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RE: dBa vs dBc- more

Posted by Bold Eagle on March 23, 2009 at 08:01:45:
The idea behind dB A, B, & C weighting was to match the sound level meter response to uman hearing at three different sound pressure levels. The Robinson and Dadson equal loudness contours (replacing the older and less accurate Fletcher-Munson curves) are the inverse of these weighting curves. A weighting is used for low levels where the ear has a lot less sensitivcity in the bass and treble, B at a higher level where the ear's sensitivity to the extremes is better, and C for much higher levels. In my ancient GenRad Handbook, A weighting is intended for use below 55 dB, B for 55-85 dB and C above 85. A weighting is -30 dB at 50 Hz (an indication of just how insensitive the ear is to bass at a low level), B is -12 dB at 50 Hz, and C is -1. All three curves are about -4 dB at 10 kHz and -10 dB at 20 kHz. Oddly, the A curve rises slightly above the others (by a dB or so)at around 4 kHz. 4 kHz is the ear's most sensitive frequency and it explains why we have the "BBC dip".

Clearly, the use of A weighting curves for industrial sound level measurements in the government regulations is a gift as they should be B weighted, not A weighted as specified. The limit for 8 hour exposure used to be 85 dB A weighted - it might still be, but I haven't worked in this area in 15 years.

You might also note that most of the less expensive meters lack the B weighting setting, which is really the right one for most home level measurements. For what it's worth, the preferred level for music listening among recording engineers and musicians is 85-88 dB long term average, which is quite loud.

As to loud: 115 dB is not un-typical for a Disco or club for short term peak levels (fast meter response, no weighting). In doing the engineering for a local Disco, I measured the SPL in several local Discos. The best of them (in terms of sound quality) was a live band and long term average levels were 105 dB (no weighting, slow response) with peaks to 122 dB ( meter set to peak, fast response, no weighting). This was a very large room and the levels were measured 2/3 of the way back, which is where the band's sound man was located. The sound quality was excellent and didn't sound amplified - it was as if they could all play and sing at inhuman levels (the demon band?).

During the band's break, the house system came on and was 108 db average with 118 dB peaks; and sounded like crap.

So I don't find the THX spec of 115 dB to be at all out of line for pro systems.

Jerry