

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 Factor for gas molecule g

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 5 s for every mile. Is this a good rule to use? Use 340 m/s as the speed in air. Convert to mi/s. 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. Air density is 1.20 kg/m3 and helium density is 0.167 kg/m3. 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 a very high bulk modulus, B = 90 GPa. This results in a high velocity of sound in steel (or iron), v = 5000 m/s. 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. 5000 m/s = 5.0 km/s (10 km)/(5.0 km/s) = 2 s The sound arrives in 2 s!