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SOL 5.2 a, b, c
March 6, 2011
Sound Notes
Sound is a form of energy all around us.
All sound energy is produced by vibrations. Vibrations are the back
and forth motions of objects.
All sound moves as waves through matter (solid, liquid, gas). Sound
cannot move through a vacuum, because there are no molecules to
vibrate back and forth.
The strengthening of a sound wave is called resonance.
Sound travels in compression waves.
Compression waves are made up areas of increased pressure called
compressions, and areas of decreased pressure called rarefactions.
Think of a slinky pressed together (compressions) and then spread
apart (rarefactions).
Sound waves have different parts. The top of a sound wave is called
the crest, the bottom of a sound wave is called the trough, and the
distance between two side by side crests is called the wavelength.
When an object vibrates very quickly, the number of vibrations per
minute increase and the sound becomes higher. When this same object
vibrates more slowly, the number of vibrations per minute decrease,
and the sound becomes lower.
The number of vibrations of an object in a unit of time is called its
The highness and lowness of a sound wave is its pitch.
Longer, larger objects (trombone) tend to vibrate slower than
shorter, smaller objects (flute).
The speed of a sound depends on the kind of matter it is moving
Of the three states of matter (gas, liquid, and solid) sound waves
travel the slowest through gases, faster through liquids, and fastest
through solids.
In a gas the molecules are spaced very far apart. For sound to travel
through air, the floating molecules of matter must vibrate and collide
to form compression waves. Because the molecules of matter in a gas
are spaced far apart, sound does not move very quickly through them.
Sound travels faster in liquids than in gases because molecules are
packed more closely together. This means that when the water
molecules begin to vibrate they quickly begin to collide with each
other forming a rapidly moving compression wave. Sound travels over
four times faster than in air!
Sound travels fastest through solids. This is because molecules in a
solid are packed against each other. When a vibration begins, the
molecules of a solid immediately collide and the compression wave
travels rapidly. How rapidly, you ask? Sound waves travel over 17
times faster through steel than through air.
As humans, we move air from our lungs across our vocal chords to
produce sound waves. These sound waves create the sound we know as
the human voice. Many animals like dogs and bats also create sounds
in this way. However, marine mammals like whales pass air through air
sacks located in their heads to create sounds.
Hearing is the detection of sound.
Sonar is a type of device used to detect sound. Ships use sonar to
help them navigate their vessels, explore underwater areas of the
ocean, and to find food.
Some animals, such as bats and whales, used echolocation to help them
find food, or even to escape predators. These animals send out sound
waves and then measure how quickly they receive the sounds back to
tell how close they are to food or near an enemy.
All animals detect sound differently because different animals are
able to hear different frequencies of sound.
As humans, our hearing is adapted to hear best the sounds we use
every day.
Because animals can hear sounds at different frequencies, some
animals can hear sounds that humans can’t.
Other words to know:
Volume – How loud or quiet a sound is
Oscilloscope – A device used to take pictures of sound waves
Hertz – measures frequency
Decibels – measures volume
Absorption – The disappearance of a sound wave into a surface, usually
making the sound softer
Reflection – The bouncing of a sound wave off a surface, usually making the
sound louder