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Old 18th January 2009, 12:39 AM   #1
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Default Measuring displacement, xmax reduction?

Whats the best way to measure displacement with the woofer in the box?

I'm guessing if I look with an rta, the point at which a specific frequency band stops increasing spl with input would be close.
Then followed up with a sine wave oscillator and rta to fine tune for eq cuts to lower the xmax points on the way into the amp.
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Old 18th January 2009, 12:52 AM   #2
Mark Kravchenko --- www.kravchenko-audio.com
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Default Try this:

http://www.linkwitzlab.com/images/gr...edge-micro.gif

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Old 18th January 2009, 01:12 AM   #3
HK26147 is offline HK26147  United States
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I disregarded the Linkwitz link, because Scott said "with the woofer in the box"
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Old 18th January 2009, 02:30 AM   #4
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I should have clarified that.. in a Horn loaded box, folded.
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Old 18th January 2009, 03:24 AM   #5
Mark Kravchenko --- www.kravchenko-audio.com
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Default Can you see the woofer?

If you can see the woofer you can by following the simple advice given on the linkwitz site measure the excursion of the cone. This applies even if it is mounted in a box. It is very simple to do and actually quite accurate!

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Old 20th January 2009, 02:58 PM   #6
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Hmmm. Here's a close solution for a TH, Tedious but doable.

Materials:
Cheap laser pointer
Sharpie silver marker
clamps
Pre cut wood jig.

Make a wood jig that contours to the speaker frame, making contact at the mounting flange to below where you want the laser pointer to mount. The laser pointer should point at the
cone in a 90 degree angle to the mounting flange.
Once mounted turn it on and make a silver mark on the cone at the rest point indicated by the laser dot.
Make a mark above and below the rest point at just under XMAX
in each direction.

To use:
Hook up a VOM and an amp so you can monitor voltage driving the speaker. If you suspect the worst xmax problems as per simulation such as hornresp or akaback then you have a head start. For example 65w at 40hz for a 3015lf.
Feed a sine wave oscillator to the amp and you can visually see the cone travel while you sweep through the specific ranges.
By writing down the peak xmax frequency and the points closest that appear managable you should get you a pretty good idea of where you may want to cut frequencies in your DSP to lower xmax and also find places around those frequencies you may be able to boost to fill in the void a little better while still controlling your xmax.
If you can gain some control over your xmax issues you should be able to get a bunch more spl out of the box. Since the driver is in the cabinet you can take advantage of any assistance your cabinets compression can offer versus measuring the raw driver outside the box.
I'll post a pic when I get to the measuring part of my builds.

WARNING: Sine waves at high level will fry speakers quickly as that is CONTINUOUS power not rms. Don't crank the speaker to full power for more than a few seconds.

For those with DSP crossovers: Some such as the Behringer have dynamic eq. While it's a pain to set up, you can use it to lower larger ranges of your problematic frequencies as the volume increases. Since it's dynamic, you have the ability to do insanely loud shows without blowing stuff up while still maintaining very clean and full sound for higher end lower spl shows. Well worth the effort and time saved during setup.
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