I'm a home theatre guy conducting acoustical analyses on my home theatre. I'm using a PC to run the analysis. I'm still haven't decided on which programs to use so I'm currently testing out various demos (ETF, Smart, etc.). I'm using a Behringer UB802 mixer with two Behringer EMC8000 mics. For home theatre, you want to have each seat sound good so many acousticians (and also recommended by THX) use microphone multiplexers that will average out the readings depending on the number of mics used. An example of this is by Goldline at:
With my setup, I'm using two mics in two seating locations but I'm only using one mic per measurement. Per the PC software, I'm only using the right channel of the mixer to go into the PC. So I just pan all channels to the left and then pan to the right the channel that has the mic that will take the measurement.
Based on this, I have the following questions:
1) What if I panned to the right all mic channels?
2) Does the mixer do any averaging of the combined channels?
3) If not, what is the mixer doing to the combined channels?
I wanted to conduct a test of putting the two mics in the same location and taking a measurement of each mic and then of both and then comparing the data. Well during this exercise, I noticed that one mic was giving me inconsistent readings so I have sent it back for exchange. So it will be a few weeks before I can try this this out.
BTW, the microphone multiplexers usually cost around $1000 so using a Behringer Mixer would obviously be a cost effective approach if it works.
Hmmm... a Behringer mic exhibiting problems?? Not possible! (Sorry about being a smart-ass, it's my nature.)
First, a recommendation - lose the behringer mics. They are not test and measurement mics. Truthfully, most mics anywhere near that price range are not accurate enough to be certified test/measurement mics. However, if you have to stay on the cheap, the absolute best "under $500" test/measurment mic is the Audix TR40.
Based on what I could read from the website you provided, I'm guessing that using a mixer to sum all channels vs. using the multiplexer to average all incoming signals is two different things. Of course, I couldn't find any schematics to bear this out, it may just be a difference in terminology.
Also, the summing bus on the Behringer mixers is considered to be one of their weaknesses, so I can't strongly recommend this for your use.
I guess it all depends on how accurate you want your set-up to be. Personally, I have a dedicated theater and mastering suite (I figured, why buy two individual set-ups when they can both be incorporated into one.) The way that I set them up was:
-Apply acoustic treatment in the known trouble spots - diffusion on the rear wall, absorption of NRC1.0 (>/= 500 hz) on all early reflection spots, and bass traps in the forward corners.
-Adjust all speakers for level (excluding subwoofer)
-Run sweep tones to properly physically and acoustically align the subwoofer and crossover
-Fed pink noise through the system with Schoeps Omni mics at the 2 primary listening points and fed into PC.
-Analyzed frequency for anomolies such as peaks or troughs
-Determined best method for fixing such anomolies using either acoustic material or re-placement of individual speakers.
Ultimately, there were compromises made; I don't think this kind of installation is possible without some.
I hope this kinda helps,
Thanks for the reply Jeremy. I received another reply in the acoustics forum and they concurred that the mixer sums the inputs. So I will continue with the single mic approach.
BTW, I know that the Behringer mics are cheap but I read they are pretty flat until around 10kHz. I plan on some day to upgrade. BTW, are you familiar with Earthworks mics? Are they as good as the Audix TR40?
I plan on doing most of the steps you outlined. I have a question about acoustically alligning the sub and crossover. How did you do this? I'm assuming you play a sinusoidal sweep tone from 20 Hz to 300 Hz, but then how do you turn this infomation into the frequency domain to get a SPL vs. Freq plot? Can ETF do this? If not, what software can?
Hmm... All very good and very tough questions.
Yes, I'm familiar with Earthworks line of T&M Mics. I recently looked at purchasing some of their recording mics but decided not to based on specs and a little friendly help from one of the reseller/RO members. As far as T&M (Test and measurment) mics go, I would say that the TR40's are good - probably close to the Earthworks (having never done a direct comparison and also never having used the TR40 in a musical setting). However, for the huge price difference, I would think that the Audix is by far the better value and still a good mic.
As for aligning the sub - a giant portion was done on PC with both still tones and swept tones, largely looking for nulls or reinforced frequencies. A lot of it, however, was done by ear and by trial and error. It took me over 4 months to find the right place/settings for the sub. Compounding a normal sub's set-up is the fact that I am using a REL sub which is fed from both high-level and low-level signals. Balancing the two was perhaps the hardest part. Of course, the most frustrating thing was when I recently threw a small get together and I had a movie playing in the theater for guests to enjoy. One of the guests decided that he wasn't hearing enough bass, so he wen't and turned all of the nobs up and messed with the crossover! :evil: Fortunately, I had the foresight to write down all of the settings. That still doesn't ease my homicidal tendancies towards the individual though!
I know this dicussion had taken a tangent for this forum but I have one more question:
For sub allignment, what about playing low frequency pink noise (20Hz-300Hz) through 1 speaker (at a time) and the sub and looking at the SPL vs. Frequency with a 1/12 octave RTA with the goal have having a nice transition at the crossover frequency?
I have read some posts that there are disadvantages of using just pink noise with a RTA and that other tones like MLS were better (but I can remember the reasoning).