isobaric sub problem

[tjrad]

Hi,

OK.. So I designed and built this subwoofer using two 128h's face to face on each side of the baffle board... At low volumes, the bass is fantastic.. Very deep and tight. The problem is, as I increase the volume, the bass disappears. Yes, I did check phasing... Ha Ha!!! It is almost like a phasing problem though in that the bass output out of the sub, especially low frequencies just disappears. So my question is, has anybody ever had this problem with isobaric configuration? Could the electro-magnetics from one woofer be affecting the other - Do they need greater spacing? Each woofer works fine by itself...

Help!!! :banghead:

Thanks in advance,
Tom

[robertbartsch]

Hmmm...

..a picture would be nice...

You mean there is a single cutout on a single baffle and two speakers are mounted facing each other?

[tjrad]

Hi,

Yes.. I am using a 1" baffle board and the speakers are facing each other.

Tom

[Allanvh5150]

You say the phasing is correct but what do you think is correct? Red to red or red to black?

Allan.

[baldrick]

Remember one woofer should be wired red to red, and the other one red to black :)

[tjrad]

The inside woofer is wired red to red and black to black. The outside woofer is wired red to black and black to red. Out of curiosity, I wired the outside red to red and black to black and there was no bass at all as I would expect.

I get great bass at low volumes - ~1-2 watts or less. After that, it seems that the bass diminishes relative to volume. Actually, my wife likes it like that... Ha Ha!! I suspect that the close proximity of the two voice coils/magnets are canceling each other. But how to prove it???

[duaneage]

Face to face one driver needs to move in the opposite direction from the other. That is what seems confusing. Test with a 1.5 volt battery to see which direction they move/

[1audiohack]

It's clear to me you have the polarity part figured out.

There are just to many working examples of clamshell / isobaric subs to need to worry about magnetic issues.

What amp are you driving these with?

Are they wired in series or parallel?

What does it do listening to the subs only as you increase the volume? I mean do you think they just hit a volume ceiling and lay right down like the amp is becoming voltage limited or does the rest of the system just run away from the sub?

[tjrad]

I am running a seven channel 160 watt Marantz receiver... I wired it in parallel and series. I also ran it with two of the Marantz amps(I only run five channels in the receiver so have two extra). I currently run a passive ADS sub with those two channels/amps and it drives the sub real loud with no distortion or problems.

To answer your question, the sub keeps up for a few watts. It actually sounds great - very deep bass. As I increase the volume, the rest on the system gets loud and the sub doesn't get any louder. The depth from the sub disappears. The low frequency disappears and cones are not extending. 128h's are not very deep speakers so that is why I am thinking electro/magnetic interference... Maybe a 1" baffle board is not enough spacing...

[Allanvh5150]

Face to face one driver needs to move in the opposite direction from the other. That is what seems confusing. Test with a 1.5 volt battery to see which direction they move/

In all Iso enclosures both drivers need to move in the "same" direction weather they are front to back or front to front. Front to back - same phase, front to front - reverse phase.

Allan.

[Allanvh5150]

I am running a seven channel 160 watt Marantz receiver... I wired it in parallel and series. I also ran it with two of the Marantz amps(I only run five channels in the receiver so have two extra). I currently run a passive ADS sub with those two channels/amps and it drives the sub real loud with no distortion or problems.

To answer your question, the sub keeps up for a few watts. It actually sounds great - very deep bass. As I increase the volume, the rest on the system gets loud and the sub doesn't get any louder. The depth from the sub disappears. The low frequency disappears and cones are not extending. 128h's are not very deep speakers so that is why I am thinking electro/magnetic interference... Maybe a 1" baffle board is not enough spacing...

Ahha, so you are running a home theatre amp. Many of these go into current limit when running into 4 ohms.....Fine at low volumes but reducing at high volumes.

Allan.

[baldrick]

Remove one driver and see what happens then.

How big is the cabinet?

Running two 128h in paralell should give you 4 ohm which shouldn't be a big problem for most amps, at lest not on moderate levels.

[UreiCollector]

Ahha, so you are running a home theatre amp. Many of these go into current limit when running into 4 ohms.....Fine at low volumes but reducing at high volumes.

