What is sound? Sound is defined as a range of compression wave frequencies to which the human ear is sensitive. Sound, as the title implies travels in
waves, longitudinal waves to be exact. This wave is created by vibrating objects
transmitted through a medium, transporting energy from one location to another.
A medium is the material the disturbance is moving through. The most common medium
for sound is air but can be any material
such as water.
This animation shows the production of sound from a tuning fork. A tuning fork
is a metal object that has 2 poles that are able to vibrate when struck by a rubber hammer.
When the 2 poles are vibrating, it causes a disturbance in its medium,in this case air, causing
the molecules around it to become disturbed, therefore creating a sound wave.
The Sonic Spectrum
The Sonic Spectrum is the frequency range over which sound can be "heard".
This frequency range is very large and has no clearly defined lower limit but has a well
defined upper limit. In ordinary temperatures and pressures, the upper range of frequencies is
10^9 hertz in a gaseous medium. In solids and liquids it's much higher because there are
smaller spaces between particles in the medium. If the wavelength of a wave is smaller
than the inter-particle spacing of the medium, it is not transported. The audible range,
or the audio spectrum, which is the range of frequencies a human ear can hear, is approximately
20 - 20000 hertz. The frequencies above this is called ultrasonic; those below this is
called infrasonic.
Sound Transmission
There are 2 things needed for sound to be transmitted.
1. A source of energy (vibration)
2. An elastic medium
For sound to be transmitted, a material is required, it cannot travel through a vacuum.
This is proved by an experiment that was conducted. A ringing electric bell was placed under
a bell jar and the air was slowly vacuumed out. As the density of the air decreased the ringing
became fainter. When air was reentered into the jar, the sound became louder.
Air is the most common transmitting medium for sound. At low altitudes, sound is
transmitted very easily but at higher altitudes it's harder to transmit sound.
This is because at higher altitudes the density of air is lower therefore less energy
can be transferred from the source to the medium. This shows that sound is transmitted
better in dense air than rarefied air. Liquids are also a very good transmitter.
When 2 rocks are struck it's very easy for a swimmer to hear. The sound is even sharper
when the swimmer is close to the source of the sound. Solids also transfer sound well because
of their density.
The Speed of Sound
Sound requires time to travel from the source to the listener where as
light is almost instantaneous.
During a thunderstorm it takes a few seconds to hear the thunder after
the lightning has flashed.
Sound travels at 331.5 m/s at 0°C in air and increases about (0.6 m/s)/°C
as the temperature increases. Sound in water travels about 4 times faster than in air.
At 25°C, sound travels about 1500 m/s in water. The speed of sound is even greater in some solids such as a steel rod, which is about 5000 m/s, 15 times the speed in air.
