-> creative designs, have you ever had the chance to play with one of -> 
those theater/auditorium reinforcement systems designed to prevent -> 
regenerative feedback by using 100 plus drivers staggered around the -> 
audience, and computer switching which drivers are active quickly -> enough 
so that any given feedback path based on phasing and room -> quirks never has 
time to develop after space delays are included?
 BG>  No I haven't Terry, and until now hadn't considered just what would be
 BG> needed to make such a system work correctly.. One thing that I would
Budget to customize the algorithm to the acoustic environment?    
 BG> love to play with sometimes is  phase adjustments on
 BG> individual speakers within an array for feedback supression, coverage
 BG> pattern shaping, etc..
Acoustic theory in that respect really isn't much different than the RF work 
I do with directional arrays, other than a screwier set of wavelength to 
bandwidth relationships and lots of assymetrical non-point source sources.  
Cheap quality delay lines should make that practical for the better touring 
sound companies.  
 BG> I've played some with the early Sabine feedback suppression, but that is
 BG> trying to find the offending frequencies and stomping on them hard.. The
 BG> enough filters to remove all of the offending frequencies with narrow
 BG> notch filters, or you get a frequency response that is riddled with
 BG> narrow holes in it, kind of like a field after a artillery shelling.
 BG> This is also a compromise point with this system of EQing.
Enter noise cancelling mic systems.  How about pairs of B&K small diaphragm 
omnis that offer near point source laboratory uniformity performance?  Of 
course, they only work on very close mic'ing, with trained users.  
At least cheap wireless in the ear monitors have helped a lot.  Since this 
isn't the "pro-" echo, should we let the mundane folk consider that monitors 
usually feed back before mains?  
 BG> Another technique that can be used in a non musical situation is doing
 BG> pitch shifting the sound before hitting the amplifiers just a tad. That
 BG> confuses the feedback a little.
I much prefer the old school trick of inserting roughly one syllable of 
delay.  That does wonders testing the concentration abilities of professors.  
I'd hate to try that on a public speaker trained as a pipe organist though.  
  
As to more serious applications, it could be interesting to try scrambling 
circuits as have been used to randomize positive and negative polarity audio 
assymetry to equalize modulation in broadcast FM limiters as a minor feedback 
threshhold improvement trick.  
 BG> Perhaps to me, the most important consideration in getting system
 BG> feedback down is NOM, or Number of Open Mics, which directly effects
 BG> system feeedback "headroom". We are much too microphone happy now...
 BG> Sound consoles now have more inputs than most recording mixdown systems
 BG> need with an arsonel of microphones that often makes the studios
 BG> envious. Keep that NOM as low as you can at all times.
Ever work with automatic mic mixers?  I've installed JBL's of a couple of 
eras in broadcast talk studios and churches alike.  In live situations the 
simple reduction in space phasing problems can increase speech index 
surprisingly.  In recording I'd imagine they could work better in some cases 
than typical channel gates, especially when you consider how room acoustics 
and background noise change character with different numbers of mics, or too 
many shut off compared to average open mics.  
We agree on the importance of NOM.  I disagree that consoles with reasonably 
cheap capacity cause foolish operation though.  Silly human technical 
incompetency and hype driven business might better get that credit.  
Terry
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