MJK Sheets: Isobaric Parameters?

[kelticwizard]

I am using one of Martin's isobaric sheets for the first time. The speakers will be wired in parallel, out-of-phase, back to back.

Can someone check to make sure that I have the values correct with respect to the single speaker values? "Same" means the same as the single speaker value, "half" means half the single speaker value.

I am especially wondering what I fill in for Vd-the single speaker value or half the single speaker value.


fd=same
Re=half
Lvc=half
Bl=same
Sd=same
Vd=same????
Qed=same
Qmd=same

Any help would be appreciated. Thanks.

[Volenti]

I would assume it's Vas (though it should be labled "Vad") in which case it would be halved.

[MJK]

If you are using one of the original isobaric worksheets you enter the single driver T/S properties and the exact geometry of the enclosure and coupling volume being modelled. If you review the TL Isobaric worksheet and compare it to the TL Sections worksheet you will see almost identical results for SPL. These are the same TL designs but in the case of an Isobaric alignment of two woofers the TL cross-section is half of the original. The worksheet combines the drivers and coupling volume internally so the user does not need to do anything special with the T/S parameters.

[kelticwizard]

Thanks, Martin.

I am using the original worksheet for the Bass Reflex Isobaric. I find it more useful for designing a Daline.

Same thing applies-merely enter the single driver properties?

[MJK]

Same thing applies-merely enter the single driver properties?

Yes.

I am using the original worksheet for the Bass Reflex Isobaric. I find it more useful for designing a Daline.

You have me scratching my head with that comment. You are designing an isobaric Daline? You are using two drivers connected by a snall coupling volume in your Daline design?

[noah katz]

"These are the same TL designs but in the case of an Isobaric alignment of two woofers the TL cross-section is half of the original. "

Has that been verified to work by test?

I could see where if you think of the TL as a volume, you would halve the area,.

But if you think of it as an area, you wouldn't, because Sd is the same, so you'd need the same, not half, the port area, for example.

[MJK]

But if you think of it as an area, you wouldn't, because Sd is the same, so you'd need the same, not half, the port area, for example.

OK, lets take that logic a little further. We halve the enclosure volume but leave the cross-sectional area the same, so the length becomes half the original value for a single driver. Does that make any sense? A TL is tuned by its length.

Has that been verified to work by test?

Yes. A few years ago I designed an Isobaric TL woofer for a local guy based on some Focal 11" drivers (don't remember the exact model since this was probably 3 or 4 years ago). It was a very compact design and he was surprised at how well it performed for its size. As far as I am concerned this proved to me that the older worksheets were very accurate. The Isobaric worksheets have not been used very much so I had not planned to upgrade them to include my latest methods.

[kelticwizard]

Originally posted by MJK
I am using the original worksheet for the Bass Reflex Isobaric. I find it more useful for designing a Daline.

Originally posted by MJK
You have me scratching my head with that comment. You are designing an isobaric Daline? You are using two drivers connected by a snall coupling volume in your Daline design?

Well, we're going to try.

Using your new worksheets, I came up with a nice virtual design for a subwoofer using a single Swan 305 woofer which a cyber-friend is pleased with. Thing is, he has four Swan 305s, and is thinking of crossing them over to a MTM or bipolar 4 ohm unit. One sub for each side. So he asked me if I would try to design something similar using an isobaric pair.

The front unit will placed conventionally on the baffle, cone facing outward, and the back unit will be placed back to back, out of phase. Right now I am trying to get a chamber size and "port", (actually, line) which gives a similar freq response to the single driver enclosure. If I achieve that, I will go about trying to figure a physical design that actually has such a chamber and a "port" and fits inside a reasonable sized box.


Frankly, I don't know if I can, but you never know until you tear into it.

[noah katz]

If the area emitting sound is halved, then I'd expect the output to be halved.

This doesn't occur when using Isobarik in sealed or vented systems.

I'm not sure Isobarik is applicable to TL.

It gives the same performance in half the volume where the box spring stiffness raises Fc of the driver. It does this by doubling mass, stiffness, and motor power to compensate.

"he was surprised at how well it performed for its size. "

How did it perform compared to one double the size? If it equaled it, then I retract my speculations.

[MJK]

If the area emitting sound is halved, then I'd expect the output to be halved.

This doesn't occur when using Isobarik in sealed or vented systems.

I'm not sure Isobarik is applicable to TL.

You are speculating. When you have done the math, designed one, and heard it come back and we can talk.