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In Reply to: RE: JBL123-A/Klipsch 400 speaker build HELP posted by Cosmickat on July 28, 2012 at 13:20:20
I presume everyone noticed he said he had a 12" woofer?
My first thought was it might be too small for the application (relative to the other drivers/horns)
??
Follow Ups:
Do you have any on/off axis frequency response measurements of the individual drivers and how they sum acoustically?
Or did you use "manufacturers curves"?
Or none at all?
Yes Presto, I think you are getting to the crux of the matter. The truth is I did not measure anything before I built,DUH! mainly because the specs on this particular driver were not that easy to come by...but a friend already had them in a pair of butt ugly cabinets, I mean REALLY UGLY... but Oooh, the SOUND! It wasn't exactly hi-end, but it was very REAL, so I went ahead, and purchased a pair(123-A)on ebay, and had very solid well built cabinets made, and here we are. My only concern is did I make them too big, at around 5 cubic ft? and is this why I appear to be missing some low end frequency.
Even if "textbook" filters can get a lucky newbie "close"... you have zero baffle step compensation for these. Push them up against the wall (if they are not already) and you might find the bass situation improves.
Is your box ported or sealed? Did you use any T/S parameters to come up with this box size?
http://www.jblproservice.com/pdf/Thiele%20Small%20Parameters/Theile%20Parameters.pdf
That's where you can see the 123A T/S parameters for your speakers there. Of course, your drivers could have slightly different T/S parameters - although JBL is likely going to have decent drive-unit consistency.
Adding volume to a box is not that hard - add a chamber on the back or bottom. Reducing volume is also easy - fill the cabinet with a sealed internal void. It can be a square box made of wood - or even a fire log if you're on a budget.
When you're building a crossover for a PA woofer coupled with horns, you absolutely NEED to measure. Factory "specs" and factory response curves will not get you too far.
Cheers,
Presto
Presto, I did move the speakers closer to the wall, they were about 3' from the front wall, and after I moved them closer I think there might be a slight increase in bass frequency,but I will move them back again just to be sure. As far as the cabinet, yes they are sealed, and measure 38x16x22. there is a lower chamber filled with Ultratouch acoustic insulation, and acoustistuff from Madisound. I do have a question-what is Baffle step?
Finally, I am seriously thinking of putting a second 123-A below the existing speaker to increase bass response. I have posted a photo of the finished cabinet. Thanks!
Baffle step is a decrease in on-axis response for frequencies below a cut-off point determined by the width of the baffle. For example, your cabinets there, if they are about 15" wide, will have a baffle step frequency of about 300hz.
To correct for this, they use a technique called baffle step compensation. Since in the passive domain we can only cut and not boost, the trick is to attenuate all frequencies ABOVE the baffle step frequency in order to get a smooth response.
Since you're using an active crossover at 600Hz with no DSP capabilities, you have no baffle step compensation. If you're trying to 'compensate' by adding "more bass" at 600 down, then you're ultimately going to be creating a midbass hump between 300 and 600Hz. This of course assumes your crossover to the midhorn is summing flat in the first place...
The big question is: Does this midhorn have a passive low/high pass or just a low-pass filter. Aka, was it designed with a specific woofer in mind (like the LaScala bottom end)? Likely so... and if that is the case, you need to "tune" the crossover for the 123A, or in other words, you need to get the acoustic response of the woofer to work with the acoustic response of the midhorn so that the response sums flat. If there is already a high-pass filter in place on the midhorn, this means the crossover point, slope and "Q" (shape) of the lowpass on the woofer is anything but arbitrary. If this is the case you can experiment with the knobs on the Urei for 100 years and never get a flat response.
Cheers,
Presto
Presto, thank god I didn't try to build a jet propulsion engine, apparently my house would have been a pile of rubble by now:) But seriously, thank you for taking the time to share your amazing knowledge and expertise with me and all others who read this. I will try to make improvements that are practical, affordable and actually make the MUSIC MORE REAL.
Cheers,
Cosmickat
you'll need to take measurements. That'll take some research, but is really the best way to go about it.
If you want to simply leverage "known" response curves, get PCD (Jeff Bagby's software) and simulate- you'll need to take some steps like considering baffle step as Presto mentioned.
Bass is supposed to sound big. 6.5" is not a woofer size.
here is the result from my 1st speaker build. thanks for all your positive input.
Unless they're demagged- which can be an issue with these alnico woofers.
That said, of the 123A/2213/123A-3 I've tested, they've all been spot on to JBL's spec.
Bass is supposed to sound big. 6.5" is not a woofer size.
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