The answer lies in evaluating the space. If noise absorption is important to the room design then you may wish to look at the overall performance of a product under consideration – rather than how
performs on average for only four mid-range frequencies (which is NRC). NRC is calculated from a standardized test that is available upon request from the manufacturer. That test lists noise absorption performance over a much larger range of frequencies than is needed to calculate NRC (from 100 to 5000 hertz). This information will show how the product absorbs noise over a wide range of frequencies.