Question about sound waves?
“Every year, 100 000 hedgehogs are killed by vehicles on British roads. It has been suggested that if a whistle emitting a sound wave of 5000Hz is attached to the front of a car bumper, that when the hedgehog hears the whistle it will remain at the side of the road.
“The movement of the car forces air into the whistle and creates the sound.
“State and explain what happens to the sound emitted by the whistle as the car moves faster. How may this change be to the hedgehog’s advantage?”
So we’re obviously talking about pitch and loudness of the sound here, meaning frequency and amplitude, respectively.
Can we use v = fλ? What /exactly/ is the relationship between velocity, speed, and wavelength of a wave? As f increases, λ decreases, right? But how does v play into this? And does this only apply for electromagnetic waves, or can it work for sound and water waves, as well?
Thanks. I need to clear up on my relationship concepts.
Answer
You should read the Wikipedia article about the Doppler effect. The article describes in good detail what happens if the observer or the sound source are moving.
In classical physics, which deal with wave propagation in a medium like air the general equation is:
f = (v+v_r)/(v + v_s) * f_0, with
f_0= emitted frequency, f=received frequency, v=wave velocity in air, v_r=receiver velocity relative to air, v_s= source velocity relative to air. If both r and s approach each other f rises.
the receiver (hedgehog) velocity is negligible compared to either the car or the wave velocity, so you can assume it’s zero.
thus you can reduce the equation to:
f = (v)/(v + v_s) * f_0
Beware that this formula is only valid if source and receiver are approaching each other directly.
In Astronomy the doppler effect of electromagnetic waves is also known as red shift or blue shift, which describes the color of stars changing due to the spectral frequency shift towards the long or short wavelength end of the visible spectrum. Blue shift means that a star/light source approaches the observer (or the other way around) and red shift means the light source moves away from the observer (or the observer moves away from the light source). You can use the same formula for calculating red shift or blue shift of electromagnetic waves, if the velocity is much lower than the speed of light. In the case of relativistic velocities you have to use the formulas for relativistic color shifts.
Again, I recommend you read the Wikipedia article. It’s very informative and should be well understandable.
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