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Old 22nd December 2006, 12:44 PM   #1
Tintin is offline Tintin  Sweden
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Default Midbass hole in JX92S/GM48

Hi all, finally in the completion stage of my Jordans in GMs MLTL-48 cabinets. The sound is mighty fine already and I am tinkering with different amounts of wool stuffing at the moment. Shouty with to little but a bit dead with too much, trying to decide on a sweet spot. The recommended 0.52 lb seems quite a bit too much. How much have you other GM48 builders stuffed yours with?

But my real question was that after listening to some thest tone samples from http://www.realtraps.com/test-cd.htm I found out that regardless of stuffing the area around 60-70 Hz seems quite dead. 40-50 Hz on these samples are quite prominent but there seems to be a hole after that. Is this something more people have seen? I do not have access nor the skills to model the speaker in order to find out if this is an effect of the design or not. More exactly how do stuffing affect the frequence response? Have anyone done RL measurements on this design?

The room for listening is about 4.5m square and have 3m to the ceiling. Boxes are the triangular variety built in 1" BB ply.

The boxes still needs finishing, and I guess they will be oiled, laquer seems like a bigger hassle than oil. Attaching a WIP picture.

yours Jonas
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Old 22nd December 2006, 01:09 PM   #2
pjanda1 is offline pjanda1  United States
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That's most likely the room; the problem the tones were intended to find. Notice the graph of a 'typical' room on real traps site. And yours is square to boot! It won't sound as bad as it is (it's always been there, with every speaker you put in the room.) If you want to get rid of it, the only options aren't small. Links to diy room acoustics stuff here:

http://www.audioasylum.com/audio/twe...es/139726.html

pj

p.s. I've got a huge peak at 42-43 hz (12db) , not much from 60-70, a huge peak at 93 (12ish db), and then a big dip (only at the listening chair) between 110-140 (at points -40db) . Again, this is with any speaker. I'm thinking some bass traps are in order, I just haven't told my wife.
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Old 22nd December 2006, 01:44 PM   #3
Tintin is offline Tintin  Sweden
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Ah, yes I guess you are right, I wonder what would happen if one used a room corrector or just a good parametric eq. Besides the adverse effect of more electronics in the chain - would this have the speaker working to 'uneven' over the frequence range and cause distortions in certain wavebands?

Bass traps seems like a quite radical idea, and I am not perfectly certain that rolls of fibreglass would fit nicely into my room though

Thanks for the input!

// Jonas
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Old 22nd December 2006, 02:03 PM   #4
pjanda1 is offline pjanda1  United States
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You may be able to acheive some gains with placement and seating positon changes. If you've only measured in one spot, just spend some time walking around the room with the sine waves playing.

There are some nifty machines on the market that may be of help. Behringer has made various products that function as nifty parametric EQ's. The Feedback destoryer was one, but I haven't paid attention to the product line and I know it has changed. Any electronic gizmo certainly won't purify your signal path. It also may not do much about the nulls, as more sound cancels just as easily as less (in my meager understanding of acoustics). I think you'd have success loping off peaks. As some of these machines aren't that expensive, it may certainly be worth a shot. You could probably pick up a Behringer Feedback Destroyer used, and then sell it if you don't like it.

You can also rest assured that most folks are pretty happy with their speakers, and most haven't addressed their room problems. If they think they don't have any, there's a darn good chance they've never checked.

Remember too, that a note isn't a sine wave. So, as long as there isn't the same peak/dip an octave up (2nd harmonic of the fundamental with the issue), the audibility of such problems isn't what it might first seem. Our hearing isn't very astute down there.

The latter comments aren't meant to disuade you, just to point out that if you find the solutions too much of a pain, you may well be able to tolerate the problem. You'll sure get more out of your speakers if you address some acoustical issues.

pj
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Old 22nd December 2006, 07:10 PM   #5
Tintin is offline Tintin  Sweden
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After lots of changing position and listeing to test tones and music I realise that the effect by digitally correct the signal would not be half as good as actually correct the room physically. My room has spots where I would guess the cancelling should be around -30 db on certain frequencies. New experience to listen to test tones indeed ;-). But just as you hinted, actual music is not affected half as much. The 2'nd harmonic has some cancelling but its not very much.

Lots of things to study up on here. I think I will have a try with Digital room correction via FIR http://drc-fir.sourceforge.net/ and http://www.duffroomcorrection.com/wiki/Main_Page

If nothing else It would be interesting to see how the frequency response is in the room and learning new stuff is always fun

Jonas
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