Download Chapter 2 – The Nature of Sound

Chapter 24 –Sound
24.1 –Properties of Sound
pp. 578 - 582
TERMS TO LEARN
• Pitch – how high or low a sound is
• Infrasonic – a term that describes
sound with frequency lower than 20
Hz
• Ultrasonic – term that describes
sounds with frequencies higher than
20, 000 Hz
TERMS TO LEARN
• Doppler effect – the apparent change
in frequency of a sound caused by
the motion of either the listener or
the source of the sound
• Loudness- how loud or soft a sound is
• Decibel – the most common unit used
to express loudness.
You are the commander of a space
station located about halfway between
Earth and the moon. You are in the
command center, and your chief of
security tells you that sensors have just
detected an explosion 61,054 km from
the station. How long will it be before
you hear the sound of the explosion?
SPEED OF SOUND
• Depends on the medium through
which it is traveling.
• Changes when the medium changes.
• Sound travels faster in steel than in
water.
• Speed of sound depends on
temperature. The cooler the
temperature, the slower the speed of
sound.
24.1 The speed of sound
• The speed of sound in
•
•
normal air is 343
meters per second (660
miles per hour).
Sound travels through
most liquids and solids
faster than through air.
Sound travels about five
times faster in water,
and about 18 times
faster in steel.
24.1 The speed of sound
• Objects that move
•
faster than sound are
called supersonic.
If you were on the
ground watching a
supersonic plane fly
toward you, there
would be silence.
The sound would be
behind the plane,
racing to catch up.
24.1 The speed of sound
• Passenger jets are subsonic because they
travel at speeds from 400 to 500 mi/hr.
PITCH
• The pitch of a sound
•
•
is determined by the
frequency of the
sound wave.
High pitch = high
frequency
Low pitch = low
frequency
24.1 The frequency of sound
• Almost all the sounds you hear contain many
•
frequencies at the same time.
Humans can generally hear frequencies between
20 Hz and 20,000 Hz.
FREQUENCIES YOU CANNOT HEAR
• Sounds with frequencies lower than 20 Hz
are called infrasonic.
• “Infra” means “below”.
• Sounds with frequencies higher than
20,000 Hz are called ultrasonic.
• “Ultra” means “beyond”
• Ultrasonic waves are used to clean jewelry
and to remove ice from metal.
24.1 The frequency of sound
• Sounds near 2,000 Hz
seem louder than
sounds of other
frequencies, even at
the same decibel level.
• According to this
curve, a 25 dB sound
at 1,000 Hz sounds
just as loud as an 40
dB sound at 100 Hz.
THE DOPPLER EFFECT
• For sound waves, the Doppler effect is the
apparent change in frequency of a sound caused
by the motion of either the listener or the source
of the sound.
LOUDNESS
• Loudness is related to amplitude.
• The higher the amplitude of the wave, the
louder the sound is.
• The lower the amplitude of the wave, the
softer the sound is.
• Loudness is measured in decibels (dB).
SOME COMMON DECIBEL LEVELS
• Whisper = 20 decibels
• Normal conversation = 60 dB
• Automobile horn = 115 dB
• Rock concert = 115 dB
• Threshold of pain = 120 dB
• Jet engine 30 m away = 140 dB
SOME COMMON DECIBEL LEVELS
24.1 Recording sound
One second of compact-disc-quality sound is a list
of 44,100 numbers which represents the
amplitudes converted sounds.
24.1 Recording sound
To play the sound back, the string of numbers is
read by a laser and converted into electrical
signals again by a second circuit which reverses
the process of the previous circuit.
24.1 Recording sound
The playback circuit converts the string of numbers
back into an electrical signal.
The electrical signal is amplified to move the coil in
a speaker and reproduce the sound.
SEEING SOUNDS
• An oscilloscope
is used to
graph
representations
of sound
waves.
HOW IT WORKS
• A microphone converts the sound waves
into an electric current.
• The oscilloscope converts the electric
current into graphs. The highest point
(crests) represent compressions of the
sound wave; the lowest point (trough)
represents rarefactions.