Transcript Chpater 13

Physics Chp 13
SOUND
• Compression vs Rarefaction
• Pitch – how we perceive the frequency
• Speed depends on medium (air, water, ice)
• 3D – goes in all directions
• Doppler Effect
•
• As you or the source moves the sound
changes.
• Towards – higher
• Away - lower
• Sound Intensity
• I = P/A
• I = P/ 4πr2
• Threshold of hearing Io = 1x10-12 W/m2
• Audible Sounds
• Depends on intensity as well as frequency
• Resonance is when something vibrates at a
natural frequency
• Our Ear converts the sound wave into
electrical waves so our brain can sense the
sound.
• Standing waves
• Simplest is the fundamental.
• On a string it has two nodes and one
antinode.
• λ1=2L
• For a string the first
standing wave is
half a wavelength
and then goes up by
whole intergers
• Then it increases by additional antinodes
• λ2=L
• λ3= 2/3L
• λ4=1/2L
• Since v=fλ and the speed remains const
• fn = n v/2L n= 1,2,3….. Harmonic series
• f1 is the fundamental
• BUT a closed on one end is different
• Then the node has to hit the closed end to
reflect back with no loss of the wave
• fn = n(v/4L) n= 1,3,5……
• If you use a mid C (261.6 hz) over a closed
tube and get a resonant sound at 0.983 m and
1.64 m, which harmonics are present and
what is the likely length for the first harmonic?
Assume v = 343 m/s
• fn = n(v/4L) n= 1,3,5……
• 261.6 hz = n (343 m/s / 4(0.983m) ) n = 3
• 261.6 hz = n (343 m/s / 4(1.64m) ) n = 5
• So if n is 1 then 261.6 hz = 1(343m/s / 4L)
• L = 0.328 m
• When two frequencies are slightly off they
interfere with each other and create a pattern
of changing intensities. The difference in
frequencies gives the beat frequency.
• f1 = 256hz and f2= 259 hz
beat freq = 3hz