Understanding Sound - NC, RC, or dBA

To realize acoustics, the first important concept is that of sound level. Sound is merely pressure fluctuations in air. While sound is vibrations in a pressure medium, it is also defined as an audible wave. Since we are dealing with pressure vibrations in a medium, sound is usually defined by two different characteristics: frequency and amplitude. The frequency of sound controls the pitch that you hear (high or low) where-as the amplitude controls the loudness level.

Although many of us may have heard some of these terms before, understanding what the differences are between them may be a slight less clear. 

We all know that background noise can be rather intolerable in most situations but what levels are suitable for various spaces? How do we measure those and what do the ratings mean? 

Many of us have heard of the term Decibel when referencing loudness level. Decibel is a unit of measure that expresses the ratio of one pressure level to another on a logarithmic scale. This allows us to handle sound in much more manageable numbers.

 dBA stands for A-weighted sound pressure level (measured in decibels, hence dBA which also sometimes shown as dB(A)). Now, where does this weighting come from? Well, the human ear is more sensitive to sound at certain frequencies. The average human hearing range is between 20-20,000 Hz but toward the top and bottom of that range, our sensitivity to the sound drops off significantly. The A-weighting scale simply applies a correction factor to the level at each frequency to match what the human ear picks up. dBA was first used as early as 1933 and has been in use ever since. Although it is not the best method for rating noise in a space, it is still very prevalent in certain situations. 

NC stands for Noise Criteria. Noise Criteria defines the sound level by a set of criterion curves that were designed and developed in 1957. It was designed to be more sensitive to relative loudness of humans. This is the most common rating criteria used in the today. One disadvantage of the Noise Criteria method is that it does not account for sound quality. Sound quality or the shape of the sound across all the frequencies plays a big role in the annoyance an occupant will experience. NC also does not account for sounds below 63 Hz which frequencies must be analyzed by other means. 

RC stands for Room Criteria. This methodology was designed to account for both the sensitivity of the human ear towards noise and the sound quality of that noise. The curves on the RC chart were designed to sound well-balanced and bland. The RC method also includes a letter designation that accounts for sound quality. For example, if the noise in a space has a lot of high frequency content, the letters HF will be the designation indicating a high sound levels in the upper frequencies. The RC methodology uses the following letter designations to account for sound quality: (N) Neutral, (LF) Low-Frequency Rumble, (MF) Mid-Frequency Roar, (HF) High-Frequency Hiss. The ideal space will be aiming for a neutral ambient noise level. 

The question you may be asking is: which rating system should I use? Well, it will depend on your situation. In most indoor spaces, NC is sufficient to meet levels that will provide occupant satisfaction (and it is easier to calculate the RC).

Following the good, better, best rule. dBA is good, NC is better, and RC is best. When you have the option of selecting which rating to meet in an indoor space, it is recommended to use the Room Criteria. This will allow for the best results in the ambient noise level for the occupants in a space.

Recommended Noise Criterion - NC