As any object moves through the air, the air near the object is disturbed.
The disturbances are
transmitted through the air at a distinct speed called
the speed of sound.
Sound is a sensation created in the human
brain in response to small pressure fluctuations in the air.
Sound moves through the air as a series of waves. When the waves
pass our ears, a sound is detected. The distance between any two
waves is called the wavelength and the time interval between waves
passing is called the frequency .
The wavelength and the frequency are related by the speed of sound; high
frequency implies short wavelength and low frequency implies a long wavelength.
The brain associates a certain
musical pitch with each frequency; the higher the frequency, the
higher the pitch. Similarly, shorter wavelengths produce higher pitches.
The speed of transmission of the sound remains a constant regardless of
the frequency or the wavelength. The speed of sound only depends on the
state of the air
(or gas) not on the characteristics of the generating
Because the speed of sound depends only on the state of the gas, some interesting
physical phenomena occur when a sound source moves through a uniform gas. You
can study some of these phenomena by using the interactive
simulator. As the source moves it continues to generate sound waves
which move at the speed of sound. Since the source is moving slower than
the speed of sound, the waves move out away from the source. Upstream (in
the direction of the motion), the waves bunch up and the wavelength
decreases. Downstream, the waves spread out and the wavelength increases. The
sound that our ear detects will change in pitch as the object passes.
This change in pitch is called a
There are equations that describe the doppler effect. As
the moving source approaches our ear, the wavelength is shorter, the frequency
is higher and we hear a higher pitch. If we call the approaching
frequency fa, the speed of sound a,
the velocity of the approaching souce u,
and the frequency of the sound at the source f, then
fa = [f * a] / [a - u]
As the moving source leaves us, the
wavelength is longer, the frequency is lower and the pitch is lower. Again.
if the leaving frequency is called fl, then
fl = [f * a] / [a + u]
Sound Wave Simulator:
- Beginner's Guide Home Page