The Doppler effect states that "there is an decrease in frequency of sound, light and other forms of waves as the separation between the source and observer increases".
This leads to low pitch of sound being heard as source of sound moves away.
The Doppler effect also suggests that as a source approaches an observer, there is an increase in frequency of waves.
This is why the sound heard by a siren fades away as a police vehicle moves away.
Also, the intensity of light fades when we move away from it.
Sound wave compression affects the frequency of a moving source of sound. Frequency is greater as it approaches an observer and pitch of the sound also increases. As the object moves past the observer, the frequency decreases and the pitch is now lower. This principle is also used to find the speed of moving bodies as well as in determining approaching or receding stars based on the frequency or color shift of their light emissions.
Air pressure is the weight of air on an area. The weight of air is due to the gravitational forces between the Earth and the molecules of its atmosphere.
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