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Multi-Way Conventional loudspeakers with crossovers

Problems with Scanspeak Illuminator 12MU/8731
Problems with Scanspeak Illuminator 12MU/8731
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Old 14th September 2015, 03:48 PM   #1
Mercury3 is offline Mercury3  United States
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Default Problems with Scanspeak Illuminator 12MU/8731

I've recently completed a three way system using 12" woofer, 2 of the 12MU's per channel and dome tweeter in MMT configuration. All drivers properly counter sinked flush mounted.

My tweeters are the Scanspeak Illuminator D3004/6640

Being a novice I chose to use active crossovers (2 Pass Labs XVR-1) with multi amps to avoid problems designing passive crossover.

I used 2 midrange drivers per channel because I do like to play my music loud at times and my thinking was to bring the efficiency up a little bit closer to the Woofer and Tweeter which are both higher than single 12MU.

My baffle step on 16" width baffle ends a little over 300hz so I'm using my crossover point @ 360hz. I used the edge program for best location of drivers on the baffle.

On the top end I've experimented a lot between 2200, 2500 & 2900 using both LR 12 & 24 none of which makes any difference on the problem I'm having.

I used the proper value resistor/cap to flaten the voice coil inductance. I tried with or without but it made no difference either.

I also tried the midrange drivers in a much much smaller parts express curved box with only a 7" wide baffle just to experiment and it made no difference to the problem. I'm positive it's not a box or baffle problem. (at least on the outside)

Here is the issue:

On female vocals when certain sustained notes/pitch is hit it sounds like some harmonics or some type of ringing is being generated. This is occurring only in the midrange speakers as determined by listening only to the midrange drivers by themselves.

It's also very noticeable with certain instruments such as flute. With some kinds of music it's not noticeable and the drivers sound very good. You can't detect it with most male vocals. Even some female vocals sound good. It's only when they hit certain notes or frequency at little bit higher levels.

Prior to this I was using Focal midrange with the same electronics and I did not have this problem so I know it's not in my electronics anywhere. I did swap some amps and things to check as well but again no difference.

In my problem determination attempts I bought an OmniMic V2. With the mic and software I was able to make some better adjustments to levels and things but nothing bad is showing up that would indicate a problem. Frequency response looks fairly flat in the midrange. It has a slight rise starting approx. 750hz to about 1100hz but it's only about 3 db. I don't think that's my problem. The only other irregularity is a slight fairly sharp dip between 1700 to 1800hz. It's a very thin dip so I'm pretty sure that would not cause it either.

Over all response is fairly flat with most of the bumps/dips all in the bass due to room nodes and things which are expected. The tweeters start to roll off a little after 6k but that's another issue and they still sound good regardless.

So I'm about at the end of my rope.

I'm thinking there might be a design flaw in these midrange units. The flat ring that holds the spider looks like it's too close to the cone and to large around. I had to slope the inside of the holes and even with that it doesn't look like a lot of room for the back wave sound to flow around that ring on a 3/4 inch baffle. I think possibly I'm getting sound from that ring and spider bouncing back to the cone at certain frequencies but probably not enough which can be measured with my mic.

My next idea is to try to slope the inside of the holes some more but I'll be down to about 1/4 inch of wood left for the screws. I'm only speculating and that might not even be the problem.

Has anyone else successfully implemented these drivers? I've search and not really finding a whole lot.

Thanks!
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Old 14th September 2015, 04:09 PM   #2
ScottG is offline ScottG  United States
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Test for non-linear distortion with your OmniMic.

It's also useful to see the Impedance trace with a tool like DATS - and DATS has a "rub-buzz" test for problems with vc position and surround problems.

My guess is that it's either defective drivers or you are getting a baffle resonance due to a lack of proper chamfering. (..and more likely the later.)

Chamfering example around driver (..from Zaph's "Blog" section - note how even the inset on the test baffle uses a felt surround - something to think about in terms of the interior cabinet):
Attached Images
File Type: jpg Scan-Speak-10F-4424G00-rearchamfer.jpg (142.1 KB, 739 views)
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Last edited by ScottG; 14th September 2015 at 04:13 PM.
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Old 14th September 2015, 04:19 PM   #3
Mercury3 is offline Mercury3  United States
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Quote:
Originally Posted by ScottG View Post
Test for non-linear distortion with your OmniMic.

It's also useful to see the Impedance trace with a tool like DATS - and DATS has a "rub-buzz" test for problems with vc position and surround problems.

My guess is that it's either defective drivers or you are getting a baffle resonance due to a lack of proper chamfering. (..and more likely the later.)

Chamfering example (from Zaph's "Blog" section):
Thanks,

Yeah I did the Chamfering already. My only thought was taking even more off but I won't have much wood left. I think that spider ring is really in the way of the back wave sound.

I did buy the DATS with my OmniMic so I could experiment with it. I haven't even opened up that box yet. I'll also have to learn more about testing with my OmniMic for the non-linear distortion and other tests it's capable of.

