Chapter 26 -- Sound
26.1 The
origin of
Sound
p. 401 #1-4
Compare sound vibrations to shaking a paddle in a tub full
of ping pong balls. The movement or vibration travels
outward from the source because balls near the paddle hit
balls around them which hit balls around them and so forth.
Sound is caused by vibrations.
Sound must have a medium of matter to move. This is
usually air.
Sound waves are longitudinal waves.
Pitch is how we describe our subjective impression about
the frequency of sound. Think about a note sung high
(soprano) or low (baritone).
The range of frequencies that humans can hear is typically
20Hz to 20,000Hz
Chapter 26
vocabulary
Below 20Hz is infrasonic <20Hz
Above 20000Hz is ultrasonic >20,000Hz
Beats—a variation in loudness resulting from interference
from two sounds with slightly different frequencies.
Compression—when the molecules in a sound medium are
pushed together making higher pressure.
Forced vibration – when another object is forced to vibrate
making a louder sound
Infrasonic—lower than humans can hear. <20hz
Natural frequency—the rate at which an object will vibrate
on its own.
Pitch—“subjective experience of frequency” how we
perceive a sound to be higher or lower.
Rarefaction – opposite of a compression. An area of low
pressure in a longitudinal wave.
Resonance—when an object is forced to vibrate at its
natural frequency.
Ultrasonic – higher frequencies than humans can hear.
>20,000
Elastic (26.4) – the ability to resume its original shape quickly
Decibels (26.5) – unit for measuring the loudness of a sound
-- the intensity or energy in a sound.
26.2 Sound in Pulses from sound vibrations cause compressions and
Air
rarefactions in air. Compressions have higher pressure &
rarefactions have lower pressure.
figure 26.5
Medium does not travel. Think a “wave” at a football game.
p. 401 #5
26.3 Media
“ear to the ground” we can hear sound faster. Also, in
that
water, sound travels faster – we can hear under water.
Transmit
Sound
p. 401 #6
Generally sound moves faster in solids and liquids than in
air. This is because the molecules are closer together and
the materials are more “elastic” -- that is in the scientific
meaning
26.4 Speed
of Sound
p. 401 #7-9
working the
No medium—no sound
Light travels much faster than sound
1200km/hr or 330m/s at 0°C. At room temp, sound typically
travels 340m/s
track meet—
watch for
Sound speed depends on elasticity. By elastic, scientist
smoke from
mean the ability of a material to return to its original shape
the starter’s after a distortion.
pistol – not
sound
26.5
Loudness
26.6 Forced
Vibation
p. 401 #10
26.7 Natural
Frequency.
p. 402 #11-12
26.8
Resonance
p. 402 #13-16
Loudness can be measured by scientific instruments as
intensity. How different people perceive loudness is
subjective but related to intensity.
Sound intensity is measured in decibels (this is named after
Alexander Graham Bell). The decibel scale is logarithmic, so
a sound of 70 decibels is 1000 times more intense than a
sound of 40 dB.
A tuning fork held to a table makes a louder sound because
the table acts as a “sounding board.” This is called forced
vibration and is the principal behind all stringed instruments
such as guitars, pianos and violins. The surface that is
vibrating is larger, so the sound is louder – more air is being
moved.
Any object will vibrate at its own frequency based on the
elasticity and shape of the object. Think different shaped
handbells.
**”a natural frequency is one at which minimum energy is
required to produce forced vibrations.” P. 395 and the
least energy to continue vibrating.
Resonance – when frequency of a forced vibration matches
the object’s natural frequency. “Resonance means to sound
and resound again.”
Two tuning forks tuned to the same frequency. Strike one
and it will make the other vibrate. This is because the
compressions in the air resonate with the frequency of the
second tuning fork.
Through history resonance has cause bridge collapses and
makes “washboard” roads.
26.9
Interference
Interference can increase amplitude or decrease (cancel)
amplitude depending on where two waves overlap.
p. 402 #17-18
Interference can affect the loudness of sounds
26.10 Beats
p. 402 #19-20
Interference is the physics principle that allows for noisecancelling headphones. They produce “anti-noise” waves
that interfere with other sound waves.
When two sounds are at slightly different frequencies,
there is a fluctuation in the loudness. This can be seen
when you sing into a fan. Two tuning forks with a similar
pitches will produce this effect as well. This variation or
fluctuation in loudness is called beats.
When the wavelengths are “in-phase” the sound is louder.
When the wavelengths are “out of phase” there is
destructive interference and the sound is softer.
This could be compared to two people walking together
with different strides – sometimes they will be in step, but
sometimes out of step.
Two combs can be used to show the interference of waves
producing beats. The beat from interference will have its
own frequency because it will follow a repetitious pattern.
Vocabulary on Sound
Beats: a throbbing variation in the loudness of sound caused by interference when two tones of slightly different
frequencies are sounded together.
Compression: a pulse of compressed air; the opposite of a rarefaction
Forced vibration: the vibration of an object that is made to vibrate by another vibrating object that is nearby. The
sounding board in a musical instrument amplifies the sound through forced vibration.
Infrasonic: term applied to sound pitch too low to be heard by the human ear, that is below 20 hertz
Natural frequency: a frequency at which an elastic object, once energized, will vibrate. Minimum energy is required
to continue vibration at that frequency. Also called the resonant frequency.
Pitch: term that refers to how high or low sound frequencies appear to be.
Rarefaction: a disturbance in air (or matter) in which pressure is lowered. Opposite of compression.
Resonance: a phenomenon that occurs when the frequency of forced vibrations on an object matches the object’s
natural frequency, and a dramatic increase in amplitude results.
Ultrasonic: Term applied to sound frequencies above 20, 000 hertz, the normal upper limit of human hearing.