Bob Capotosto: Can you cover the relationship between the Transfer Function and coherence traces?

Jamie: Coherence is basically looking at how consistent your data is. In a perfect world, your measurement should be dead-on for each consecutive measurement, such as when you are measuring an electronic device like an equalizer: no noise, high repeatability, and good coherence.

Jamie (continued): Coherence can be affected by noise, reverberance (late arriving info), bad measurement delay setting, time misalignment, distortion… To figure out if the bad coherence is being caused by noise, simply turn your measurement noise up. As the level goes up, coherence should get better. When you hit the point that it no longer improves from turning the noise up, you have reached the point where the poor coherence is not due to room noise.

Jamie (continued): What you are left with then is the product of reverberance, cancellations, distortion, etc. Keep in mind that you shouldn't expect to see great coherence in a medium-sized room. Remember to increase your averages to stabilize your measurements and give you better s/n (signal to noise ratio). You won't see the coherence trace go up, but you will be able to trust your data more. With 64 averages, I trust data at 20 percent Coh (Coherence) as long as it appears stable.

Harry: Does this apply to 3.5?

Jamie: Since 3.5 doesn't use overlapping FFT's, you are basically looking at more data in fewer averages. In SIM, we used the following guidelines, and this would certainly be comparable to 3.5. At two averages, we wanted 90 percent or better; at four averages we wanted 60 percent or better; at eight averages, we wanted 40 percent or better; and at 16 averages, we wanted 20 percent coherence or better. I guess the only drag with 3.5 and coherence is that it is not live Coh.

AlanH: Reference mics... even Behringer is making one. Audix, Earthworks... How do the lower priced ones stack up?

Jamie: As you pay more for a mic, you get: flatter response, extended range, lower self-noise, and better consistency from mic to mic. There's a lot of mics that will work well with SMAART. The question is: how flat do you need it? What frequency range do you need? What mic-to-mic consistency?

Jamie: To be honest, most live measurement situations do not need a B&K (Bruel & Kjaer) 4007, although it is a great mic. The key: do you need a hyper-flat response above 16 kHz? What is it worth to you? And, will you freak out if that mic is stolen or bashed with a chair cart?

Chris Kathman: The chair cart raises its ugly head!

Jamie: That is probably why SIA sells the Earthworks mic. It is a great balance between performance and price, a great value. We also sell the Audix, which is also a good measurement mic at a great price. I guess it just depends what you need out of your mic.

Larry Elliott: There was some thought a while back that there would be a (Apple) Macintosh version of SMAART. Is this still being considered?

Jamie: The short answer is no, not really. The problem is that we don't have the resources to go multi-platform. It's an economic issue. For the price of developing and maintaining the code, we would have to crank the price way up. Sorry. And I am/was a pretty devoted "Mac Guy". It just isn't in the cards right now.

Larry Elliott: I seem to get "adequate" performance on a G4 Powerbook with Virtual PC Version 4.

Jamie: That's what we've been hearing. I think that's great news. Unfortunately, that also makes justifying doing a Mac version harder.

SRS Audio: Jamie, I was in your SMAART class held at NSCA. My boss asked me the other day if I could precisely measure reverb time with SMAART?

Jamie: The NSCA class was way over-booked. My bad, I guess. Anyway. I guess the key here is "precise". There are questions of how you excite the room, what frequencies you are specifically talking about? The T-60 can be amazingly different at differing frequencies.

SRS Audio: Speech range, mostly.

Jamie: You would need to break it down into bands, and even then, it's a bit of a subjective call in selecting a starting and stopping point. I guess I'm the wrong guy to be asking about this, in a way, because I do not work as an acoustician on a regular basis.
There seems to be a fuzzy art/science line in these measurements.

Harry: Many people say the great thing about SMAART is you can use the show as a source. How can this be done when both sides are on, and usually different distances from the mic?

Jamie: I guess it all has to do with how isolated you can make your measurement. You can also use a mono source and place the mic in the exact center. The key is NOT TO MAKE DECISIONS ON A BAD MEASUREMENT. I just wanted to say that loud to remind people that a squiggly line doesn't mean you have a useful measurement.

Chris Kathman: Yeah, people! What he said!

Jamie: I also believe that, if you do your job setting up, you can generally make adjustments by ear during the show. I regularly turn off TF (Transfer Function) mode and switch people over to RTA during a show. Put the RTA off the Cue buss, and let the mixer use it to police his signal. RIGHT ARM! FAR EAST!

Devin DeVore: What about measuring distortion? It would be good information to know how the distortion changes with different SPL and different array set-ups, Also, Dr. Don preaches well about critical distance.

Jamie: Distortion is a hard thing to measure with SMAART. So many elements go into your measurement, and there are so many things that could be affecting your coherence, which is the main indication of distortion in our measurements. Looking at distortion versus level is probably a measurement best suited for a lab, and maybe an AP.

Jamie (continued): But that is not to say it isn't a critical factor in our listening experience, just that it is something that we probably won't be using a SMAART measurement during a show to figure out. And, you can bet that if the good Dr. is preaching about it, then it is definitely a key issue. I guess one of the things we are talking about here is measuring at a distance that isn't in the high SPL of the horn throat, and not being back buried in the reverberant field.

Jamie (continued): For the purposes of our measurements, let's stay in the linear range of the devices when we measure them. Yeah, we might take them up to their limits as the show goes on. But remember that SMAART is expecting a linear system when it makes it's measurement, so non-linearities like clipping and the limiters that are there to protect us from clipping are best kept out of your alignment measurements.

Devin DeVore: Any suggestions on phase steering with SMAART and EQ'ing for phase? Can you or Dr. Don add some case studies, or another SMAART class with a focus on phase steering?

Jamie: Phase determines how two signals will add. The whole game of system engineering and system alignment is controlling and managing the interactions between elements of your system. The key ingredients in phase are polarity, phase shift (from EQ's, crossovers, etc.), and timing. Control those and you control how and where your system adds and subtracts.

Jamie (continued): Take the example of a stack of subwoofers. If you simply put them in a nice vertical stack, they will tend to narrow their pattern in the vertical. If you want to affect this pattern, say open the coverage up vertically, you can delay subs incrementally as you go up, effectively acting like arcing your subs in an upward direction.

Jamie: While not perfect, this will open up the coverage because you are shifting the equal time arrival point upward. The same effect can be applied to two stereo stacks of subs to de-emphasize the big LF build-up in the center of the room, which is the equal time arrival point. You can play with delaying one side of the subs to push that equal time arrival point - the focus - to the side.

Jamie: T he thing to keep in mind, always, with these types of things is that they are imperfect substitutions for true physical solutions to your problems. Steering is taking control of the elements that determine how a system interacts. Level and phase.

Moderator: Thank you, Jamie.

Jamie: Thanks. Let's do this again. Maybe I'll be a little less shell-shocked
Hasta and peace.

Chris Kathman: Those who wish to may now assemble next door in the Live Sound chat room. Your moderator was "Another Dave"! We will return soon with Mr. M.L. Procise in the hot seat. Good night, this has been a production of Universal Concept Incorporated (UCI).