Asked by sara

You are traveling in a car toward a hill at a speed of 40.8 mph. The car's horn emits sound waves of frequency 243 Hz, which move with a speed of 340 m/s.The frequency with which the waves strike the hill is 254.7Hz and the frequency of the reflected sound waves you hear 271.2 Hz.
What is the beat frequency produced by the direct and the reflected sounds at your ears?


271.2-243 is not the correct answer
Answered by bobpursley
actually,it is. they may want the sum frequency, in mixing two sound, one gets the sum and difference.
Answered by sara
could you help me more with this?
Answered by Damon
The question is misleading I think. I gave the obvious answer as well yesterday Bob but it is not what they want.
It says you hear 271.2 Hz coming back from the hill, but I think it may mean that the frequency of the waves reflected is 271,2 and you will hear a higher frequency in the moving car.
The 271.2 is from a stationary source and you are approaching it at 40.8 mph which is 18.2 meters/s
fl = (v+vl) fs/v = (340+18.2)271.2/340
= 285.7
so the two frequencies you hear in the car are
285.7 and 243
and the beat is 42.7 Hz
Answered by sara
I am sorry..but this is not the right answer either
Answered by Damon
Ah well I surrender ! I think the problem is English, not Physics.
Answered by Damon
I assume that you are plugging into so computer , try rounding to 43 Hz
Answered by Damon
Hey, are you sure those waves do not hit the hill at 256.7, not 254.7 ??
vl = 340 * 243/(340-18.2)
Answered by sara
yes they hit it at 256.7..i typed it wrong...i am still not getting the correct beat frequency
Answered by Damon
Well they hit the hill at 256.7 and come back at that same frequency in the air.
Now an observer in the car will hear them at:
256.7(340+18.2)/340 = 270.4
not 271.2
so I get
271.2 - 243 = 28.2
Answered by Damon
Excuse me
270.4 - 243 = 27.4 !!!
Answered by Damon
That question was poorly written and riddled with errors. Burn the book :)
Answered by sara
thank you soooo much...the answer is correct..thanks again for working on it for this long
Answered by Damon
You are welcome, like I said sacrifice that book to some deity !
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