Transcript 17

Chapter 17: Waves - II (Summary)
Are sound waves longitudinal or transverse?
Frequency ranges: audible, infra sonic and ultra sonic
What creates sound? A sound transducers.
Speed of sound waves: (elastic property/inertial property)1/2
v = (B/r)1/2
Periodic sound waves:
s(x,t) = sm cos(kx-wt)
Dp = Dpm sin(kx-wt)
Dpm = r v w sm
Notice that pressure and displacement are 90o out of phase!
Interference, path lengths, phase difference:
Dr/l = f/2p
When is the interference constructive? Dr = n l
When is the interference destructive? Dr = (n+1/2) l
What if the two sources are not “simultaneous” in time?!
Spherical and plane waves: At large distances, a small part
of a spherical wave acts like a plane wave.
Intensity: power per unit area. The rate at which the energy
being transported by the wave flows through a unit area
perpendicular to the direction of travel of the wave.
I = ½ r v (w sm)2 = (Dpm)2/(2 r v)
The intensity falls as 1/r2
Sound level (in decibel):
b = 10 log(I/I0)
As a rule of thumb: when the intensity is multiplied by 2
the sound level increases by 3 dB.
Standing waves in air columns:
Displacement node (i.e. pressure antinodes) at closed end.
Displacement antinodes (i.e. pressure nodes) at open ends
A- open at both ends:
fn = n v/2L (n = 1, 2, 3 …)
B- closed at one end:
fn = n v/4L (n = 1, 3, 5 …)
Doppler effect: (medium stationary)
f’ = (v +/- vD)/(v -/+ vS) f
What if the air( i.e. medium) is moving?!