Sound is a wave-form phenomenon which is produced by vibration. When you talk,
your vocal cords vibrate creating waves which travel through the air. When you
whistle, your lips vibrate. Musical instruments have reeds or strings that
vibrate. When you are sick or yell a lot, your vocal cords may become swollen to
the point of where they have difficulty vibrating. When this occurs, you have
laryngitis and you lose your normal voice.
For sound waves to travel they need a conductor. All matter can conduct sound waves, but some matter conducts better than others. For example, air is a better conductor of sound waves than water.
A vacuum is a place that has no matter. Without matter, sound waves cannot occur. "In space no one can hear you scream." On the moon, verbal communication had to occur electronically.
When sound waves are produced, they take time to travel. This is known as the speed of sound. The speed of sound is the speed at which sound waves travel. Sound does not travel at the same speed in all kinds of matter. In normal air, the speed of sound is 1,087 feet per second. This is equal to about 740 miles per hour.
Not all sound waves can be perceived by people. We can only hear sound waves from 20-20,000 hertz. Hertz is the scientific term for vibrations or cycles per second. Vibrations slower or faster than this cannot be heard by people. However, other animals, such as dogs, can hear vibrations that are faster than this. A dog whistle, produces vibrations above 20,000 hertz.
Sound which can be heard has qualities. One such quality is pitch. Pitch refers to how high or low a sound is. The shorter the vibrating object, the higher its pitch. Shorten a guitar or violin string, and the pitch becomes higher. Strike a short string on a piano and it produces a high pitched sound. The thicker a vibrating object, the lower the pitch. The thick strings on a guitar or a piano produce low pitch sounds. Men have thicker vocal cords than women, and their voices are usually lower in pitch. The tighter the vibrating object, the higher the pitch. Tighten a guitar string and the pitch will become higher. Helium tightens human vocal cords and causes people to talk in a higher pitch.
Another quality of sound is loudness. The greater the amplitude of the sound
waves, the louder the sound. Loudness is measured in units called decibels (db).
A sound of zero decibels is the quietest sound which can be heard by the human
ear. A sound of 120 decibels is the loudest sound which can be heard by the human
ear without producing pain. Prolonged exposure to any sound above 90 decibels can
cause hearing loss. Normal conversation is 65 db. Conversation in my classes is
about 91 db. A rock concert, 120 db. A jet airplane, 140 db. A whisper, 20 db.
The external, fleshy portion of the ear which is trumpet-like in shape is called the pinna. Its purpose is to collect vibrations and channel them into the auditory canal. There they strike the tympanic membrane or ear drum which starts to vibrate. On the other side of the ear drum are the ossicles. The ossicles are the three smallest bones of the body. Their names are the hammer, anvil and the stirrup. Their function is to amplify the vibrations of the ear drum. The stirrup changes vibrations to piston like pressure against a structure called the oval window of the cochlea. The cochlea is a long, curled, fluid filled canal which is lined with millions of microscopic hair cells which are connected to nerve fiber. If the ripples caused by the piston-like pressure exerted by the stirrup bends certain hair cells, the connecting nerve fibers are activated. These nerve fibers feed into the auditory nerve which takes the sound message to our brain. Sounds are realized in the temporal lobes of the brain.
There are two general kinds of deafness, conduction deafness and deafness due to
auditory nerve damage. Conduction deafness usually involves damage to the
ossicles or the ear drum. This type of deafness can usually be helped by using
a hearing aid or corrective surgery. However, there is generally not much which
can be done to correct auditory nerve damage.
In the ear is found another sensory mechanism called the vestibular sense. The
vestibular sense is our sense of balance. The receptors for the sense of balance
are three semi-circular canals and two adjoining sacs. All of these receptors are
fluid-filled and lined with millions of tiny hair cells. These hair cells are
connected to associated nerve fibers which send their message to the vestibular
nerve which relays this message to the brain.
The vestibular receptors tell us about rotation, acceleration, and changes in gravitational pull so that we can maintain balance. The functioning of the vestibular receptors is analogous to a glass full of water. If you had a glass of water and were seated in a dragster, what would happen to the water as you accelerated rapidly to travel a distance of a quarter mile in less than five seconds? Right, you would get wet because the water would be forced to the back of the glass by the force of acceleration. A similar thing occurs in the vestibular receptors when you accelerate. The fluid in these receptors is forced backward, bending the microscopic hair cells, activating the associated nerve fiber sending a message to the vestibular nerve and brain that you are accelerating.
When you rotate your body the fluid in the vestibular receptors is slung outward by centrifugal force. This bends those hair cells, activating their associated nerve fiber and the vestibular nerve tells the brain that you are rotating. When you go up rapidly in an elevator, the fluid in the vestibular receptors is compressed. The force of this compression bends hair cells, activating associated nerve fiber and the vestibular nerve tells your brain that you are moving upward. When you go down in a fast moving elevator, the fluid in the vestibular receptors goes up bending those hair cells, activating their associated nerve fiber and the vestibular nerve tells the brain that you are going downward. The vestibular receptors tells our brain about our body position so that we know how we are oriented even if our eyes are closed.
It is a malfunction in the vestibular receptors which causes motion sickness. This malfunction causes the vestibular nerve activate the vomit center in the brain. Usually motion sickness, such as sea sickness, car sickness and air sickness can be controlled with a mild central nervous system depressant drug such as Dramamine. In space travel, the vestibular sense must adjust to the lack of stimulation produced by weightlessness.
Our sense of taste is called gustation. The receptors for our sense of taste are specialized nerve cells called taste buds. Taste buds respond to four primary tastes: sweet, salty, bitter and sour. The taste buds for sweet and salt are near the tip of the tongue. The taste buds for bitter and sour are to the sides and back of the tongue. Much of what we call taste is actually taste combined with smell. When your nose is stopped up and you cannot smell your food, it does not taste the same. To demonstrate this interrelationship between taste and smell, blindfold a person and have them pinch their nose closed and stick out their tongue. Place a few drops of lemon juice on their tongue. They can only tell you that it is a sour liquid. Allow them to open their nose and they will identify the liquid as lemon juice.
Our sense of smell is called olfaction. The receptors for our sense of smell are found in the olfactory bulb of the brain. Our sense of smell tells us about chemical qualities of the air we breath.
For us to be able to detect a chemical with our sense of smell, the chemical must
fit one of our receptors site on the olfactory bulb. If it fits, it activates
nerve cells that tell our brain about the chemical. If it does not fit, we cannot
detect it. Carbon monoxide and methane, for example, are both deadly chemicals
for which we lack a receptor site. Therefore we cannot detect either carbon
monoxide or methane with our sense of smell.