This from the University of NSW, Physics Department.
The filters used for dBA and dBC
The most widely used sound level filter is the A scale, which roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve. Using this filter, the sound level meter is thus less sensitive to very high and very low frequencies. Measurements made on this scale are expressed as dBA. The C scale is practically linear over several octaves and is thus suitable for subjective measurements only for very high sound levels. Measurements made on this scale are expressed as dBC. (snip)
They say the "C" weighting scale is only good for "subjective measurements only for very high sound levels".
In Queensland, the acceptable level for most licensed venues is 82 dBA at the nearest residence likely to be effected. Hardly what you'd call a high sound level.
"only for very high sound levels"
While most audio engineers are familiar with the A-weighting curve, which is said to reflect the 'equal-loudness contours' derived initially by Fletcher and Munson (1933) and later Robinson and Dadson (1956), few seem to realise that these curves relate only to the subjective loudness of pure tones, not noise. Furthermore, recent experimental work casts doubt on their accuracy.
Nevertheless, it will be noted that A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz (and thus gave more relative emphasis to the lower frequencies), and it can be assumed that a better match was not aimed for originally because steep filters were more difficult to construct in the early days of electronics. Topology also can amplify the perceived level off the premises, including reinforcinment due to reflection and echos. Also lower frequencies carry further - and 'rock music' has the lower frequencies boosted massively, which the A scale is highly inefficient at capturing. This factoring - the A scale to put it simply - is not relevant when the bass frequencies are massively artifically boosted. 'Music' as played comprises many simultaeneous tones, and also much percussive energy with amplified drum beats too.
At one stage before calling The Mayor personally on her mobile, I measured an amplified drum solo at over 90 Db(C) several hundred meters off the premises, and in an area shielded by houses and trees.
While people are considerably more sensitive to noise in the region of 6 kHz than they are to tones of equivalent level the music pumped out at 'rock music festivals' tends to have massive bass boosting - which is clearly shown by Mr Pidgeon telling me on the mobile that he was reading nearer the 60Db level at the source on the A scale, while several hundred meters away levels in excess of 90 Db werere read on the C scale. This means that since there is an expected increase in sound level with increasing proximity, the sound level could clearly be well in excess of 100 dB (C) at the source - last year I received information that some stall holders on site had headaches and temporary deafness - irrespective of what the meter was reading on tee dB (A) scale! The A scale discards massive amounts of the bass boost!
Levels of 90 Db (C) and 60 dB (A) were noted under the streetlight next to the Olive 'Peace Tree' in East Creek Park - a few hundred meters off the premises. Similar levels were also recorded between houses several hundred meters from the source which should have shielded the noise - but because of the massive boost given to bass frequencies, and the massive wattage of amplification used, there was no attenuation with distance - something that is to be expected with low and ultra low frequencies. Levels approaching the 90 dB level were also noted at the far south end of East Creek Park - a considerable distance further - no noticeable dropoff in level was noted over that distance, which makes me believe that the topology has an efect.