Allan.
I agree with this comment...it is possible that the amplifier is unhappy with the low impedance load (2 drivers in parallel).

[tjrad]

I'm sure the amp is not a problem.. I currently run an old 4 ohm ADS sub and can really crank it with no problems. I am also running L112's fronts/center and L100's for rears and the amp can make my ears bleed... Ha Ha!!!

As I said, when I tested the 128h sub, I tried different configurations, parallel, series and running two amps, one attached to each driver and yes, phasing was correct.

This weekend, I will pull off the top driver and test with just one driver at a time. If that works, then, I will experiment with the clamshell spacing or building a chamber to have the drivers face the same direction...

btw: The cabinet is made of 1" MDF and yields 2.5 cu ft after bracing and displacements.

Thanks for all of the responses...

[djnagle]

Have you tried changing the location of the sub? You might be getting some type of cancleation.

[spkrman57]

If 2 drivers are sharing the same enclosure and wired so one is pushing and the other is pulling at the same time(Simple cabinet-not Isobarek), the bass will be enhanced at certain frequencies and cancel out at others.

Also when turned up the bass output will decrease as they will not add to each other.

If the bass response is dropping with increased signal power I would check the polarity of the drivers once more to be sure.

If this is not the case just disregard my input.

Regards, Ron

[Earl K]

tjrad,

- I have zero experience or hands-on-knowledge at working with this approach ; but if you're confident that you've followed all the published guidelines & protocols for making what to ought to be a successful isobaric / then I would suspect that something is amiss with the woofers .

- Maybe the increased acoustic load has pooched a surround on one or both 128Hs. I'd pull them and give them both a good inspection .

- When you researched this type of loading / were any recommendations made as to what sort of woofers were best suited to the approach ? ( perhaps the 128H just doesn't fit the parameters ) .

:)

[duaneage]

The 128H is best in about 1.7 cu feet. An isobaric reduces the volume by 1/2 so .85 of box would be about right. This driver is a vented box driver and should really not be used in a sealed arrangement. A volume that small may be tough to build around a 12 inch driver pair and tuning would require a long duct tube, probably 8-10 inches or so.

Isobaric loading is not that efficient, and the foam surrounds on the driver edge might not support the pressure generated by this config. I have build isobarics using rubber surround drivers and the woofers had fairly high xmax as well. Some drivers are better at this than others.

In my opinion you run the risk of overextending the drivers because the box might be way too large for the drivers. Run them side by side in 3.3 cu feet with two 4 inch ducts tuned to about 35 hz and hear the difference.

[Allanvh5150]

Something that no one has thought to ask.....Is the cabinet closed, vented or bandpass?

Allan.

[tjrad]

Hi Guys, Finally got back to this... At the risk of being thrown off the forum for being an idiot, I will tell you what happened.. On my receiver, I can set the peak output of the subwoofer. I thought that this was adjusted high but as it turns out, it was turned down. As a result, the output of the sub would max out real quickly and not keep up with the rest of the system. Unfortunately, I didn't find this out until I pulled my subwoofers apart and ruined the gasket on one of them.. So right now, I only have one of the woofers in the cabinet. It does sound really good and plays loud. I can't wait until I can get both put back in there. This leads me to my next question, anybody know where I can get foam gaskets for the 128h. I called Orange County and they only have cardboard gaskets...

Thanks for all of your responses and sorry about being an idiot.. Ha Ha!!! :banghead:
Tom

[tjrad]

Just a followup on this issue... Well, when all is said and done, the sub sounds really good. I choose a bigger ported cabinet(2.5 cu ft) to get better extention. Using a sound pressure meter located on the back of my couch, I was able to run 20-40 hz signals through it. From 29 hz up, it is solid. It is still useful down to about 27 hz but rolls off quickly. As I have three L112's as fronts and two L100's as backs, the sub is only for LFE and not used for music. I run the L112's/L100's full range. Running music through it sounds OK but certainly not as tight as I would want. A smaller cabinet would be better for music. Anyway, I recommend trying this configuration(two 128H's clamshell) if you have the time and of course, the speakers.