NR and NRC

[Lindalou84]

Hi - Going through the Ballast Sample problems and came across this:

Which of the following statements about noise reduction are FALSE?
A. noise reduction is improved with an increase in the transmission loss of the wall separating the two rooms
B. The stiffness of the separating wall has some effect on noise reduction
C. to improve noise reduction, place absorptive materials on both sides of the separating wall
D. An increase in wall area separating the two rooms is detrimental

Debating between C and D, I chose D which is wrong. C is correct. Can someone clarify why an increase in wall area separating the two rooms is detrimental when in comes to Noise Reduction? Sounds a little dramatic
I see that the wall barrier area is in the numerator of the equation NR = 10log (S/A) which means a higher NR (?) Isn't a higher NR better than a lower NR when it comes to sound? Or am I thinking NRC?

Thank you in advance.

[AJD]

If you plug numbers into the formula for noise reduction, NR = TL-10log(S/A), you will see why answer D is true. If you increase the value of S, then the NR goes down, which is detrimental.

Having said that, I am unclear as to why placing more absorptive material on both sides of the wall would be considered to be a false answer. I think that adding absorptive material would help to increase NR. It certainly would seem to be beneficial in the room that is receiving the noise as it would increase the value of A (total acoustic absorption) in that room. If material was added to the wall of the room producing the noise it would increase the value of TL (transmission loss) for the wall, would it not? Can anyone shed some light on this aspect?

[Lindalou84]

In the formula S= area of the barrier wall A= absorption. If i plug in a number I get a higher NR with a higher area...which is why I'm confused.

Regarding your question - absorption and transmission are different. Adding more absorption does not prevent or lessen transmission.

[AJD]

My apologies. I wrote "A", when I had intended to write "S". I just corrected my initial reply. When I plug larger numbers for "S" into the equation, I get a reduction in NR.

Ex.
If I use 100 for "S" I get the following:
NR = TL - 10log(S/A)
NR = 50-10log(100/200)
NR = 50-10log(.5)
NR = 50-(-.301)
NR = 50.301

And if I add a larger value for "S" (in this case 400) I get the following:
NR = 50-10log(400/200)
NR = 50-10log(2)
NR = 50-(.301)
NR = 49.699

Thank you for distinguishing between sound absorption and transmission. That was very helpful.

[Lindalou84]

Hmm..i must be doing something wrong in the order I'm calculating then...thanks!