Sound Waves
Sound is created when an object is struck, plucked, strummed or vibrated in some way. Before we hear the vibrations, the sound must travel from the string to our ears. The sound does this by traveling as a sound wave.
The molecules closest to the strings are excited by the energy of the
vibrating string and those molecules then excite the molecules next to
them. This energy travels outward from the string through the molecules
into the air.
It is important to understand that the molecules themselves do not travel
from the string to our ears. It is the energy that travel outwards. Think
of waves in the ocean. They water itself moves up and down, not forward.
Frequency and Pitch
An object is "vibrating" when it is moving back and forth at
a steady pace. One complete back and forth motion of an object is equal
to one vibration. The number of vibrations occurring each second is the
"frequency" and is measured in Hertz (Hz). Frequency is heard
as pitch. A high frequency is perceived as a high pitch, a low frequency
is heard as low pitch.
Time to Frequency Conversion
We can measure the length of a frequency by dividing the velocity of sound
by the frequency. This length is the "frequency wavelength".
Higher frequencies have a shorter wavelength than lower frequencies. Humans
have the ability to hear objects vibrating between 20 Hz and 20,000 Hz.
The Speed of Sound
The velocity, or speed, of a sound wave changes, depending on what it
is traveling through and on what the temperature is. As the temperature
rises sound travels faster.
In air and at room temperature, sound waves travel at speeds of 1130ft/
sec.
Sound travels 4,800 ft/sec in water, and even faster through steel, 16,000
ft/sec.
Compressions and Rarefactions
As the string moves it collides with surrounding molecules. As the molecules move together and then apart from each other, the air pressure between the molecules changes. The phenomenon of the molecules moving together is known as a compression. As the space between the molecules decreases, the pressure between the molecules increases. Compressions produce crests in the sound wave.
When the molecules move away from each other the air
pressure decreases. This phenomenon is known as a rarefaction. Rarefactions
produce troughs in the sound wave.
Together, compressions and rarefactions make the sound wave which travels
through the air and into your ears. The speed at which these compressions
and rarefactions occur determines the sound wave's frequency, or pitch.
The faster they are vibrating, the higher the frequency and a higher pitch.
Sound Waves Lose Energy
The differences in pressure are produced by energy created by the excited
air molecules. As the energy travels outward, the molecules which were
previously excited gradually stop vibrating. As the sound wave travels
outward it will eventually lose energy until it is undetectable. Changes
in humidity, altitude, or temperature all affect the speed that the sound
is traveling. Next, what happens when sound waves interact?