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!!!

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.