Calculating Sound Pressure Level

[tilds]

So, the question is this:

A major road is to be constructed adjacent to the house. The noise source on the road is 45m away from the rear facade of the house. It is predicted at a distance of 4m from the noise source the SPL will be 85dB. Calculate the SPL at the rear facade before the barrier is erected.

So far, I am getting two very different answers by using two equations. The first equation being SPL = 20logp/p0, and the second one is one I've found online which is SPL2 = SPL1 - 20log r2/r1 where SPL1 is the known value, i.e. 85, r2 = the given distance, i.e. 45 and r1 = the known distance, i.e. 4

By working backwards using the first equation, I've managed to figure out what the variable of p is as this was not given to me, I got the answer for p as 0.00281 (rounded to 3 sig figs) at 45m. Therefore, when put into the first equation, you get an SPL value of 43dB (rounded as a whole number)

However, when using the second equation: SPL2 = 85 - 20log 45/4, you get an SPL value of 64dB (rounded as a whole number).

Which one is correct?!

Any help will be greatly appreciated!

 

[stapel]

A major road is to be constructed adjacent to the house. The noise source on the road is 45m away from the rear facade of the house. It is predicted at a distance of 4m from the noise source the SPL will be 85dB. Calculate the SPL at the rear facade before the barrier is erected.

What is "SPL"? Are you referring to "sound pressure level" (as here)?

So far, I am getting two very different answers by using two equations. The first equation being SPL = 20logp/p0

For what do the variables in this equation stand?

...and the second one is one I've found online which is SPL2 = SPL1 - 20log r2/r1 where SPL1 is the known value, i.e. 85, r2 = the given distance, i.e. 45 and r1 = the known distance, i.e. 4

What do you mean by "known" versus "given"? Do you perhaps mean "corresponding to SPL_2 and SPL_1, respectively"? Are you sure about the "minus" sign on the right-hand side of this equation?

How do SPL_1 and SPL_2 relate to SPL (with no subscript)? What equation did you book give you for this situation?

By working backwards using the first equation, I've managed to figure out what the variable of p is as this was not given to me, I got the answer for p as 0.00281 (rounded to 3 sig figs) at 45m. Therefore, when put into the first equation, you get an SPL value of 43dB (rounded as a whole number)

However, when using the second equation: SPL2 = 85 - 20log 45/4, you get an SPL value of 64dB (rounded as a whole number).

Which one is correct?!

Unfortunately, it is not possible for us to troubleshoot work that we cannot see, nor can we correct or confirm answers without being clear on the inputs. So, when you reply to the answers above, please also include a clear listing of your work. Thank you!