Download Chapter 12: Sound

Section 4: The Nature of Sound
Sounds
 All sounds are caused by something that
vibrates.
 Your vocal cords vibrate in relation to
controlled bursts of air and produce sounds
to give rise to speech.
 Sound waves are longitudinal waves that rely
on moving matter (a medium) to transmit
the energy.
Moving through a Medium
 Sounds travel at different speeds through
different mediums.
 Sounds move fastest through solids as the
molecules are closer together.
 Liquids transmit sound at a slower speed
than in solids.
 Sounds travel the slowest in gases as the
molecules are most spread out.
 As sound is a mechanical wave, it requires a
medium. Sound is not transmitted in outer
space as there is not enough matter.
Hearing Damage
 What commonly occurs in hearing damage is that
hearing has been damaged due to constant
exposure to loud sounds.
 Damage occurs in the inner ear at the hair cells in
the cochlea.
 Hair cells can be easily damaged or destroyed by
continuous exposure to excessive volume levels.
 These sensitive hairs do not grow back if lost and
the resulting loss of hearing is permanent.
Intensity of Sound
 The amount of energy of sound is measured in its
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intensity. Intensity is a measure of amplitude and the
distance from a sound source.
Intensity is measured in a unit called the decibel.
The decibel scale measures sound intensity levels and
can be used to prevent hearing damage.
Generally prolonged exposure to sound levels above 90
dB (decibels) cause permanent hearing damage. See
figure 15 on page 515.
Hearing damage is slow but cumulative and
permanent. Watch your ipod volume level!
Frequencies of Sound
 Recall that frequency is a measure of how many
wavelengths pass a point per second.
 In sound, frequency describes the pitch of a sound.
 A wave with a low frequency has a low pitch like a low
bass note.
 A wave with a high frequency has a high pitch like high
treble note.
 Humans vary in the range of frequencies they can hear,
but generally our hearing range is from 20 Hz to
20,000 Hz.
Infrasound and Ultrasound
 Infrasound is a range of frequencies that are below
human hearing (< 20 Hz).
 Ultrasound is a range of sound frequencies that is
above 20,000 Hz.
 Ultrasound is what is utilized in underwater sonar
as the distance of objects is measured in the time it
takes for an ultrasound echo (reflection) to return.
 Ultrasound is used in medicine to map structures
in the body (like a fetus) and can be used in some
operations to break up matter (kidney stones).
The Doppler Effect
 Sound is a series of compressions and rarefactions
as carried in a sound wave.
 As objects move toward you, the pitch of the sound
seems to rise. This phenomenon is caused by the
sound wave compression being closer together.
 As an object moves away from you, the pitch seems
to drop lower. This is because the sound waves are
becoming more spaced apart.
 The Doppler effect is defined as the change in
pitch or wave frequency due to a moving wave
source.
Doppler Effects
 If you are moving past a stationary sound source,
you will also experience the Doppler effect.
 Basically either you or the sound source must move
to cause the Doppler effect.
 The Doppler effect can be used to detect the speed
of moving cars (radar guns) or to track wind
movement in dangerous storms.