Question

The acoustical absorption of a wall panel in each octave band is tabulated below. The Noise Reduction Coefficient of the wall panel (rounded off to two decimal places) is \(\underline{\hspace{1cm}}\).

Hint:

NRC uses only four octave bands: 250, 500, 1000, and 2000 Hz—never the low or very high frequencies.

Answer 1

Correct Answer: 0.6

The Noise Reduction Coefficient (NRC) is defined as the average of the absorption coefficients at the four octave-band centre frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz.
From the table:
\[ \alpha_{250} = 0.5, \alpha_{500} = 0.5, \alpha_{1000} = 0.7, \alpha_{2000} = 0.8 \] The NRC is calculated as:
\[ \text{NRC} = \frac{0.5 + 0.5 + 0.7 + 0.8}{4} = \frac{2.5}{4} = 0.625 \] Rounding off to two decimal places gives: \[ \text{NRC} = 0.63 \]