Sound Chapter 15 Sound Waves Produced by vibrating

Sound Chapter 15

Sound Waves • Produced by vibrating object: strings, vocal cords, reeds, air columns • Frequency of sound wave equals frequency of vibration • Frequency directly related to pitch • Humans hear sounds between 20 Hz and 20, 000 Hz

Sound Waves • Sounds below 20 Hz are infrasonic; above 20, 000 Hz are ultrasonic • Sound waves are longitudinal waves • Composed of compressions and rarefactions, high and low pressure areas • Sound needs a medium; can travel through solid, liquid or gas but not in a vacuum

Speed of Sound • Depends on the medium it is traveling through • Travels fast in liquids and much faster in elastic solids than in air • Speed in air depends on temperature and humidity

Speed of Sound • Ranges from about 330 m/s to 350 m/s (or about 1100 ft/s or 760 mph) • Faster in warm air due to more kinetic energy and humid air due to more density • In aviation, speed of sound is called Mach 1, twice speed of sound Mach 2, etc.

Sound Intensity & Loudness • Intensity is measurable quantity proportional • • to square of wave amplitude Loudness is physiological measure of sound intensity Depends on hearing of listener Varies logarithmically with intensity Measured on decibel scale (d. B)

Sound Intensity and Loudness • Decibel change is equal to 10 times log of the ratio of two sound intensities • Increase in loudness of 20 d. B means intensity increases by factor of 10 squared or 100 times • Change of 30 d. B means factor of 10 cubed or 1000

The Doppler Effect • The apparent change of frequency due to • • motion between wave source and receiver Heard as change in pitch of sound of moving objects Occurs with all types of waves Used by police radar to check speed Astronomers measure color shift in starlight

Forced Vibration and Resonance • Forced vibration is setting an object into vibration by contact with another vibrating object • Principle behind sound production in stringed instruments • Most objects have a natural frequency of vibration that depends on size, shape, elasticity

Forced Vibration and Resonance • Natural frequency is also resonant frequency of object • When forced to vibrate at its natural frequency, resonance occurs and wave amplitude (loudness) is greatly increased • Important in design of buildings, bridges, musical instruments • Principle behind TV and radio tuners

Resonance in Tubes • Air column in a tube or pipe will vibrate— organ pipes, wind instruments, blow across bottle • Resonance occurs when wave fits in length of tube

Interference of Sound Waves • Can occur between 2 sources (like speakers) or between one source and its reflection • Constructive and destructive interference affects loudness of sounds heard at different locations • Destructive interference can be used to cancel noise

Beats • Caused by interference between two sounds of nearly the same frequency • Heard as pulsing loudness • Number of beats heard per second (beat frequency) equals difference between two original frequencies sounded • Can be used to tune musical instruments to a reference pitch

Review: Sound • • produced by vibration longitudinal wave-compressions & rarefactions must have medium speed depends on medium, temperature and density • loudness measured on logarithmic decibel scale

Review: Sound • Resonance due to forced vibration at natural frequency increases wave amplitude & energy • Interference can occur changing loudness • Beats are caused by interference between two sound sources of nearly identical frequencies

Important Terms • • infrasonic ultrasonic longitudinal wave compression rarefaction medium pitch • • intensity loudness decibel scale forced vibration natural frequency resonance beats