Transcript 13-2 sound intensity and resonance

13-2 sound intensity and resonance
• Relate sound intensity to amplitude
• Explain why resonance occurs.
The energy of a sound wave is its
intensity (loudness)
• The amount of energy which is transferred to the medium is
dependent upon the amplitude of the wave.
• For example, if more energy is put into the plucking of the
string (that is, more work is done to displace the string a
greater amount from its rest position), then the string vibrates
with a greater amplitude.
•
The amount of energy which is transported past a given area of the medium per unit
of time is known as the intensity (loudness) of the sound wave. The greater the
amplitude of vibrations of the particles of the medium, the more intense that sound
wave is.
As a sound wave carries its energy through a
medium, the intensity of the sound wave
____________ with increasing distance from the
decreases
source.
The mathematical relationship between intensity
and distance is sometimes referred to as an
inverse square relationship.
The scale for measuring intensity is the decibel
scale.
Class work – today’s date
1. A system consists of an oscillator and a speaker that
emits a 1,000.-hertz sound wave. A microphone
detects the sound wave 1.00 meter from the
speaker. The microphone is moved to a new fixed
location 0.50 meter in front of the speaker.
Compared to the sound waves detected at the 1.00meter position, the sound waves detected at the
0.50-meter position have a different
a. wave speed
b. frequency
c. wavelength
d. amplitude
2. Light is to brightness as sound is to
a.
b.
c.
d.
color
loudness
period
speed
3. A stationary research ship uses sonar to send a 1.18 × 103-hertz
sound wave down through the ocean water. The reflected sound
wave from the flat ocean bottom 324 meters below the ship is
detected 0.425 second after it was sent from the ship. [show
work]
a.
Calculate the speed of the sound wave in the ocean water.
b. Calculate the wavelength of the sound wave in the ocean
water.
c.
Determine the period of the sound wave in the ocean water.
Forced vibration and resonance
Natural Frequency
• Nearly all objects, when hit or struck or plucked or strummed
or somehow disturbed, will vibrate. The frequency or
frequencies at which an object tends to vibrate with when
disturbed is known as the natural frequency of the object.
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Forced vibration
• If you were to take a guitar string and pluck it, you would hear
a small sound; On the other hand, if the string is attached to
the sound box of the guitar and you pluck it, the sound
produced would be much louder.
• This is because the vibrating string force the bridge of the
guitar to vibrate, and the bridge force the sound box to vibrate
and the sound box forces air particles inside the box to vibrate.
This forced vibrations are called sympathetic vibrations.
• The tendency of one object to force another adjoining or
interconnected object into vibratio is referred to as a forced
vibration. The forced vibration causes an increase in the
amplitude and thus loudness of the sound.
Vibration at natural frequency
produces resonance
• Resonance - when one object vibrating at the
same natural frequency of a second object
forces that second object into vibration.
Condition for resonance:
1. when the frequency of the periodic force
equals to the natural frequency of the object
it applied to.
2. The amplitude of the original wave is big
enough.
Examples of resonance
• A non vibrating tuning fork, having a natural
frequency of 256 Hz, will resonate when a vibrating
tuning fork with a natural frequency of 256 Hz is
brought near it.
• It is possible for an opera singer to Shattering a glass
by maintaining a note with a frequency equal to the
natural frequency of the glass.
• Collapse of the Tacoma Narrows Bridge due to high
wind induced resonance.
• Pushing a child on the swing with the same rhythm as
the swing will make the swing go higher.
In conclusion
• In conclusion, resonance occurs when two
interconnected objects share the same vibrational
frequency. When one of the objects is vibrating, it
forces the second object into vibrational motion. The
result is a large vibration. And if a sound wave within
the audible range of human hearing is produced, a
loud sound is heard.
Class work – today’s date
1. In a demonstration, a vibrating tuning fork
causes a nearby second tuning fork to begin
to vibrate with the same frequency. Which
wave phenomenon is illustrated by this
demonstration?
a.
b.
c.
d.
the Doppler effect
nodes
resonance
interference
2. Which phenomenon occurs when an object
absorbs wave energy that matches the
object's natural frequency?
a. reflection
b. diffraction
c. resonance
d. interference
example
3. A student in a band notices that a drum
vibrates when another instrument emits a
certain frequency note. This phenomenon
illustrates
a. reflection
b. resonance
c. refraction
d. diffraction