Transcript Introduction to Waves and Sound

Introduction to
Waves and Sound
Chapters 14 and 15
Periodic Motion
 Motion that occurs in a regular cycle.
 Period – the time it takes to complete one
cycle (T)
 Amplitude – the maximum distance the
object moves from equilibrium
 Can occur back and forth (like a
pendulum) or up and down (like a weight
suspended from a spring)
Resonance
 An increase in amplitude that occurs
when small forces are applied at regular
intervals to a vibrating or oscillating
object.
 Some examples include:
 Pumping your legs while you swing.
 Jumping on a trampoline.
 Bridges collapsing when soldiers cross in
unison.
Wave Types
 Wave – a disturbance that carries energy
through matter or space
 Transverse
 Wave vibrates perpendicular to the direction of
motion
 Ex. Light waves
 Longitudinal
 Waves vibrate along the direction of motion
 Ex. Sound waves
 Surface waves
 A combination of both other wave types
 Ex. Water waves
Measuring Waves
 Speed – displacement divided by time (v)
 Wavelength – distance between identical
parts of a wave (λ)
 Phase – particles that have the same
displacement and the same velocity
 Frequency – how many oscillations occur
in 1 s (f)
Diagram of a Wave
 Transverse
crest
amplitude
wavelength
 Longitudinal
rarefaction
trough
Wave Behavior
 Amplitude measures energy
 2 x the amplitude is 4x the energy
 Medium affects speed:




Temperature
Depth
Tension
Density
 When the medium changes, wave speed changes
 This results in part of the wave being transmitted, and
part being reflected back into the original medium
 The reflected wave will be inverted if the new medium
has a slower speed than the original
Superposition
 Occurs when more than one wave is in the same
place at the same time.
 Results in wave interference
 2 types:
 Constructive – in phase waves create a new wave with
larger amplitude
 Destructive – out of phase waves create a new wave
with less amplitude
 Standing wave – special type of interference
where parts are exactly in phase (antinodes) and
parts are exactly out of phase (nodes), resulting in
a wave that appears not to move
 Faster vibrations result in more nodes (harmonics)
When Waves Hit a Barrier
 Normal – a line perpendicular to the barrier
 Reflection
 The wave returns back into the medium it came
from
 Law of Reflection – the angle of incidence is equal
to the angle of reflection as measured from the
normal
 Refraction
 The wave changes direction (gets bent) because of
a change in speed
Sound Waves
 A longitudinal wave caused by something
vibrating causing molecules (usually in
the air) to vibrate.
 Typical sound wave speed: 340 m/s
 Changes in warmer temps or in more solid
materials
 Must be in a medium to travel (no sound in
space)
Hearing
 Vibrating air molecules cause the ear drum to
vibrate, in turn causing bones in the ear to
vibrate.
 Next, the vibration gets transferred to tiny hairs
in the inner ear.
 Losing these hairs is a typical reason for hearing
loss
 These hairs trigger nerve impulses which send
messages to the brain to process the sound.
 Humans can detect and process sounds between
20-16,000 Hz typically
Perception of Sound
 Pitch – how high or low a sound is
 Caused by the frequency of sound
 Loudness – how strong a sound is
 Caused mostly by the amplitude
 Measured in decibels (dB)
 10 dB is barely audible to humans, 110 dB is
the average rock concert
 Long term exposure to sounds around 100
dB causes permanent hearing loss
Doppler Effect
 A change in a sound’s apparent frequency due
to motion of the source or receiver.
 Red shift – frequency decreases because the
source is getting further away
 Blue shift – frequency increases because the source
is getting closer
 The faster the motion, the more dramatic the effect
 Used in animals (like bats and dolphins) and in
medicine (like sonograms)
Interference in Sound
Waves
 Resonance
 An increase in sound produced occurs when forced
vibration equals the natural vibration (constructive
interference occurs)
 Affected by length of air column, size of vibrating
surface, etc
 Beats
 Alternating periods of constructive and destructive
interference that result in a throbbing effect