Physics
Name______________________________
Chapter 12 Sections 1 and 2 Notes – SOUND
All sounds begin with a __vibration_____.
Sound waves can not travel through a vacuum; they require a medium, so they are
_mechanical____________________ waves.
Sound waves are also _longitudinal___waves, thus particles in the medium travel
___parallel_____ to the wave motion.
The region of high molecular density and high air pressure is called a
__compression______. The region of low molecular density and low air pressure is called
a ________rarefaction_______.
Audible sound waves (that the average human ear can hear) have frequencies
between__20__ and ___20,000____Hz (cycles per second).
Sound waves below the audible range, below ___20______Hz, are called
_____infrasonic_________waves. These low frequency waves have
____long____wavelengths.
Although humans can not hear these low frequency sound waves, other creatures can, and
sometimes use such low frequency sound waves to communicate.
Sound waves above the audible range, above __20,000__Hz, are called
_____ultrasonic____________waves. These high frequency sound waves have
___short______ wavelengths.
Although humans can not hear these high frequency sound waves, other creatures can, and
sometimes use these high frequency sound waves to detect objects. Ultrasonic waves are
also used in medical sonography to view soft tissue and in SONAR to locate objects.
These high frequency waves have __short__ wavelengths, and thus most are reflected,
enabling a picture to be formed (sonography) or the time and hence location, shape, and
size of the object to be determined (echolocation, SONAR).
Pitch is how __high___ or _____low_______ we perceive a sound to be. It depends upon
___frequency______ as well as other factors such as background noise and loudness.
Pitch and frequency are __directly____ related.
The speed of sound depends on the ___medium____ in which the sound waves travel. In
general, sound travels faster in __solids____ than in liquids and faster in liquids than in
____gases_________. This occurs because the particles are closer together in solids than
in liquids, and closer together in liquids than in gases. As the sound wave travels, the
motion of one particle is transferred to another particle. This happens faster when the
particles are closer together.
The speed of sound also depends on ____temperature____. As the
____temperature_____ rises, the particles collide more frequently allowing the sound
wave to spread faster. This increase in the speed of sound is most noticeable in gases.
Sound waves propagate in __3_________ dimensions. As these spherical waves move
away from their source, they become almost __parallel__. At distances from the source
that are much greater than the wavelength, the spherical sound waves can be treated as
____plane waves____. These ____plane waves_________ can be treated as 1
dimensional waves traveling in the same direction.
The Doppler Effect
Whenever there is relative motion between an object that is emitting sound waves and an
observer, the Doppler Effect occurs. If a moving police car with its siren blaring
approaches a stationary observer, the wavelength of the sound waves decreases and the
frequency increases. Increased frequency leads to ___higher____ pitch. As the moving
vehicle passes and moves away from the stationary observer, the wavelength of the sound
waves increase and the frequency decreases. Decreased frequency leads to
___lower___ pitch.
When a tuning fork is struck, the prongs will eventually stop moving because the kinetic
energy of the vibrating prongs changes to _sound____ energy.
Intensity
As sound waves travel outward from their source, energy is _transferred from one particle
to the next___. The rate at which this energy is transferred per unit area of the plane
wave is called intensity.
Intensity = (ΔE/Δt) /area_=__P/area__=___P/4πr2___
P=
power in watts, rate of energy transfer
Units: W/m2
r = radius of sphere in meters
Intensity _decreases___ as the distance from the source increases.
Intensity determines the volume, but the volume is not directly proportional to the intensity.
(nearly logarithmic)
Relative intensity is the ratio of the intensity of a specific sound wave to the intensity at the
threshold of hearing._
Relative intensity is measured with the decibel, a dimensionless unit.
Conversion of Intensity to Decibel Level
Intensity (W/m2)
Decibel (dB)
Example
1.0 X 10-12
threshold of hearing
1.0 X 10-11
breathing
1.0 X 10-10
rustling leaves
1.0 X 10-9
whisper
1.0 X 10-8
refrigerator humming
1.0 X 10-7
normal conversation
1.0 X 10-6
laughter
1.0 X 10-5
vacuum cleaner
1.0 X 10-4
traffic
1.0 X 10-3
lawn mower
1.0 X 10-2
snowmobile
1.0 X 10-1
rock concert
1.0 X 100
threshold of pain
1.0 X 101
machine gun
1.0 X 102
jet engine at 100 feet