Would basically be a self-modifying ambisonics system. It looks doable, though possibly a bit expensive.
They have a test tone and each one has a little micro dick (only way I can really describe it) that pokes out during the test tone playback to pick up reflections.
More here: http://www.nytimes.com/2004/02/19/technology/how-it-works-a-...
Cinemas (and concert venues) do calibrate their sound, here's a sample link that touches on what's involved:
http://education.lenardaudio.com/en/17_cinema_7.html
I'm not sure a combined mic/speaker system makes much sense -- it would appear easier to just model what's important: how the sound sounds where the listener is -- which is typically anywhere other than on top of the speaker.
Model a human head w/ears, but mics in each -- and hook that up to a system that plays/records/compares audio. While dynamically adapting (noise-cancelling when there's construction next door, the garbage truck rolls by?) -- I'm not sure that makes much sense. We're used to acoustics changing a bit -- but having a half-decent base-line would probably be great.
Though if you also have some extra microphones, then you can do dynamic adjustments on top and even account for people moving through the space so that they do not block sound or even so a stereo recording follows them around.
It's a very delicate technique though. When you're talking about phase-aligning soundwaves you're sensitive to sub-millimetre alignment of your transducers.
I listened to a stereophile friend eager to give advice, and convinced myself to add a self-powered subwoofer to the existing setup. Adding one shook the house but (to my ear) muddied the sound of music when no explosions were taking place. I could blame the equipment, or more properly myself, and replace some or all of the former. But I thought there must be some way to modulate all this unruly gear so that it produced a provably balanced and pleasant sound.
I will skip over some intermediate steps where I tried to learn the science and engineering of acoustics, using a neutral reference microphone and several fine pieces of audio software available to all of us interwebbily. I learned that I was overdriving the subwoofer, which made sense, but I couldn’t seem to go from spectrum graphs and suchlike to an actual balanced sound system. My final setup worked out quite satisfactorily and is an affordable alternative to building the computer-driven speaker cluster for room modeling.
> I have been looking at the idea of doing lots of small speakers with microphones on them attached to a fat lump of computer hardware, that you stick all over and they bleep each other and build an acoustic model of the space they are in, then adjust for it properly.
When I played in a band, we had to work with the club’s sound engineer or bring our own to get a balanced sound setup before the audience showed up. The more recent gigs showcased an interesting technology, a system that generated pink noise through the actual sound system, from the original sound sources before the mixing board, all the way through power amplifiers to the speakers at front of house. The engineer put a reference microphone in a strategic position, turned on the pink noise generator, and twiddled equalizer knobs to get a roughly even spectrograph (plus unavoidable “sound-guy magic” to de-equalize somewhat for specific gig purposes like dancing or seated listening) for the whole room, using the actual speakers and sound-reinforcement equipment we would be using.
This seemed to me to be the thing I needed in my home theater setup. It would let me balance and compensate for the (now) ten speaker cabinets in the living room (including the powered subwoofer), their real-life positions, the walls and floor coverings, the furniture, and even the cat. I think I could also do it with some combo of REW (Room Equalization Wizard) software, reference mike, and pink noise streamed through the amplifier. But while I was trying to get that approach figured out, I found a cheap piece of audio hardware with all that functionality built-in – no need to dedicate a computer to EQing the sound.
This is the Behringer DEQ-2496, a configurable digital equalization gizmo. It has a room equalization function built in, which is a home version of the room-equalization setup we saw in the clubs, and as a bonus it basically automates the sound engineer’s fingers on the equalizer sliders. I am not sure if it’s the latest model supporting this function, as I got mine used, but it was well below $200 and probably the best money I’ve spent on pretending to be an audiophile.
You set up the room for your listening preference (for me, this involves rolling the softest armchair into the center of the room the best to enjoy all those gunshots and explosions), put the reference mike about where your head will be, activate the automated room EQ process on the DEQ-2496, and sit down in your regular seating position to read for a while. It sends pink noise through the power amplifier and speakers and starts modifying an equalization curve to produce a “flat” room signal across all audible frequencies.
After about 15-20 minutes the EQ settings stop changing and the Behringer has generated the EQ setup for acoustically-neutral room sound. You can save the setup and create others (e.g., for group seating), or modify it manually to match your preferences (bigger bass for film audio, slight high-frequency boost for classical music listening, etc.), and save the modified versions as well. All my digital sound sources pass through it (game consoles, video, and music) in digital format and are equalized before they hit the amplifier and speakers – just as it worked in the clubs.
There are usable reference materials on the web to make up for the DEQ-2496’s obtuse user interface and execrable user manual. It’s not quite the super-flexible software-driven system I might have gravitated toward, but it does what I needed without tying up a computer.
