I thought I'd try putting a maths/physics post for those people that
aren't already familiar with it, just for fun.
This URL on the Doppler Effect formula might be interesting to some
people. It's not too difficult and requires no prerequisite
university-level knowledge (only knowing what frequency and velocity is).
https://en.wikipedia.org/wiki/Doppler_effect
f= [(c +- Vr) / (c +- Vs)] f0
where
f observed frequency
f0 emitted frequency
c velocity of waves in medium
Vr velocity of receiver relative to medium, positive if receiver is
moving towards source, otherwise negative
Vs velocity of source relative to medium, positive if source if moving
away from receiver, otherwise negative
There is a slight confusion in the wikipedia article, I don't know if
it's a mistake, since Vs is "positive if source if moving away from
*receiver*" not "positive if source if moving away from *medium*"?
At 20 °C (68 °F), the speed of sound in air is about 343 metres per second.
For example if a car drives past with noise from its engine as it is
traveling away from you, and you are standing on the side of the road,
the medium is air (assumed to be not moving), Vs is positive, Vr is
zero, c=343m/s, so that f is less than f0 since we have f=a number
divided by a larger number, which means, when the car is driving away
from you, the perceived frequency of it's horn or engine is smaller, so
it has a lower pitch. When the car is driving towards you, Vs is
negative, and so f is greater than f0, since f=a number divided by a
smaller number, which means the perceived frequency of it's horn or
engine is greater, so it has a higher pitch. So as it drives towards you
it gets higher in pitch and then gets lower in pitch after it passes you
and moves away.
Enjoy
Jevan.
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