a railroad locomotive is at rest with its whistle shrieking then starts moving toward you. does the frequency?

Question

that you hear increase, decrease or stay the same. how about the wavelength reaching your ear? how about the speed of sound in the air between you and the locomotive?

Answer 1

Frequency and wavelength are inversely correlated, such that a higher frequency has a lower wavelength while a lower frequency has a higher one. That is a rule of thumb when you consider that the speed of a wave is equal to the frequency multiplied by the wavelength (assuming that sound has a constant speed given standard atmospheric conditions).

Now the best way to imagine what happens is to first picture a sound wave as it moves away from a source. If that source is stationary, then the sound wave is some value X. But what happens if the source moves towards a stationary receiver? The sound waves become "compressed" in a sense because now the source is also moving in the direction of the wave. Picture a "slinky" and imagine pushing it away from you. If you move in the direction of the slinky just as you push it, the slinky is compressed, and the wave length is compressed as well.

Since frequency is inversely related to wavelength, then we expect that the frequency of the slinky wave is higher.

The same idea is behind the Doppler effect of any sound where either the source or the receiver are in motion.

So the frequency increases when the receiver is stationary but the source moves towards it, while the wavelength decreases. The speed of sound is constant at a given pressure and temperature.

Answer 2

It is the Doppler effect. U will heat the true frequency at the closest approach. Coming toward u it will be higher than the true frequency. After it passes it will decrease,

Answer 3

As it is approaching you, the frequency increases until it is next to you and then begins to decrease.