[This excerpt is paraphrased from Chapter 42, "Sound and Vibration Control," of the 1991 HVAC Applications ASHRAE Handbook.]

The RC rating of a noise is usually based on sound pressure level data at center frequencies of 31.5 to 4000 Hz and consists of two descriptors. The first descriptor is a number representing the spectrum's speech interference level (SIL), and is obtained by taking the arithmetic average of the noise levels in the 500-, 1000- and 2000- Hz octave bands. The second descriptor is a letter denoting the sound's "quality" as it might subjectively be described by an observer. These steps describe how to determine an RC rating:

1. Plot the octave-band noise spectrum on an RC chart.
2. Calculate the SIL by arithmetically averaging the sound pressure levels at the 500-, 1000- and 2000-Hz octave band centers.
3. Draw a line with a slope of - 5 dB per octave in the frequency range from 31.5 to 4000 Hz, and passing through 1000 Hz at the SIL calculated in Step 2. This is the reference curve for evaluating the sound quality of the spectrum.
4. Draw one line 5 dB above the reference curve extending from 31.5 to 500 Hz. Draw a second line 3 dB above the reference curve, extending from 1000 to 4000 Hz. The range between these two lines and the reference curve represents the noise spectrum's maximum permitted deviation above the reference curve to receive a neutral (N) rating.
5. Judge the sound's quality by observing how the spectrum's shape deviates from the boundary limits of the reference curve set in Step 4. Use the criteria described below to choose the appropriate letter descriptor.
6. Assign the spectrum an RC rating - i.e., the numerical part of the rating corresponds to the level of the reference curve at the 1000-Hz octave band center; then append the letter descriptor determined in Step 5.

Characterize the subjective quality of the room's background noise based on the following criteria.

Neutral (N). The levels in the octave bands centered at 500 Hz and below must not exceed the octave-band levels of the reference spectrum by more than 5 dB at any point in the range; the levels in the octave bands centered at 1000 Hz and above must not exceed the octave-band level of the reference spectrum by more than 3 dB at any point in the range.

Rumbly (R). The level in the octave bands centered at 500 Hz and below exceeds the octave-band levels of the reference spectrum by more than 5 dB at one or more points in the range.

Hissy (H). The level in the octave bands centered at 1000 Hz and above exceeds the octave-band level of the reference spectrum by more than 3 dB at one or more points in the range.

Acoustically Induced Perceptible Vibration (RV). The cross-hatched region in the 16-to-63-Hz octave band frequencies on an RC chart indicates sound pressure levels at which walls and ceiling can vibrate perceptibly - rattling cabinet doors, pictures, ceiling fixtures and other furnishings in contact with them.