1) When standing at a certain distance from a jet on the ground on which all thr

Question

1) When standing at a certain distance from a jet on the ground on which all three engines arerunning at equal power, the sound level is heard at the threshold of pain, 120 dB. (A) If the pilotshuts down two engines but leaves the third at the same power, what is the sound level that isheard? (Hint: intensities, not sound levels, add.) (B) When at a different distance, the same jet isinitially heard at 95 dB. At that point after two engines are shut off, what sound level is heard?

2) You have tuning forks (which oscillate at a fixed frequency stamped on the fork) for frequenciesof 294 Hz and 300 Hz. (A) What beat frequency is heard when you play both forks at the sametime? (B) If you have an (untuned) string which makes for a beat frequency of 8 Hz against the 294Hz fork and 2 Hz against the 300 Hz fork, what frequency is the string emitting? (C) If the string is 91.44 cm long, has a mass per unit length of 0.4 g/m, and is vibrating in its fundamental mode, what change in string tension is needed to bring the string into tune with the 294 Hz fork? (Either anabsolute change in newtons, or a multiplicative factor to the tension, makes an acceptable answer.)

For question 2, i only need help for c

Explanation / Answer

1)

intensity of sound is given as

I = Io 10db/10

I = (10-12) 10120/10 = 1

when two engines are shut , intensity heard

I' = 1/3

Sound level is given as

db = 10 log (I/Io) = 10 log (0.33/(10-12)) = 115.2

b)

intensity of sound is given as

I = Io 10db/10

I = (10-12) 1095/10 = 10-2.5

when two engines are shut , intensity heard

I' = 1/3

Sound level is given as

db = 10 log (I/Io) = 10 log (10-2.5/(10-12)) = 95

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