Bob,

Allow me to step in here and take an educated guess about 90 dB's interest in the specifics of the 105 dB discussion.

90 dB's handle here is an indication of some of his past experience and possibly thinking of in this instance: hearing conservation.

90 dB Time Weighted Average is the daily exposure limit set in play by OSHA with a 5 dB exchange rate, though more conservative organizations recommend 85 dB TWA with a 3 dB exchange rate. These are calculated using A-weighted measurements, normalized to an 8 hour day.

Lots of people throw around dB values, without much knowledge as to how it's calculated, what weighting is used, time response, etc. etc. etc.

Someone who is familiar with the details of SPL measurement technique, like 90 dB, has an instant distrust of persons making statements about SPL, and this is exacerbated by the prevalence of folks that think that their iPhone/Android SPL meter app has any kind of precision or accuracy (these are different in measurement theory), and most of them are quite far off, make no indication of whether weighting is being used, time averaging in play, and so on.

Even more dangerous are folks that are semi-informed on the topic. I've lost track of the number of times someone has shown me a peak level hit on their iPhone SPL meter and complained that I'm damaging someone's hearing because I had an instance in a show that hit 93 dB on their app (I run live sound now and then).

105 dB, at someone's ear, in a sustained situation has an allowed exposure time of 1 hour in a day, before it would tip over the TWA of 90 dB. To see a time weighted average calculator, you can go here: https://www.noisemeters.com/apps/occ/twa-dose.asp

Regardless,

I find it quite intriguing someone is using the difference encoding in a stand-alone amp/cabinet. It makes quite a bit of sense to me how this would sound like more spatial sound would result from this kind of an arrangement. You know, there is a bit of this going on in Dolby Surround and Pro-logic stuff as well. The ability to encode more than 2 channels of information, in a purely 2 channel signal is a hallmark of the Dolby Surround signal encoding/decoding: http://en.wikipedia.org/wiki/Dolby_Pro_Logic

But the difference there is the encoding/decoding is done intentionally, whereas this system is using the differences of the left and right signals, no matter the source, to crank out a 3rd signal and the common signal to crank out a 4th signal, at a spatially 90 degree oriented speaker. I totally buy that this could create a 'spatial' sound at much more locations on a stage, than signals being created by L/R separated speakers, where because of proximity to either speaker, your perception is dominated by the signal coming from that closer speaker.

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