Sound WavesSources of SoundPressure WaveSpeed of SoundHot AirThunderHelium VoiceOn TrackSound WavesSound WavesSources of SoundSources of SoundSound begins with a vibration.Sound begins with a vibration.•Pencils on glass Pencils on glass •SpeakersSpeakers•Vocal chordsVocal chordsAir is pushed by the vibration – this is sound.Air is pushed by the vibration – this is sound.The sound reaches our ear and vibrates our eardrum.The sound reaches our ear and vibrates our eardrum.Pressure WavePressure WaveSound waves are longitudinal waves.Sound waves are longitudinal waves.Pressure varies in the air as sound energy is Pressure varies in the air as sound energy is transmitted.transmitted.•Pressure is easier to measure than displacementPressure is easier to measure than displacement1 wavelengthxx – propagation directionReal displacements in the air represent less than 1% of the wavelength.Speed of SoundSpeed of SoundThe speed of a wave on a The speed of a wave on a string is related to tension string is related to tension and density.and density.Look at them in generalLook at them in general•restoring force: tension restoring force: tension FFTT•Inertia: density Inertia: density = = mm//LLIn liquids and solids:In liquids and solids:•Restoring force: bulk Restoring force: bulk modulus modulus BB•Inertia: density Inertia: density In gases:In gases:•Restoring force: pressure Restoring force: pressure PP•Inertia: density Inertia: density •Factor for gas molecule Factor for gas molecule //TTFLmFv /Bv /Pv Hot AirHot AirTemperature affects the speed of sound in gases.Temperature affects the speed of sound in gases.•At 0 At 0 C the speed in air is about 331 m/sC the speed in air is about 331 m/s•Each degree adds 0.60 m/s in airEach degree adds 0.60 m/s in air•Less affect in liquids or solidsLess affect in liquids or solidsCombine facts to get an equation for speed in air as a Combine facts to get an equation for speed in air as a function of temperature function of temperature TT ( (C).C).•About 340 m/s at 20 About 340 m/s at 20 CCm/s )60.0331( Tv ThunderThunderA common rule of thumb is A common rule of thumb is that thunder follows lightning that thunder follows lightning by 5 s for every mile.by 5 s for every mile.Is this a good rule to use?Is this a good rule to use?Use 340 m/s as the speed in Use 340 m/s as the speed in air.air.Convert to mi/s.Convert to mi/s.•340 m/s = 0.34 km/s340 m/s = 0.34 km/s•(0.34 km/s)(1 mi/1.6 km) = (0.34 km/s)(1 mi/1.6 km) = 0.21 mi/s0.21 mi/sFind the distanceFind the distance•(0.21 mi/s)(5 s) = 1.06 mi(0.21 mi/s)(5 s) = 1.06 miFrequency is related to the Frequency is related to the speed of sound.speed of sound.The frequency will change The frequency will change by the square root of the by the square root of the ratio of densities.ratio of densities.Helium VoiceHelium VoiceWhen you inhale helium When you inhale helium form a balloon your voice form a balloon your voice gets higher. Air density is gets higher. Air density is 1.20 kg/m3 and helium 1.20 kg/m3 and helium density is 0.167 kg/m3. density is 0.167 kg/m3. A person makes a sound A person makes a sound with wavelength 64 cm and with wavelength 64 cm and frequency 536 Hz. What frequency 536 Hz. What frequency is that same frequency is that same wavelength in helium (ignore wavelength in helium (ignore )?)?PvffTv1//Hz 1440/ HeairairHeffOn TrackOn TrackThe speed of sound varies in The speed of sound varies in different materials.different materials.Steel has a very high bulk Steel has a very high bulk modulus, modulus, BB = 90 GPa. = 90 GPa.This results in a high velocity This results in a high velocity of sound in steel (or iron), of sound in steel (or iron), vv = 5000 m/s. = 5000 m/s.The sound of a train 10 km The sound of a train 10 km away on a railroad track away on a railroad track would take 30 s.would take 30 s.If you listen to the sound in If you listen to the sound in the rail directly it gets there the rail directly it gets there faster.faster.•5000 m/s = 5.0 km/s5000 m/s = 5.0 km/s•(10 km)/(5.0 km/s) = 2 s(10 km)/(5.0 km/s) = 2 sThe sound arrives in 2 s!The sound arrives in 2 s!/Bv
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