Low SPL at 20hz for 12" XLS Peerless - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 27 Old 12-24-07, 04:17 AM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Maybe it's easier to see the difference if I put the two graphs together? Red line = sub in corner; blue = new sub and listening position. Difference between the two lowest dips (28hz vs. 39hz) is only 4.5dbs
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post #12 of 27 Old 12-24-07, 04:27 AM
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Re: Low SLP at 20hz for 12" XLS Peerless

You are using scaling other than the 'standard' in use here, 45-105 on the left hand side, up to 200 on the bottom. that might worsen how it looks though...anyway best if you stick to the norm so we can get a better feel.

there is a pull down menu that allows you to set the graph limits, up to the right top somewhere, just press buttons till you find it (mine is not open, not hard for you to find it).
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post #13 of 27 Old 12-24-07, 07:38 AM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Here we go - I think I got the dimensions right. Top graph compares sub in corner (blue) with sub out from corner and a different listening position (red). Yes, it does look worse. Two bad dips at 127hz and 192hz too, with the sub in the corner.

Hang on - I just checked a reading I did with the sub in the corner but with the new listening position and it appears that the corner is not the problem but the listening position (see bottom graph). Both readings taken with new seating position, red = sub out from corner, green = sub in corner.

Looks like the major difference is the peak at 65hz with the sub in the corner. But which sub position would be better to start from before eq-ing?

Cheers
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Low SPL at 20hz for 12" XLS Peerless-corner-side-positions2.jpg  

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post #14 of 27 Old 12-24-07, 11:40 AM
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Re: Low SPL at 20hz for 12" XLS Peerless

Regarding your original question about low 20hz output in post #1.

With a bass reflex alignment, most of the output around the tuning frequency +/- a half octave or so comes from your resonance device, be it passive radiators or the more common port. Inversely, at the same frequencies, the active driver is at a minimum of output. When you place the mic so close the active driver, you effectively ignore the output of the PRs, in your case...so minimal output around 20hz is a "good thing" based on the way you originally measured.

The only way to compare your measurements to the designers (post #3) is to measure under the same conditions...same distance, room boundaries or lack thereof, same drive level, etc.

-Brent
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post #15 of 27 Old 12-24-07, 03:58 PM
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Re: Low SPL at 20hz for 12" XLS Peerless

Quote:
brent_s wrote: View Post
Inversely, at the same frequencies, the active driver is at a minimum of output. When you place the mic so close the active driver, you effectively ignore the output of the PRs, in your case...so minimal output around 20hz is a "good thing" based on the way you originally measured.

The only way to compare your measurements to the designers (post #3) is to measure under the same conditions...same distance, room boundaries or lack thereof, same drive level, etc.

-Brent

Good point!, and if the PR's are on the other side of the box then the mic is even less likely to pick up any output.

Perhaps do a series of runs, close mic near the driver, close mic near the PR(s), and watch how the output 'swaps' at different frequencies, and maybe use the sum feature of REW to have an integrated graph of all outputs.

We are only interested in the SUB at this point....do those graphs include the mains?? The sub should roll of by two hundred hz shouldn't it, if not it will be contributing a lot in competition with the mains....make sure the mains are off for the measurements.



Edit I see that somehow you have gone from Log graph to linear (a button down near the bottom RHS of the graph area I think), so in addition to making the scale right (45-105, 20-200 hz) make sure the graph is LOG.
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post #16 of 27 Old 12-25-07, 05:51 PM Thread Starter
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Re: Low SPL at 20hz for 12" XLS Peerless

Ok, I think I have the graph right finally. Thanks for the feedback regarding my box's low SLP at beginning post. That makes perfect sense regarding the close mic minimum SLP at 20hz. So what is the best/ideal distance to measure my SLP to see how flat the box response is and what sort of performance I'm getting from it (even just to get a rough idea)?

Also, regarding room response, which would be the better curve (ie. better listening position) to start from for eq-ing?

Cheers
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post #17 of 27 Old 12-25-07, 06:09 PM Thread Starter
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Re: Low SPL at 20hz for 12" XLS Peerless

Also, yes these graphs do include mains. I'll do another test with the mains off. Also , your idea with measuring close mic 3 times and averaging sounds good but a little complicated. Not wanting to appear lazy but I wondered if there might be an easier way...

Cheers,
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post #18 of 27 Old 12-25-07, 06:49 PM Thread Starter
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Re: Low SPL at 20hz for 12" XLS Peerless

Ok now what's happening? Green line is with main speakers turned off, purple line is with mains turned on. Crossover is set at 80hz on the receiver and on full (200hz) on the sub amp.

Seems to be a big dip at 28hz (output = 46db) with the sub only, which is greatly ameliorated with the mains turned on (output = 55db). But surely the mains won't be working that low - specs say they only go down to 70hz (rears) and 52hz (fronts)?

Confused
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post #19 of 27 Old 12-25-07, 08:03 PM
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Re: Low SPL at 20hz for 12" XLS Peerless

stop worrying, you're getting your knickers in a knot over not much...

the two different dips you see, look how narrow they are, the difference is simply a measurement artifact, do three different measurements without changing a thing and you will see the 'dip's' lowest value change each time.

Re doing two or three measurements, I'm certainly not saying it will answer all your questions, but the effort of doing two or three sweeps suely is not the deciding factor? ie no effort at all really, compared to what you may potentially learn (or not ha ha).

In any case, my own view is that what you get at the LP is what is important, but if you really want to test your build against the design, then what is a few more measurements? Plus each time you do it you become more familiar with REW and that is not a bad thing in itself.

In any case, there would be benefits to eq'ing the bass for you. Nexct time you do a measurement, add the target curve for the sub, will help you get a feel for what's reqd. It is one of the check boxes below the measurement panel I think, and you can change the parameters in the pull down box over on the left, I think from memory.
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post #20 of 27 Old 12-26-07, 06:38 AM Thread Starter
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Re: Low SPL at 20hz for 12" XLS Peerless

Thanks, Terry. For all the measurements I have taken so far I have run 4 sweeps each. I tried designing a 'hard knee' house curve and seeing if I could straighten out the response - but the dips and peeks are huge (+/-24db). Any suggestions as to how to boost the dips (just ordered a BFD today)? Could I flatten everything down near the dips and then just raise the gain on the sub? At the moment it's only on about 30% gain.

I have two concerns:

1. Could the dip be caused the the shape of the room (L-shaped)?

2. Could I have damaged something when I built the sub? I left a sign wave running for a very long time (hours) thinking I was breaking in the driver before reading that this is a very bad idea. The sub amp got very hot...

(I do sound a little neurotic don't I?)

I know this is a little off topic now... Should I start another thread?

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