posted by on .

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

actually,it is. they may want the sum frequency, in mixing two sound, one gets the sum and difference.

could you help me more with this?

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

I am sorry..but this is not the right answer either

Ah well I surrender ! I think the problem is English, not Physics.

I assume that you are plugging into so computer , try rounding to 43 Hz

Hey, are you sure those waves do not hit the hill at 256.7, not 254.7 ??
vl = 340 * 243/(340-18.2)

yes they hit it at 256.7..i typed it wrong...i am still not getting the correct beat frequency

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

• typo - ,

Excuse me
270.4 - 243 = 27.4 !!!

That question was poorly written and riddled with errors. Burn the book :)

thank you soooo much...the answer is correct..thanks again for working on it for this long