The Speed of Sound
The Speed of Sound, source: Air & Space/Smithsonian.
The speed of sound varies with temperature. At sea level Mach 1 is around 742 mph. It decreases with altitude until it reaches about 661 mph at 36,000 feet, then remains at that speed in a band of steady temperature up to 60,000 feet. Because of the variation, it is possible for an airplane flying supersonic at high altitude to be slower than a subsonic flight at sea level. The transonic band extends from around Mach .8—when the first supersonic shock waves form on the wing—to Mach 1.2, when the entire wing has gone supersonic.
A sound wave is a disturbance of pressure which travels through a medium, such as air, water, and earth, by means of particle interaction. As one particle becomes disturbed, it exerts a force on the next adjacent particle, thus disturbing that particle from rest and transporting the energy through the medium. Like any wave, the speed of a sound wave refers to how fast the disturbance is passed from particle to particle.
The speed of sound (a) is equal to the square root of the ratio of specific heats (g) times the gas constant (R) times the absolute temperature (T).
a = sqrt [g * R * T]
Since the speed of a wave is defined as the distance which a point on a wave travels per unit of time, it is often expressed in units of meters/second. In equation form, this is speed(s)=distance(d)/time(t). Distance is usually measured in Meters, and time is measured in seconds, thus giveing us the equation Speed=(meters)/(seconds)
The Doppler effect is a phenomenon observed whenever the source of waves is moving with respect to an observer such as a car passing by a person of a plane flying overhead. The Doppler effect can be described as the effect produced by a moving source of waves where there is an apparent upward shift in the frequency or ammount of waves, for the observer and the source are approaching and an apparent downward shift in frequency when the observer and the source is receding.
The Doppler effect can be observed to occur with all types of waves. We are most familiar with the Doppler effect because of our experiences with sound waves. Such experiences may have been as you stand on the side of the highway and a police car with it's sirens blazing passes by and listening to the siren as it approaches and as it passes and travels away.