Poor results with some AC12 subs

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davehyder

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Poor results with some AC12 subs
« on: 17 Mar 2010, 05:51 am »
Greetings all!
First-time user from an old-time (long retired) Motorola engineer and long-time user of ACI stuff - Sapphires and others scattered all over the house and the kids' homes as well. Thanks Mike for the years!

However: I have four DV12 woofers in good shape (I'm the original owner) and have never succeeded in getting out of them what I think they should be giving. I'm using them as subs with a pair of nice sats (a North Creek Scan-Speak design). Subs have their own amp, and an electronic crossover between the preamp/tuner and the two power amps. The filters are configurable, as it's my design, so anything is possible there. They've been 2-pole Butters, Bessels, and at the moment are 4-pole Linkwitz-Riley jobs, but that's not pertinent right now....

The woofers are two per channel, wired in series (these had two 8-ohm coils, and are wired in parallel) for an 8-ohm, two-woofer box each side. Our problem is very poor bass output; we have a rising output from around 40 Hz up to 200 Hz or so. This is measured with a good SPL meter - but it agrees with my ears - and without any crossover, although you can clearly hear the issue with a crossover in there, of course. Should be cracking the sheetrock with these babes.

The amp output looks great - the 'scope shows a clean sine wave of constant amplitude at all times and in all cases (we're driving the CD input of the preamp with a signal generator). Several amps have been used as well, all quality stuff.

Each dual woofer box is well-built, braced MDF of common-sense design and 7 ft3 total, for about 3.5 ft3 (cubic feet) per driver. We've carefully checked all wiring and verified everything's in phase, even to the point of hooking up a power supply and checking the direction of movement of each cone. All is well there. Calculates to a rolloff of around 31 Hz or so.

Does anyone have any suggestions or thoughts as to why this performance is so wanting? I've not seen such a response from a woofer - lousy output below 100 Hz, and surprising output up into the 400 Hz range or above.

Thanks in advance for any thought!

Cheers and good listening,
Dave

Joe_K

Re: Poor results with some AC12 subs
« Reply #1 on: 21 Mar 2010, 01:39 am »
Hi Dave,
That's a weird situation. Up to 400 cps doesn't surprise me though.
A lot of spl meters roll off on the low end, are you certain yours doesn't? But you're using your ears too? Clean bass can be deceiving!
A lot of sub amps have EQ built in, tweeked to the system, but you're describing your situation as much worse than needing some EQ.
Have you tried them w/o the satellites? One sub at a time? So there's no cancellations?
What about your room? How big is it? Could they be in a nasty null?
Have you tried taking advantage of room gain? ie; placing them at a floor / wall intersection (woofers side by side, both close to the floor) or corner loading them?
Just some thoughts,
Let us know what you find out,
Joe

poseidonsvoice

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  • Science is not a democracy - Earl Geddes
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Re: Poor results with some AC12 subs
« Reply #2 on: 21 Mar 2010, 02:47 am »
Joe has stated a number of good points.

The one thing you need to do is get a decent calibrated mic (a Behringer ECM8000 from Cross-Spectrum is one good example) and measure your subs in room, then post that graph. You might be dealing with a big null or peak somewhere. I certainly don't use my ears until I'm done measuring to make sure it is reasonably flat (i.e. +/- 2dB) below 200 Hz. If not, then its time to move subs, use room treatments and optimize. Finally, applying parametric EQ can help tame those peaks. How big is your room, where are your subs, shoot us a picture and maybe we can help.

My $.02

Anand.

warnerwh

Re: Poor results with some AC12 subs
« Reply #3 on: 21 Mar 2010, 03:09 am »
Your room, both dimensions and acoustics, has a major impact on the bass. Also where you sit will have a significant effect. If any dimension of your room is the same or a multiple of another then that can also cause cancellations. Try measuring in different parts of the room. As a matter of fact move around the room and see where the best bass is.
Without bass traps any system will have ringing if it is capable of low bass. I really doubt it is the subwoofers. And of course don't be afraid of equalization. In that region peaks and dips in any room can be + or - 30db easily. Cleaning up the worst peaks will not only sound like you have better bass but the midrange will be improved to a significant degree. Fixing the bass region will be some of the best money you have spent on your system.

dave hyder

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Re: Poor results with some AC12 subs
« Reply #4 on: 22 Mar 2010, 04:23 am »
All:
Sincere thanks for the replies and obvious concern!
Yes, things have been thoroughly checked without the satellites (mains) and direct without the electronic crossover, every conceivable perturbation. Scope on the amp output, all the good equipment agrees with the ears. Important point at age 62 - ha!

Up until now there has been little/no opportunity to move the subs, as they're safely tucked away firing up into a nice 4" slab of marble hearth below the fireplace, down in a readily accessible basement area that can have as many amps and cubic feet of enclosure as anyone could want! The WAF factor of this arrangement is very high indeed, but it's also quite limiting as to sub placement.
However, today we drug out an old but fine cabinet that mounts one of the DV12 woofers in a down-firing configuration and we're moving that around. If successful, we will begin negotiations on placement! The good thing here is that the wife dearly loves the system and runs it 16 hours/day on really good music. This is a definite plus.

So thanks for the encouragement - it'll be worth the effort. I don't know what a good bass trap entails (hope it's not needed), but may be. We have proven that we have a major suckout or null in the main listening position, so some study will be needed.

But to answer your question: the room is 20x15, with a 12 foot vaulted ceiling. There's a 2' deep bay window along one long side. Don't see obvious issues there, but the existence of a huge null says all that needs to be said, theory notwithstanding.

Thanks again for all the thoughts! We'll go to work.

Sincerely,
Dave in Seattle

Big Be

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Re: Poor results with some AC12 subs
« Reply #5 on: 31 Mar 2010, 02:18 am »
I had a similar problem with AC-12 subs.  I built them in the late 80's, 4 cf each.  I eventually solved it with great results.  This problem could occur with any sub, by the way.

Part 1: How much damping material did you put in the boxes?

Part 2 depends on your answer.

Kenneth Baker

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Re: Poor results with some AC12 subs
« Reply #6 on: 19 Dec 2012, 11:15 pm »
I stumbled upon this thread, yes kind of old... But I can hardly resist commenting, especially since I don't see a resolution to the problem. 

Step one, involves a re-wire of the box in order to get the drivers to "see" the same voltage and share current (series -parallel) instead of having the same current and splitting the voltage (paralell-series, as they do in the setup that was explained above)... The total impedance will be similar, but the first driver coil will not be affected by the second driver's voicecoil's reactive components as much... Total voltage drop equals the sum of the series voltage drops... Don't want that to be driver fighting driver...

Turn your 8/8 ohm driver coils into 16 ohm coils (series), then wire that driver in parallel to the other driver (16 ohm) for a total of 8 ohms... And both drivers will experience the same volts and since they are the same drivers (x 2), they will equally share the current of your amp channel(s). I have only done the opposite (as you were doing) for midrange/linearrays.

Second, and crazy until you try... Put you sub where you will LISTEN FROM and go find where it is heard as a clean and even sound... And that is about the best place for the sub to be... Or a place to start!