Hi Tim,
I just read a reply from Mark Seaton on this subject. I wrote the reply below before Mark answered, but neglected to post it till now.
Go take a look at Mark's last post.
I agree with his points completely. Following his suggested protocol looks good to me
That said I stand by what I wrote below.
The finesse comes in when deciding how far to push the speakers while we test them and deciding how useful the measurement is.
Timmahh wrote on Sat, 05 June 2004 18:42 |
In the process of building 5 or 6 pair of Labs. 4 pair for a guy, and 1 or 2 pair for our personal use. When we have 5 pair complete, was wondering if any of the Professionals would be willing to come up to Michigan, Saginaw Bay City area, and do some stacking measurements using Smaart. .
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Well I may not be a professional, but I have played one on TV.
I don't have Smaart, but I do own Praxis, which works a little better for investigating speaker performance in any case. (Several types of Distortion measurements including something experimental)
If you talk to Ken O. he should know I live in Bridgeport. (In any case tell Ken I said "Hey")
Timmahh wrote on Sat, 05 June 2004 18:42 |
We are thinking of starting with one pair, then 2 then 4 then 6 ect.... and taking measurements along the way to be posted for those out there wondering how they would preform in various set ups, i.e. vertical side by side, horizontal stacking ect..... Most likely this will be done outside, unless we can find an inside place willing to donate an area large enough and with the open time to do some inside tests. let me know.
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I tried this very setup with four of Al's LAB subs some time ago.
We had the worst luck in wiring and equipment snafus you can imagine. That and other problems I won't go into.
For outdoors the problems are noise (are you in the country or downtown and next to an expressway) and wind.
In Michigan finding a calm day out in the country is a real crap-shoot.
A light wind not only raises the noise floor, it also can easily bottom the microphone diaphragm.
All that being said I highly recommend outdoors for low frequency measurement.
Makes scheduling a real problem
Finding an indoor space "large enough" will be a serious problem. To measure down to 20hz we need a MINMUM of one wavelength or about 57 feet in all direction (except down, we will measure in a half-space unless someone is going to spring for a crane) before the first reflection hits the measurement mic and several wavelengths is better. This works out to about 30 feet to a surface at an absolute minimum.
Also while talking to Tom Danley on this very subject he recommended being at least 20 feet away from a LAB sub array. IOW the bigger the source (in this case the size of the horn mouth as we stack more and more boxes) the further the measurement mic must be away to be out of its near field effects.
So we are looking for a room 80'x 60'x30' and double is better. The ceiling height is usually the limiting factor. For this size room you can forget doing anything but on-axis measurements
We did the original Michigan Sub woofer shootout in Fenton, Michigan kindly hosted by Joe Heslip the winter before last indoors in a room with about a 12' ceiling and found it to be a real compromise for low frequency measurement.
For what you are proposing indoors would not make sense unless we can use the Sliver Dome.
Timmahh wrote on Sat, 05 June 2004 18:42 |
Too Tall just posted a message about "guidelines" for use for a single Sub/ pairs/ ect... and the possible results using a modeling program. Maybe we could work this together to get modeling reading compaired to actual use readings...
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It would be nice
First we need to be clear as to what can and can not be done with a measurement program versus a modeling program.
In the horn modeling program they predict cone movement at a specific voltage input. Unfortunately I know of no way to "directly" measure how far the cone has actually moved and what is going on except in terms of distortion and SPL.
IOW I can't shine a laser on the cone and tell how far the cone has physically moved referenced to a specific voltage input. This means I can't check the prediction model directly. Especially with a horn we are relying on the modeling program to predict cone movement referenced to volts input and SPL output.
So we are confined to comparing SPL measured (by Praxis) compared to SPL predicted By McBean or something similar.
I know there is a direct link between SPL, the cone's Sd and how far it is traveling back and forth from center. But it is a link away from physically measuring how far the cone is moving and if it is exceeding Xmax or more dangerously Xmech (Mechanical travel limit before the driver tears itself apart)
Perhaps a subtle difference until you start doing real measurement experiments.
Where the rubber meets the road is when we crank up a big power amp and run the voltage output up to the point where the horn modeling program predicts Xmax..
We can't physically tell how far the cone is moving other then through predicted SPL.
As we play around up near the limit of the system all we can do is hope distortion shows up before the driver starts to physically hurts itself under long term use.
For a reference on what is "safe" limit for using LAB subs under different configurations I REALLY wish we could hang a sensor on a driver that charts its real physical movement off center.
Considering that the preferred failure mode is exceeding Xmech having a direct physical measurement would be best.
In the mean time perhaps we will find either distortion or compression will show up at Xmax and well before Xmech..
Timmahh wrote on Sat, 05 June 2004 18:42 |
I'm sure a few of the Michiganders in this perticular area would like to make it if time permits them too. But was hopeing some of the more Prominant people involved with this site would/could be willing to come up n lend their expertise on this endevour. As its been stated on various posts already; Inquiring Minds Want To Know.
We are thinking the 1st or 2nd weekend of July at this point. Of course there will need to be some co-ordination of minds, time, and the like, not to mention enough power to do this. we have some but nowhere near enough to run 4 - 6 pair. Any thoughts or interest out there? Everyone would be welcome to attend, but are hoping some of you experts can make it too. let me know.
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For people in the sound business during the summer weekends are "workdays" (at least we HOPE they are), but perhaps a Sunday might work. for some. For me it doesn't matter. I have other issues.
So did you guys turn on the tablesaw and make some sawdust yet?
Once you get some working models you are welcome to bring them out to Bridgeport where I can check their performance for frequency and impedance.
Too Tall