period two
Sound Perception and Rating Methods
shall be X,” where X is a single-number descriptor conveying the desired quality of sound.
The most frequently used single-number descriptors are the A-weighting network, noise criteria (NC), and room criteria (RC). All three share a common problem, however: they unavoidably lose valuable information about the character, or quality, of sound. Each of these descriptors is based on octave- band sound data which, as noted earlier, may already mask tones. Further, the process of converting from eight octave bands to a single number overlooks even more sound data.
Despite this shortcoming, the single-number descriptors summarized in this clinic are valuable tools for defining sound levels in a space, and are widely used to specify the acoustical requirement of a space.
A–B–C Weighting
| 0 | C | | | | | | |
| | | | | | | |
dB | -10 | B | | | | | | |
response, | | | | | | |
| | | | | | |
-20 | | | frequency responses for sound |
| | | meter weighting characteristics |
relative | | | |
-30 | A | | | | | | |
| | | | | | |
| | | | | | | |
| -40 | | | | | | | |
| 20 | 50 | 100 | 200 | 500 | 1,000 2,000 | 5,000 | 10,000 |
One simple method for combining octave-band sound data into a single- number descriptor is A-,B-, or C-weighting. The weighting curves shown in Figure 23 compensate for the varying sensitivity of the human ear to different frequencies.
A-weighting, which is most appropriately used for low-volume (or quiet) sound levels, best approximates human response to sound in the range where no hearing protection is needed. B-weighting is used for medium-volume sound levels. C-weighting is used for high-volume (or loud) sound levels where the response of the ear is relatively flat.
