can you explain breafily about Resonance ?
The word resonance comes from Latin and means to "resound" - to
sound out together with a loud sound. Resonance is a common cause
of sound production in musical instruments. One of our best models
of resonance in a musical instrument is a resonance tube (a hollow
cylindrical tube) partially filled with water and forced into
vibration by a tuning fork. The tuning fork is the object that
forced the air inside of the resonance tube into resonance. As the
tines of the tuning fork vibrate at their own natural frequency,
they created sound waves that impinge upon the opening of the
resonance tube. These impinging sound waves produced by the tuning
fork force air inside of the resonance tube to vibrate at the same
frequency. Yet, in the absence of resonance, the sound of these
vibrations is not loud enough to discern. Resonance only occurs
when the first object is vibrating at the natural frequency of the
second object. So if the frequency at which the tuning fork
vibrates is not identical to one of the natural frequencies of the
air column inside the resonance tube, resonance will not occur and
the two objects will not sound out together with a loud sound. But
the location of the water level can be altered by raising and
lowering a reservoir of water, thus decreasing or increasing the
length of the air column. As we have learned earlier, an increase
in the length of a vibrational system (here, the air in the tube)
increases the wavelength and decreases the natural frequency of
that system. Conversely, a decrease in the length of a vibrational
system decreases the wavelength and increases the natural
frequency. So by raising and lowering the water level, the natural
frequency of the air in the tube could be matched to the frequency
at which the tuning fork vibrates. When the match is achieved, the
tuning fork forces the air column inside of the resonance tube to
vibrate at its own natural frequency and resonance is achieved. The
result of resonance is always a big vibration - that is, a loud
sound.
Another common physics demonstration that serves as an excellent
model of resonance is the famous "singing rod" demonstration. A
long hollow aluminum rod is held at its center. Being a trained
musician, teacher reaches in a rosin bag to prepare for the event.
Then with great enthusiasm, he/she slowly slides her hand across
the length of the aluminum rod, causing it to sound out with a loud
sound. This is an example of resonance. As the hand slides across
the surface of the aluminum rod, slip-stick friction between the
hand and the rod produces vibrations of the aluminum. The
vibrations of the aluminum force the air column inside of the rod
to vibrate at its natural frequency. The match between the
vibrations of the air column and one of the natural frequencies of
the singing rod causes resonance. The result of resonance is always
a big vibration - that is, a loud sound.
The familiar sound of the sea that is heard when a seashell is
placed up to your ear is also explained by resonance. Even in an
apparently quiet room, there are sound waves with a range of
frequencies. These sounds are mostly inaudible due to their low
intensity. This so-called background noise fills the seashell,
causing vibrations within the seashell. But the seashell has a set
of natural frequencies at which it will vibrate. If one of the
frequencies in the room forces air within the seashell to vibrate
at its natural frequency, a resonance situation is created. And
always, the result of resonance is a big vibration - that is, a
loud sound. In fact, the sound is loud enough to hear. So the next
time you hear the sound of the sea in a seashell, remember that all
that you are hearing is the amplification of one of the many
background frequencies in the room.
Resonance and Musical Instruments
Musical instruments produce their selected sounds in the same
manner. Brass instruments typically consist of a mouthpiece
attached to a long tube filled with air. The tube is often curled
in order to reduce the size of the instrument. The metal tube
merely serves as a container for a column of air. It is the
vibrations of this column that produces the sounds that we hear.
The length of the vibrating air column inside the tube can be
adjusted either by sliding the tube to increase and decrease its
length or by opening and closing holes located along the tube in
order to control where the air enters and exits the tube. Brass
instruments involve the blowing of air into a mouthpiece. The
vibrations of the lips against the mouthpiece produce a range of
frequencies. One of the frequencies in the range of frequencies
matches one of the natural frequencies of the air column inside of
the brass instrument. This forces the air inside of the column into
resonance vibrations. The result of resonance is always a big
vibration - that is, a loud sound.
Woodwind instruments operate in a similar manner. Only, the source
of vibrations is not the lips of the musician against a mouthpiece,
but rather the vibration of a reed or wooden strip. The operation
of a woodwind instrument is often modeled in a Physics class using
a plastic straw. The ends of the straw are cut with a scissors,
forming a tapered reed. When air is blown through the reed, the
reed vibrates producing turbulence with a range of vibrational
frequencies. When the frequency of vibration of the reed matches
the frequency of vibration of the air column in the straw,
resonance occurs. And once more, the result of resonance is a big
vibration - the reed and air column sound out together to produce a
loud sound. As if this weren't silly enough, the length of the
straw is typically shortened by cutting small pieces off its
opposite end. As the straw (and the air column that it contained)
is shortened, the wavelength decreases and the frequency was
increases. Higher and higher pitches are observed as the straw is
shortened. Woodwind instruments produce their sounds in a manner
similar to the straw demonstration. A vibrating reed forces an air
column to vibrate at one of its natural frequencies. Only for wind
instruments, the length of the air column is controlled by opening
and closing holes within the metal tube (since the tubes are a
little difficult to cut and a too expensive to replace every time
they are cut).
Resonance is the cause of sound production in musical instruments.
In the remainder of Lesson 5, the mathematics of standing waves
will be applied to understanding how resonating strings and air
columns produce their specific frequencies.
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