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

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post #1 of 27 Old 12-23-07, 02:26 AM Thread Starter
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Low SPL at 20hz for 12" XLS Peerless

Low dbs at low hz - Do I have something wrong with my subwoofer?

I have just built my first subwoofer. Fantastic! I used a design I found online which uses a 12" XLS Peerless driver and two 12" Peerless Passive Radiators in a 50 litre box, with a 240 watt (8ohm) amp. Fb is tuned to 20hz. Sounds great but when I check the output using RoomEQ I seem to be getting a lot of loss in dbs at lower frequencies (see photo) - 51db at 20hz as opposed to 79db at 40 and 60hz.


The measurement was taken with the RS SPL meter less than 1/2 inch away from the cone and the sub 6 feet away from any walls. Calibration corrections were loaded and measurement level was set to 76db.

Is everything normal? If not what should I check?

Also, is it ok to have one of the passive radiators facing the wall (2inch clearance) with the sub in the corner?


Cheers,
Blue
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Low SPL at 20hz for 12" XLS Peerless-low-dbs-low-hz.jpg  


Last edited by Blueeyedfrog; 12-23-07 at 02:32 AM.
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post #2 of 27 Old 12-23-07, 02:46 AM
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Re: Low SLP at 20hz for 12" XLS Peerless

why not try a measurement from the listening position.

I think a close mic measurement has as it's main reason is it removes the influence of the room, in other words a quick and easy way to get an 'anechoic' measurement in the bass.

Subs get room gain etc, so measure with the room and see how they compare.
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post #3 of 27 Old 12-23-07, 03:06 AM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Thanks Terry,

Here is the sub measured at the listening position - quite a different story.

No problem down near 20hz, with the assistance of room gain, but a nasty dip at 40hz. Does that mean my sub's ok? Also, what should I do about the dip at 40hz?

The designer said he managed to get the following from this design:

anechoic
SPL 104db @ 20Hz
SPL 109db @ 30Hz
SPL 112db @ 50Hz

How should I measure my sub to get a comparison - crank up the volume to 112db at 50hz and do a sweep down to 20hz (meter close to the driver)?

Blue
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post #4 of 27 Old 12-23-07, 07:12 AM
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Re: Low SLP at 20hz for 12" XLS Peerless

yeah, different result, and one more likely to explain your Fantastic! comment with the little fella running around in circles.

I'm no expert so very little extra help I can give. I would not think it very necessary to have to have the levels the same as stated by the designer to see if the response is close to the designers claim, the graphs should at least follow a very similar pattern (I think). Taking a look at some of the graphs of Ilkkas tests may help you there?? A browse of the ones he's measured may very well include a DIY 12 peerless somewhere, possibly not with two PR's tho.

I see where you got the idea from, I said a close mic gives an 'anechoic' measurement, well I think I'm reasonably close but really think you should wait for advice from the more knowledgable sub guys than go off what I say. For example, Ilkka does his measurements outdoors and the mic on the 'groundplane' or somesuch, way over my head, but it may explain the differences you see.

In the end, does it matter? Try moving your sub around in different positions - or the trick of putting the sub on your seat and move the MIC around, and observe the changes in the response.

After all, it is what you get in room that really matters, and unless you do a rebuild there is not much you can do about it at this stage.

You could find a location which maintains the extension yet helps the dip, then any final tweaking can be done with eq.

What type of amp are you using...often if it's a plate amp they can have different settings which give different boosts etc. If so, fiddle and play.

Good luck.
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post #5 of 27 Old 12-23-07, 08:01 AM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Thanks for the suggestions. I'm using an MWAVE 350Watt RMS Subwoofer Amplifier Module - it looks exactly the same as the Jaycar amp but who knows? Unfortunately only crossover and gain to play around with. The specs say that it has a Bass boost: 6db @ 35Hz.

Hoping to get a BFD soon (anyone have one for sale?) but apparently you only apply limited gain to the dips (a few db) due to clipping? As far as I understand eq is best used to reduce peaks not increase dips?

I guess my main concern is that I'm wondering if my sub is performing as well as can be expected or if I have missed something...?

Anyway, I'm still very pleased with the sub - it's the first I've ever owned and sounds a million times better than the B&W ASW600 I audition a few months back - very disappointing. I'm not saying the B&W sub is not good, it's probably a better audiophile sub than mine (less distortion etc.) but for my application in an HT setup my DIY sub seems to give a lot more grunt (quantity verses quality?) - and the 12" B&W subs were completely out of my price range. This sub has cost me about 3/4s the price of the ASW600 so I'm very pleased. Couldn't have done it without forum help though. Thanks everyone!

