Tonal noise is commonly referred to as discrete frequency noise and is characterized by spectral tones that are pure tone in nature. Pure tones are wave forms that occur at a single frequency. Tonal noise is generated by rotating equipment at a predictable frequency relating to the rotational speed of the shaft and the number of compressor vanes, fan blades, engine pistons, gear teeth, etc. The fundamental tone (F) may also manifest itself at progressively lower intensity levels at integer harmonic multiples (2F, 3F, etc.). Tolerance levels for tonal noise are generally at a lower threshold.
Spectral data measured using frequency filter sets is necessary to asses tonal content. Characterizing the source noise frequencies in full octave bands (see example above right for a transformer) does not provide the degree of spectral definition of fractional 1/3 octave bandwidths (see above left for same transformer example).
Discernable tones shown in 1/3 octave format can disappear in a full octave analysis. Narrow band frequency analysis is used to precisely identify tones. Examples of tonal noise include fans, rotary lobe blowers, compressors, gears, transformers, saws, and piston driven engines. Noise control treatment strategies for tonal noise sources must target the discrete frequencies.