What are special audible charcteristics?

Special audible characteristics are described in the New Zealand Standard for the Assessment of Environmental Sound (NZS 6802:1991) as noise that is tonal or impulsive and is likely to arouse adverse community response at lower levels than noise without such characteristics.
Frost fans typically exhibit these characteristics. In a quote from a 2004 Marshal Day report (04W142) on an Amarillo frost fan;

"The blade noise is characterised by a "chopping" sound which would generally be interpreted as having special audible character. The degree of chop varies with rotation, ranging from mild (about 70% of the rotation period) to distinct (about 25% of the rotation). The remaining 5% of the rotation the blade noise is barely audible and no chop is heard."

and

"At 513rmp blade speed, the blade noise is significantly reduced, as is the amount of chop. With less blade noise, the engine note is predominant, and is tonal by the definition of NZS 6802:1999"

This nicely describes the noise as being both 'impulsive' (chop) and 'tonal' (engine). For this reason the report found that the frost fan exhibited special audible characteristics.

The reason that this is important is that a 5dB penalty on the noise level is warranted if the frost fan exhibits a special audible character (at least in Marlborough). This is following the guidelines of the NZS 6802.