Chapter 20
Sound
Basics of Waves
• Amplitude: The height of the wave from
center to crest or center to trough.
• Wavelength: The physical length of one cycle a
wave.
• Frequency: The number of waves in 1 second.
Types of Waves
• Transverse Waves: A wave that moves
perpendicular to its medium.
• Longitudinal Wave: A wave that moves parallel
to its medium.
Origin of Sound
• Sounds waves are produced by elastic material
objects.
• Vibrations in elastic material create pressure
disturbances that propagate through a given
medium.
• Frequency corresponds to pitch; how many
waves there are in a second.
Nature of Sound in Air
• Sound is a wave that moves by causing
particles to move to and fro.
• High and low pressure zones are created.
• The high pressure portion of a sound wave is
called compression.
• The low pressure portion of a sound wave is
called rarefaction.
Mediums for Sound
• Elasticity is the ability of a material to
change shape without permanent
deformation.
• Sound travels 4x faster in water then air
and 15x faster in steel.
• Liquids and crystalline solids are excellent
conductors of sound.
• There is no sound in a vacuum.
Speed of Sound in Air
• Speed of sound in air is dependant on
humidity, temperature, and wind conditions.
• No matter how loud all sound travels at the
same speed.
• Speed of sound in dry air at 0ºC is 330m/s or
738mph.
Reflection of Sound
• Reflection of sound is called an echo.
• Multiple reflections of the same sound are
called reverberations.
• Theaters, Opera houses, and stages are
designed to maximize reflections for the best
sound quality.
• The study of sound is call acoustics.
Refraction
• Since the speed of sound is dependant on
temperature, moving through different
thermal regions causes the speed of sound to
change, this is called refraction.
• Utilizing refraction and reflection make ultra
sound images possible.
• Dolphins and bats use echo location
techniques.
Energy in Waves
• Sound is a weak form of energy transfer.
• Sound energy is dissipated as thermal energy.
• When struck objects ring at their natural
frequency.
• Natural frequency is the frequency that
objects “want” to vibrate at.
Resonance
• When an object vibrates at its Natural
Frequency it is in Resonance.
• All objects with springiness have a resonance
frequency.
• When crossing bridges soldiers break step to
avoid hitting the bridge's resonance
frequency.
Interference
• Waves can add together or cancel out.
• Phase is a term that describes how two waves
are positioned from each other.
• Constructive interference happens when two
waves are in “phase”.
• Destructive interference happens when two
waves are out of “phase”.