# Grade 11 Sound Intensity Question

1. May 12, 2013

1. The problem statement, all variables and given/known data
If Phrank's new speakers produce a sound of 90dB at 10m,

a) How many decibels would be required for the sound to be one hundred times the intensity?

b) How close would Phrank need to stand for the intensity to be at 60dB?

2. Relevant equations
β2 - β1 = 10 log (I2/I1), where I2/I1 is equal to r1^2/r2^2

3. The attempt at a solution
I got 110dB for the first question and 100000m for the second. I'd just like someone to verify my answers. I'm not very confident on the second.

For both I just used the formula. For the second I used these values:
β2 = 60
β1 = 90
r1 = 10

2. May 12, 2013

Another one:

1. The problem statement, all variables and given/known data
You are observing a little light that emits a beep and a flash at the same time. You move backwards until the moment before you no longer hear the beep. You measure the beep to be at a frequency of 512Hz with a wavelength of 0.66m, and the time between the flash of the light and you hearing the sound is 3s. How close do you need to be to the contraption to hear the sound at 30dB?

2. Relevant equations
β2 - β1 = 10 log (I2/I1), where I2/I1 is equal to r1^2/r2^2
d = vt
v = fλ

I think that's it.

3. The attempt at a solution
I got 32m. I used the v=fλ to find the speed, which I then put into d=vt to find the distance at which you can't hear the beep anymore. Then I put that in the first equation and solved for r2 using these values:

r1 = What I got from d=vt
β1 = 0
β2 = 30

Please actually try to solve both of the questions I posted instead of just posting "makes sense to me" or something.

3. May 12, 2013

### haruspex

Agree with 110dB. Pls post your working for the second part.
Pls put new questions on separate threads.