Okay with work it'll take me some time to get to this but I'll check into all three of these things.
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Old 14th September 2015, 09:34 PM   #4
Lojzek is offline Lojzek  Croatia
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Quote:
Originally Posted by Mercury3 View Post

I'm thinking there might be a design flaw in these midrange units.
My guess is that if there is anything wrong with it, it's the XO filter.
Ever tried building a 2 way with one 12mu and tweeter passively?

You ought to get a dsp based active filter product.

Once I had a chance to repair a passive 3 way diy speaker project
that had a sort of reverberating female voices at certain frequencies
and I've noticed it on Spanish Harlem by R.Pidgeon. It turned out
that midrange-tweeter filter was badly designed. The driver units
were Focal polykevlar.
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Old 14th September 2015, 09:37 PM   #5
LineSource is offline LineSource  United States
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For a TMM topology with both MM crossed at 2200Hz+ lobing and comb filtering effects of polar beaming and uneven power response can be audible. The sound stage "moves around". Some notes in a chords sound louder/softer as more power beams into a smaller room volume. To the listener, a slow sine wave sweep can change sound stage position and amplitude.

You might experiment with disconnecting the lower M and re-adjusting SPL levels for TMW.

You might decide on a 3.5 way crossover, where the lower M has a lower crossover frequency to supply the extra cone surface area needed at the lower midrange frequencies.
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Old 14th September 2015, 10:03 PM   #6
Mercury3 is offline Mercury3  United States
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Thanks guys.

I just finished increasing the Chamfering behind the driver rear holes. I have it at least as good and maybe a little more than the pictures on Zaph's Blog that Scott showed.

I'd hoped that was it but sadly no change.

I think the next easiest thing to try is disconnecting the lower driver as LineSource suggested.

Gonna try that and I'll report back.
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Old 15th September 2015, 02:41 AM   #7
ScottG is offline ScottG  United States
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Bummer..

Yes, always listen to the driver by itself (singularly) on a large baffle - even if the baffle is just made of cardboard (..though properly "damped").
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Old 15th September 2015, 06:33 PM   #8
Mercury3 is offline Mercury3  United States
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Update:

I ran only the single top midrange (MT instead of MMT) and problem is less pronounced but absolutely still present. I think it's so noticable to me because of using 4 drivers)

Hooked up the 12MU's full range fed right off my CD player to power amp. (eliminated the crossovers and all other electronics) Running the drivers full range with no crossovers the same problem is very much still present. This rules out any crossover problems which I was already pretty sure was not the case.

Played with the DATS'2. Doesn't really show anything and I didn't really expect it to. No voice coil or issues at all on any of the four driver with Buzz/test. The impedance sweep looks very flat from about 200 hz up. No irregularities to that.

Resonant frequency in the cabinets approx.90hz (doesn't really matter I measured it so thought I'd list)

I'll play with my OmniMic V2 more when I get some time. As I reported earlier though the frequency response sweep actually looks pretty damn good after playing around with various crossover and level settings.

I think the drivers is a bad design with that wide/flat ring completely blocking the back wave from the cones back. It's bouncing back to the cone at certain frequencies between that gap. I really think these are bad driver design and it's less pronounced when using only one so it could almost go unnoticed and chalked up to driver colorations.

Last edited by Mercury3; 15th September 2015 at 06:37 PM. Reason: spelling
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Old 15th September 2015, 07:56 PM   #9
Lojzek is offline Lojzek  Croatia
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I'd like to see the impedance sweep of both of the midranges
in the box without any filters or Zobels. From 10 Hz to 20kHz.

Got any stuffing to damp the standing waves in the box?

edit: the distance from the cone to the ring is less than 20 mm, right?
The standing wave would be like 8,6 kHz. You are crossing nowhere near.

Last edited by Lojzek; 15th September 2015 at 08:02 PM.
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Old 15th September 2015, 09:01 PM   #10
Mercury3 is offline Mercury3  United States
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Quote:
Originally Posted by Lojzek View Post
I'd like to see the impedance sweep of both of the midranges
in the box without any filters or Zobels. From 10 Hz to 20kHz.

Got any stuffing to damp the standing waves in the box?

edit: the distance from the cone to the ring is less than 20 mm, right?
The standing wave would be like 8,6 kHz. You are crossing nowhere near.
I can do that impedance sweep for you later when I have a day off. I'll have to open the rear and remove the zobels to take it. Note that with the zobals it's pretty darn flat after 200hz.

Yes inside the box I have a combination of fiberglass lining the back and all sides with wool fiberfill in the center and behind the drivers.

I had also thought changing the inner stuffing as my next easiest thing to try. Perhaps lining some felt on all the walls prior to stuffing might make a difference.

Thanks everyone for the ideas. These drivers were expensive so I'd like to make them work. It's kind of hard for me to believe the design is bad but that damn spider ring sure is wide and right under the cone. I dunno......
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