What I like most is that it deals with the real world sound of my setup and room – if I change my setup or furnishings (as has happened a couple of times), I just re-run the balancing process and use the new settings as the baseline for my preferred room EQ. If my subwoofer, say, is turned too high the Behringer sets up a balancing cut in the affected frequencies. If the carpet soaks up some high-frequency signals, they’re boosted to compensate.
If my audiophile buddy gushes about the spectacularly flat frequency response of some new bookshelf system, I relax knowing that even my second-hand and slightly dinged-up speakers have been magically upgraded to subjectively flat (or not, as I wish) frequency responses – in the real world of my living room, not just in the lab or the store. It even has a full panel display of das blinkenlites should I need confirmation things are working properly. Very satisfying!
Amplifier designers are avoiding capacitors in signal path to minimize harmonic distortion. Now imagine you would put such a beast as EQ in the path...
To address your point directly, equalization (the feature of the Behringer DEQ 2496 I am using and discussing) does change the incoming signal, or it would not be a very useful audio component. My hands-on experience with oscilloscopes and digital audio analysis suites indicates that equalization does induce phase changes in the signal – you can see that by comparing input and output signals, or alternatively by doing the simple math that creates band equalization in the digital domain. These phase changes are pretty much unavoidable by-products of the audio filtering process, whether analog or digital.
Better ears than mine sometimes claim to hear the effects of phase retardation, and I can conceive of extreme situations where this might be plausible. Those with golden ears can test themselves by doing a controlled blind ABX test with a conventional EQ system (as I am led to understand the Behringer implements) and a linear-phase EQ system, and publish results from which we all may learn. But I cannot hear phase retardation effects with current audio equipment and I would be interested to see the blind ABX tests that showed them to exist at all in normal human hearing. Ethan Winer has addressed the real-world impact of phase shifting in high fidelity audio on several occasions – for example:
“More to the point, phase shift occurs naturally and unavoidably in all speakers and crossovers, and in every EQ circuit, with no obvious detriment as far as I can discern. Phase shift can be an important factor in speaker and crossover designs, but only because a tone whose frequency is near the crossover point is radiated by two speakers at once. In that case phase shift could cause the two acoustic outputs to partially cancel each other as they combine in the air in front of the speakers. But any time you stand in front of a loudspeaker and then simply take one step backward, you are inducing a large amount of phase shift at the higher frequencies.” [1]
As to the more general implication that the Behringer DEQ 2496 introduces distortion, specifically the kind of distortion I would hear and dislike, this claim may still be argued. I will say that I have not yet run the full suite of digital audio analytics on the signal coming out of the Behringer DEQ 2496, and it may (against the general practice and standards of commercial audio equipment) distort the heck out of the incoming gunshots, explosions, and piccolos. You cannot prove it by me, and if I live up to my responsibilities I will check the manufacturer’s specifications with a digital audio reference suite some quiet week in the future.
I would note, however, in case anyone else is concerned about the capacitors that may be magnifying harmonic distortion in the Behringer signal path, that its EQ process in my setup is conducted entirely in the digital domain, from SP/DIF digital input to TOSLink output. Only the infinitely delicate fingers of the Fourier transform and its ilk have touched those bits, and the reputedly foul influence of capacitors appears, if at all, downstream in the amplifier.
References
[1] From “Dispelling Popular Audio Myths” (http://ethanwiner.com/myths.html, retrieved 2015-05-11).
Modern digital EQ algorithms don't add any distortion worth talking about. But even in the worst case, it's still an infinitely small amount compared to the wild distortions that your room imposes on the sound.
If EQ can compensate for even a modest chunk of the room's influence, the net result is a huge drop in overall distortion.
For stage shows, having a proper computer driven sound system with a large array of small transducers would let you do stuff like having the audio follow the position of the instruments and voice accurately, as well as doing some really silly things, like rotating 3d soundfields. It takes some reasonably serious hardware and knowhow though. A good starting point is here - http://www.york.ac.uk/inst/mustech/3d_audio/ambis2.htm
edit - that Behringer thing sounds really cool though, makes me wonder how hard it would be to code something like that in pure data or similar. https://puredata.info/
As for re-implementing the Behringer in code, I don’t think there are any significant technical barriers for a modern PC, or even a high-end mobile device. I have not used Pure Data, but if it’s adequately performant it looks like a fine platform from which to start.
My original plan was to execute the room response mapping with REW (Room Equalization Wizard) on my PC. Then I would hard-code a set of appropriate digital filters into a software plug-in that I could attach to a digital audio I/O board in a PC or equivalent. That filter system could run headless after that, since all the human interaction ends with REW.
The sorry part for my project was realizing I had (at least in part) to dedicate a general-purpose computer to a single minor use. I gave up designing this system in code when I knew I would it meant yet another little PC with suitable digital audio input / output hardware, built to sit up behind the television with all the other gear that gets turned on whenever a game, movie, or song is needed. But my weakness need not be your own.
More power to you in your future endeavors! I look forward to visiting your place once you've got your project up and running.