The Physics Of Sound
Sound surrounds us at all times. The ring of an alarm clock or the whistle of
birds may wake us up in the morning. Through out the day, we hear a variety of
sounds; for instance, the banging of pots and pans, the roar of traffic, and the
voices of people. When we fall asleep at night, we might listen to the sounds of
frogs croaking or the wind whistling. All sounds have one thing in common. This
being that the vibrations of an object makes every sound. When an object
vibrates, it causes the air around it to vibrate. The vibrations in the air
travel out from all directions of the object. They then reach our ears, and the
brain reads them as sounds. Many sounds we hear travel through the air; however,
they can also travel through solid objects like the earth. Our voice is made in
the larynx, which is a part of the throat. Two small pieces of tissue are spread
across the larynx. These pieces, called vocal chords, make the opening smaller.
Air from he lungs quickly passes trough the tightened chords, causing them to
vibrate. The vibrations make the sound of the voice. The tighter the chords are,
the faster the vibrations and the higher the resulting sound. If a rock is
thrown onto a still pond, several waves travel out from the place where the rock
hit the surface. Likewise, sound moves in waves through the air or some other
medium. The understanding that sound travels in the form of waves may have
originated with the artist Leonardo Da Vinci in 1500. Generally, waves can be
spread transversely or longitudinally. In both cases, only the energy of wave
motion is spread through medium. No portion of the medium actually moves very
far. As the waves caused by the moving object travel outward, they are carried
by a medium. This movement causes compression. As the movements move backward it
is called rarefractions. Sound requires a medium to travel; therefore, it cannot
travel in space, which is a vacuum with no medium. The number of compressions
and rarefractions per second is called frequency, The more rapidly and object
vibrates, the higher the frequency. Frequency is measured in hertz.
As the
frequency of a sound wave increases, the wavelength decreases. Wavelength is the
distance between one point on a wave to the corresponding point on the next
wave. Most people hear sounds with frequencies that fall between 20 and 20,000
hertz. Many animals can hear sounds about 20,000 hertz. A person’s voice can
have a range of about 85 to 1,100 hertz. When sound waves leave one medium and
enter another in which the speed of sound differs, the direction of the waves is
altered. This change in direction results from a change in the speed of waves
called refraction. Sound waves can also be refracted if the speed of the sound
changes according to their position in a medium. The waves bend toward the
region of slower speed. Sound travels farther when the air is cooler. During the
day, the ground is warmer than the air above. Sound waves are bent away from the
ground into the cooler air above, where their speed is slower allowing the sound
to be heard over longer distances. The spreading out of waves as they pass by
the edge of an obstacle or through and opening is called diffraction. It occurs
whenever a sound wave encounters an obstacle or opening. Diffraction enables
sound to be heard around a corner, even though no straight path exists from the
source of the sound to the ear. If sound travels at about the same speed in both
materials with the same density, little sound will be reflected. Instead, most
of the sound will be transmitted into the new medium. If the speed differs
greatly in the two mediums and their densities are greatly different, most of
the sound will be reflected. The intensity of a sound is related to the amount
of energy in the sound waves. Intensity depends on the amplitude of the
vibrations making the waves. Amplitude is the distance that the object producing
the sound travels as it vibrates. A more intense sound will have greater
amplitude. The loudness of a sound is how strong the sound is to us when it hits
our ears. At a certain frequency, the more intense the sound is, the louder it
seems. Equally intense sounds of different frequencies are not equally loud.
Resonance is the reinforcing of sound. It occurs when a small, repeated force
produces larger and larger vibrations in an object. To produce resonance, the
repeated force must be applied with the same frequency as the resonance
frequency of the object. Sound makes life more interesting. Without it things
would be dull and boring. Because of sound we are able to verbally communicate
with others, show emotions, and express our beliefs. Sound is a very important
factor in the lives of many.
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