1.A wall between two rooms is 20 x 9 ft. The wall construction has Transmission Loss 50 dB at 1000 Hz. In the wall is a small window of size 4 x 2 ft. The window has a tested Transmission Loss of 20 dB at the same frequency. The receiving room has 200 ft^2 Sabine absorption. What is the Noise Reduction NR of the wall without the window?
2.Two identical offices are separated by a wall 15 x 8 ft. Sabine absorption in the offices is 150 ft2 . In one office the Sound Intensity Level is 72 dB. What transmission loss is required for the common wall if the SIL in the second office is to be 35 dB?
1. To calculate the Noise Reduction (NR) of the wall without the window, we need to determine the Transmission Loss (TL) of the wall and the Receiving Room Factor (RRF).
First, let's calculate the TL of the wall:
TL_wall = Transmission Loss of the wall construction = 50 dB at 1000 Hz
Next, let's calculate the RRF using the Sabins formula:
RRF = Receiving Room Factor = 10 * log10(Receiving Room Absorption / Total Surface Area)
Here, the Receiving Room Absorption = 200 ft^2 Sabine absorption, and the Total Surface Area = 20 ft * 9 ft = 180 ft^2 (without the window).
RRF = 10 * log10(200 / 180) = 10 * log10(1.11) ≈ 0.46 dB
Now we can calculate the NR of the wall without the window:
NR = TL_wall - RRF
NR = 50 dB - 0.46 dB ≈ 49.54 dB
Therefore, the Noise Reduction (NR) of the wall without the window is approximately 49.54 dB.
2. To find the required Transmission Loss (TL) of the common wall between two identical offices, we need to calculate the Source Room Factor (SRF) and the Receiving Room Factor (RRF).
Given:
Size of the wall = 15 ft * 8 ft
Sabine absorption in the offices = 150 ft^2
Sound Intensity Level (SIL) in one office = 72 dB
Required SIL in the second office = 35 dB
The difference in SIL between the two offices determines the required TL:
Required TL = SIL_source - SIL_receiver
Required TL = 72 dB - 35 dB = 37 dB
To calculate the SRF, we can use the same formula as in the previous question:
SRF = Source Room Factor = 10 * log10(Source Room Absorption / Total Surface Area)
Here, the Source Room Absorption = 150 ft^2 Sabine absorption, and the Total Surface Area = 15 ft * 8 ft = 120 ft^2.
SRF = 10 * log10(150 / 120) = 10 * log10(1.25) ≈ 0.80 dB
Now we can calculate the RRF:
RRF = Receiving Room Factor = 10 * log10(Receiving Room Absorption / Total Surface Area)
Here, the Receiving Room Absorption = 150 ft^2 Sabine absorption, and the Total Surface Area = 15 ft * 8 ft = 120 ft^2.
RRF = 10 * log10(150 / 120) = 10 * log10(1.25) ≈ 0.80 dB
Finally, the required TL of the common wall is the sum of the SRF and RRF:
Required TL = SRF + RRF
Required TL = 0.80 dB + 0.80 dB = 1.60 dB
Therefore, the required Transmission Loss (TL) of the common wall between the two offices is approximately 1.60 dB.