Sound. Sound Waves travel as compressions & expansions Alternating regions of compressed and...

Chapter 14Sound

14.1 – Sound Waves & Beats• Sound Waves travel as compressions &

expansions• Alternating regions of compressed and expanded

air• These regions move away from source as

longitudinal wave

Sound waves exhibit sine wave behavior

Speed of sound• Same for all frequencies• Remember v=fλ …. And v only depends on

medium, not f

Speed of Sound• Normal atmospheric pressure and temperature

speed of sound is 343 m/s.

• Speed of sound is determined by properties of medium that it travels through

• Speed of sound increases with temperature

• More rigid/stiff an object is… the faster sound will travel through it• Steel – 5960 m/s• Plastic – 2680 m/s• Fresh Water @ 20° C – 1482 m/s• Air – 340 m/s

Lightning/Thunder• Use speed of sound to calculate how far lightning

is away from you

Human Hearing• The pitch of a sound is simply the frequency

of the sound wave• Higher frequency, higher the pitch• Humans can typically hear pitches ranging from

20 Hz (infrasonic) to 20,000 Hz (ultrasonic), but that range diminishes as you age… typically at the high end

• Loudness of a sound is affected only by amplitude

Beats • Two tones of slightly different frequency are

sounded together. This produces a fluctuation in the loudness of combined sounds.

• Amount of beats per second is equal to the difference in frequencies. • Fbeat = |f1-f2|

• Two tuning forks…. One 340 Hz another 310 Hz are sounded together. Beats will occur at a frequency of 30 Hz

14.2 Standing Sound Waves• Guitar strings, flute• Blowing air across the open end of a bottle• In general a standing wave in a bottle has a node

at bottom and antinode at the top

Harmonics for pipe open at one end

• First Harmonic w node at bottom and antinode at top would be ¼ of a wavelength fitting in the bottle

• So λ= ¼ L and f1 = v/4L

• In general,

Harmonics for pipes open at both ends

• Must have antinode at each open end

14.3 – The Doppler Effect• The frequency of a sound wave will change do to

the perceived motion of the sound source• Waves bunched when approaching (high F, low λ)• Waves spread out when receding (low F, high λ)

Doppler Effect

• Waves From an object that is approaching have short wavelength, which means they have a higher freq. And a higher pitch

• waves coming from an object that is moving away from you have a longer wavelength and therefore a lower freq, and lower pitch

• Cars approaching and leaving

Bow shock/sonic Boom• Sometimes the source of a wave can travel

faster than the waves it is creating• http://

www.youtube.com/watch?v=6o0zmafxTmE&safe=active

• Can be easily seen with a speed boat• http://

www.youtube.com/watch?v=2Sv4o4Kktm4&safe=active

• Sonic boom created when a object or plane breaks through the sound barrier

Human Perception of Sound• Loudness is determined by Intensity

• Doubling loudness corresponds to increasing the intensity by a factor of 10.

• 2x as loud means a 10 dB increase in Intensity