Chapter 12: Sound
Learning Objectives:
Objective 1: Understand how sound is produced p408
 A vibrating source pushes molecules in air back and forth, creating areas of compression and
rarefaction. So for example, your vocal cords would be a vibrating source that is used to create
areas of compression and rarefaction.
 When a molecule moves, it collides with the next one and makes it move too. An example of this
is hitting your hands together. Notice you don’t get a sound until your hands collide.
 The energy of a sound wave travels away from the source trough a series of molecule collisions
parallel to the direction of the wave. This is best observed when you consider how a singer has to
change the shape of their mouth often to create a different sound.
 Reference Source: http://answers.yahoo.com/question/index?qid=20071127032833AAvoKtx
 There are three conditions that must be met in order for sound to occur:
I. You need a vibrating source such as vocal cords or stretched strings
II. A medium – sound cannot travel through a vacuum
III. A receiver – such as your ears. Example: A deaf person can’t hear the sound even if you
have met both conditions.
Objective 2: Understand the Human Ear p420
 Big Picture: The outer receives original (unmodified) sound and sends it to the inner ear the
(modifies) the sounds and sends the message to the brain.
 Analyze:
I. Sound waves travel down the ear canal of the outer ear ending at a thin, flat piece of
tissue known as the ear drum.
II. The ear drum vibrates in response to the sound and then transfers the vibrations to the
small bones in the middle ear known as the hammer, the anvil, and the stirrup. The middle
ear simply serves as a bridge found the sound.
III. The middle ear then in turn transmits the vibration to the cochlea and the basilar
membrane, both which are found in the middle ear.
IV. From the basilar membrane, nerve impulses are then sent the brain for interpretation of
the sound.
Objective 3: Explain rarefaction vs compression p408
 We already know that sound comes out as a wave. These two components help
to determine the characteristic of the sound wave. A more compact sound will
come off as full and rich. A more stretched sound will be more thin and higher
pitched.
 Rarefaction – When the wave has been stretched or its overall length has been
increased. At this particular region, you will find a low molecular desnsity as
well as low air pressure.
 Compression: When the wave has been compacted or its overall length has
been decreased. At this particular region, you will find a high molecular density
as well as high air pressure
Objective 4: Explain the characteristics of sound p408 – p411
 Sound waves are longitudinal waves (vibration is parallel to wave motion)
 Can be drawn using a sine curve where the crest represents compression points (high pressure)
and the troughs represents rarefaction points (low pressure).
 Audible sound waves range from 20 – 20,000 Hz; greater than 20k Hz is ultrasound which can be
heard by dogs. This is often observed by blowing the whistle. Humans don’t hear the sound, but
dogs run directly to the sound.
 Frequency determines the pitch of the sounds (high and low)
 Sound waves propagate (to transmit, reproduce, or spread) in three dimensions
I. Think of this as three people standing in three different corners of a sound source. Will
each person still hear the sound? YES because it will transmit out in three different
directions.
 The speed of the sound can vary depending on the media that it has to travel.
I. Sound cannot travel through a vacuum.
II. Sound waves can also travel trough liquids and solids. Sound waves move faster through
liquids and solids than through gases.
III. Sound waves will travel faster during higher temperature than lower temperatures. Think
of this in terms of particle collision.
Objective 5: Explain the Doppler Effect p412
 Named after the Australian physicist, Christian Doppler
 This is a system that is used to determine location of objects based on the frequency of the
sound.
 It measures how fast the wave signal returns back to the system.
 A similar concept is used by bats to determine the location of objects.
Objective 6: Explain intensity and be able to solve math problems using the formula p414- p417
 The rate at which energy is transferred (power) through a unit area of the plane wave
 In layman’s terms intensity is like the strength at which the sound reaches the receiver
 Distance and intensity are inversely proportionate
 SI Units for intensity are watts/m2
 We can also explain intensity in terms of power using the “Intensity of a spherical wave formula”
I = P/42 I = intensity P = power
Objective 7: Explain characteristics of intensity p414- p418
 Intensity (strength) and frequent (speed) can determine which sounds are audible
 Loudness is not directly proportionate to intensity. Think of an earthquake. Although the
earthquake is felt with great intensity, notice that it’s not heard. You only hear the objects
moving in response to the earthquake.
 Police officers can use a device to measure how loud your music is. This helps them to interpret if
you are in violation of a noise ordinance. They compare the intensity of your music in relation to
set level and it gives them a measurement in decibels. Decibels is a dimensionless value.
Objective 8: Understand the concept of resonance p419
 The increase in amplitude of oscillation of an electric or mechanical system in response to a
periodic force whose frequency is equal or very close to the natural frequency of the system. For
the most part it acts as constructive interference.