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aISO Recommendation 266 and USAS SL6-1960 listed sets of preferred numbers which were recommended for use in acoustic design. Most filters are now designed on this basis. The tables give octave-band and one-third octave-band filter characteristics, using preferred numbers as the center frequencies from which the band-edge frequencies were computed. Older filter sets used slightly different frequencies; they are interchangeable in all ordinary work.

aISO Recommendation 266 and USAS SL6-1960 listed sets of preferred numbers which were recommended for use in acoustic design. Most filters are now designed on this basis. The tables give octave-band and one-third octave-band filter characteristics, using preferred numbers as the center frequencies from which the band-edge frequencies were computed. Older filter sets used slightly different frequencies; they are interchangeable in all ordinary work.

Each NC curve describes a set of noise conditions; the acoustic environment suitable for a need is specified with a single number. For example, a concert hall or auditorium should be NC-25 or better; a private office is NC-35. That is, octave-band SPLs measured in these areas should not exceed, at any frequency, the values specified by the appropriate NC curve.

These criteria are related to the psychological characteristics of the ear and, consequently, the shape of the curves is like the equal-sensation curves. For broad-band noise, if 7 to 9 units is subtracted from the sound level measured in dBA, the difference approximates the NC-curve rating. If the ear can detect some frequency which seems to dominate an otherwise uniform background noise, this rough criterion is not acceptable. An octave-

band analysis is needed, especially if specifications are being checked.

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