Decibel
Denotes the relative difference between the intensity of one sound and the lower intensity of a reference sound; equals 10 times the common logarithm of the ratio of the two intensity levels: dB = 10 log10 (N/Nref). Commonly used reference values are 1012 watt (1 pW) for sound power and 20 micropascals for sound pressure.

Frequency
Number of cycles that occur in one second. (A "cycle" is the complete sequence of motion comprising a sound wave.)

Octave Band
A frequency range with an upper limit that's twice the frequency of its lower limit.

Sound
Audible emissions resulting from the displacement/vibration of molecules in an elastic medium such as air or, in an HVAC context, the building structure.

Sound Power
Acoustical energy, measured in watts, emitted by a sound source. It's a calculated value unaffected by environment and distance.

Sound Pressure
An audible atmospheric disturbance that can be measured directly; its intensity is influenced by the surroundings and distance from the sound source.

Tone
A sound of distinct pitch, quality or duration with a narrow frequency range.

For more acoustics basics, consult the "Sound and Vibration" chapter of the ASHRAE Fundamentals Handbook or the Trane Acoustics in Air Conditioning manual (FND-AM-5).

Continue on to How to Determine the RC Noise Rating.