I'll try looking at some graphs of Ilkka's tests as you suggest.

Last edited by Blueeyedfrog; 12-25-07 at 06:03 PM.
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post #6 of 27 Old 12-23-07, 03:55 PM
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Re: Low SLP at 20hz for 12" XLS Peerless

I would really suggest placing the sub in different locations and see what you come up with. that dip at 40 is pretty bad and i bet very noticeable.

Do you know what you tuned the passive radiators to? I cant think of much you could have done wrong other then maybe add too much weight to the passives thus changing the tune.

The reason you dont want to boost the signal in the BFD is that if you are playing at spirited levels it will be sending allot more power during those freqs then others even though SPL wise it is the same output, thus the amp will be allot more likely to clip/over drive the sub.

The big peak at 20 and maybe the dip at 40 could be caused by the placement of the sub, with one radiator facing directly into a wall and the others not, the reflections in the sound could be canceling each other out at 40 and doubling up at 20. or the 20 could just be room gain.

I dont know if i would believe that the amp has a 6db boost at 35hz, as it really dosent look like it in either graph.
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post #7 of 27 Old 12-23-07, 05:23 PM
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Re: Low SLP at 20hz for 12" XLS Peerless

Hello, I got the same problem and resolved it by speaker placement. It really works for that kind of problem. Boosting a frequency more than 4 db will drain power from your amp, just try to move your sub.

Good luck !

Jean-Pierre Imbeau


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post #8 of 27 Old 12-23-07, 07:46 PM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Quote:
ccdoggy wrote: View Post
I would really suggest placing the sub in different locations and see what you come up with. that dip at 40 is pretty bad and i bet very noticeable.

Do you know what you tuned the passive radiators to? I cant think of much you could have done wrong other then maybe add too much weight to the passives thus changing the tune.

The big peak at 20 and maybe the dip at 40 could be caused by the placement of the sub, with one radiator facing directly into a wall and the others not, the reflections in the sound could be canceling each other out at 40 and doubling up at 20. or the 20 could just be room gain.
Thanks cdog,

Yes, I think I am having problems with my PR tuning - I actually started another thread here:

http://www.hometheatershack.com/foru...built-box.html

I also took the above room readings with heavier weighting on the passives than I have now - will try measuring again and play around with sub and listening position. But do you think I still may have a problem with my 'anechoic' readings (suggesting PR tuning problem?)?

Cheers,
Blue
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post #9 of 27 Old 12-24-07, 12:17 AM
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Re: Low SLP at 20hz for 12" XLS Peerless

in reality im not really sure that even having the mic that close eliminates the reflections and what not that you need to get rid of for a real anechoic reading. If possible you could take it outside and measure it where there is really nothing close by to reflect loudy.

Although looking at that top graph it dosent look that bad at all. The second graph is the trouble maker.

check the weights and see what you come up with, Whats it supposed to be tuned at?
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post #10 of 27 Old 12-24-07, 04:08 AM Thread Starter
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Re: Low SLP at 20hz for 12" XLS Peerless

Ok, some I've just spent the whole day shifting my subwoofer around .

See attached graph. The best response (if that is the correct term?) I have been able to get is with the sub out from the corner (reduces peak at around 24hz just a little) and the listening position moved 2 feet forward (gets rid of that nasty drop at 40hz). This gives me a somewhat flatter response however, I loose 13db boost at 25hz (from 95db down to 82db). I presume a flatter response is better than big peaks and curves, even at the expense of higher dbs in places, yes, no?

Also, while the new position reduces the nastry drop of 30db at 40hz, it only marginally makes things better - I still get a big drop (about 25db) at 28hz (so the dip appears to have shifted. Still its better than the 30db drop at 40hz I suppose...

Glad to hear that you think the close mic reading seem ok. I'll take the sub outside in the next few days and see what readings I get. Should I measure close mic at 75db as I have been doing inside? Also, I only have a small driveway - how far away from the garage doors should I be (other that far as possible)?

In answer to my PR tuning dilemma - follow the link I pasted in one of my previous replies above detailing my weights/tuning etc. - the box should be tuned to 20hz fb. Essentially there doesn't seem to be much difference in room performance ranging from 335gms - 400gms extra mass per PR (designer of the sub recommended 365 extra each) but my box is about 1 litre smaller - so I guess that would mean I should have slightly more mass?

Missed out on an ebay BFD so will have to wait a couple of weeks to buy one new
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