Honors Physics
Name: ________________________
CHAPTER 16 NOTES:
WAVES AND SOUND
16.1 The Nature of Waves and 16.2 Periodic Waves
Wave – a traveling disturbance
Waves carry energy from place to place (waves transfer energy)
There are two basic types of waves:
1. transverse – particles move perpendicular to the wave motion
(examples: radio waves, light, microwaves)
Sketch:
2. longitudinal – particles move parallel to the wave motion
(example: sound)
Sketch:
medium – material through which a wave travels
pulse wave – a non-periodic disturbance
periodic wave – patterns that are produced over and over again by the source of the wave
Amplitude (A) – maximum distance traveled from equilibrium
Wavelength (λ) – horizontal distance between two successive crests
Period (T) – time required for one complete cycle
Frequency (f) – number of cycles per second
f= 1
T
T= 1
f
speed =
distance
time
= wavelength
period
Wave speed:
v=λf
The medium determines the wave speed
16.3 Speed of a wave on a string:
v=
F
m
L
16.5 The Nature of Sound
Sound is a longitudinal wave created by a vibrating object
Condensation – region of increased pressure in a sound wave
Rarefaction – region of lower pressure in a sound wave
Pure tone – sound with a single frequency
Audible sound waves – sound waves that can be heard (f = 20 to 20,000 Hz)
Infrasonic sound waves – sound waves with frequencies less that 20 Hz
Use: rhinoceroses and elephants use infrasonic waves to communicate
Ultrasonic waves – sound waves with frequencies greater than 20,000 Hz
Use: bats and dolphins use ultrasonic waves for echolocation and medical
ultrasounds
16.6 The Speed of Sound
• The speed of a sound wave depends on the properties of the medium the wave
travels through.
• Sound travels slowest through gases and fastest through solids. The speed of
sound in liquids is somewhere in between the speed in gases and the speed in
solids.
• The speed of sound in air is about 343 m/s at room temperature.
16.7 Sound Intensity
Power (P) – amount of energy transported per second by a sound wave (units: J/s or
Wattts (W))
Sound Intensity (I) – sound power that passes perpendicularly through a surface divided
by the area of that surface (units: W / m2)
I= P
A
If a sound is emitted uniformly in all directions and no walls, ceilings, floors, ect.
are present to reflect sounds:
I= P
4 π r2
Threshold of Hearing – smallest sound intensity that the human ear can detect
(about 1 x 10-12 W / m2)
16.8 Decibels
Decibel (dB) – a measurement unit used for comparing two sound intensities
Intensity Level (β)
#I &
! 2 " !1 = (10dB)log % 2 (
$ I1 '
" I%
! = (10dB)log $ '
# Io &
“log” denotes the logarithm to the base 10
Io = 1.00 x 10-12 W / m2
(threshold of hearing is used as a reference)
An intensity level of zero does not mean that the sound intensity is zero.
It means that I = Io.
Experiment shows that if the intensity level increases by 10 dB, the new sound seems
approximately twice as loud as the original sound.
16.9 The Doppler Effect
Doppler Effect – the change in frequency or pitch of the sound detected by an observer
because the sound source moves relative to the observer.
Car at rest
car moving to the right
Doppler Effect Formulas
Source moving toward stationary observer:
"
%
$ 1 '
f o = f s$
v '
$1! s '
#
v&
Source moving away from stationary observer:
!
$
# 1 &
f o = f s#
v &
#1+ s &
"
v%
Observer moving toward stationary source:
! v $
f o = f s#1+ o &
"
v%
Observer moving away from stationary source:
" v %
f o = f s$1! o '
#
v&
For all of the above equations:
v = speed of sound
vs = speed of source
vo = speed of observer
fs = frequency emitted by the source
fo = frequency heard by the observer