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Ratio of velocity of car and sound

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A car moving towards a vertical wall sounds a horn. The driver hears that the sound of the horn reflected from the cliff has a pitch half-octave higher than the actual sound. Find the ratio of the velocity of the car and the velocity of sound.

In this what does they mean by 'half octave'
According to me in this the frequency will change as
$$f' =\frac{v + v_0}{v} f$$ (doppler's effect)

Where v and v$_0$ are velocities of sound and car respectively.

But how how can we proceed ?

asked Jan 5, 2017 in Physics Problems by koolman (4,286 points)
reshown May 16 by sammy gerbil
"Half octave" is ambiguous. An octave increase is double, which indicates a logarithmic scale. In this case a half octave increase would be a factor of $\sqrt2$. On the other hand the question-setter might mean an increase in frequency of half as much as the octave increase. So this is an increase by a factor of $\frac32$. I have no idea which is intended. You will need to examine the book to find out. (Or look at the book solution.)

Apart from this, what is your difficulty? Why can't you proceed?

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