Isobaric - less then $20

[Brian Steele]

Have you confirmed that you've hit the target Fb by comparing the impedance response of the built sub against the predicted impedance response?

[ODougbo]

I did not measure with a mic, but reasonably sure the predicted curve is pretty close to the actual response.

E.g. @ 25hz the bass is very strong, I did have to move back from the speaker about 15'. Also my impression was the speakers were pretty good; everything was rattling: ducts, table, test equipment, basically different parts of the room at different frequencies.

What do you think about the 10" PR, is this Iso box and speaker(s) a candidate?

Thanks for the link to the diy subwoofer page, I’ll be reading up on that.

[Don Hills]

I've said before that mounting drivers using a tunnel is a sub-optimal method. The "tunnel" graph illustrates why. It's from the model I did of your MCM 55-2421 design. Note that the "outer" (visible) driver (red line) will hit its excursion limits well before the inner driver (black line). The green line shows the theoretical response for drivers with no space between them. Mounting the drivers face-to-face improves the performance considerably ("clamshell" graph).

[ODougbo]

Thanks for the graph Don. I did make many, many boxes and just went with the ones I liked and thought worked best. The clamshell box was okay, it had a bit more output, however something about the tunnel gives the box/woofer(s) a solid sound.

I do have two more of these 6 1/2 “ woofers and was going to build another box, maybe with a 10" PR if it looks good in WinISD. The fact that there is a PR makes life a whole lot easier as far as removing the back (active) woofer; I’ll go with a clamshell.

[Don Hills]

Originally Posted by ODougbo

... I did make many, many boxes and just went with the ones I liked and thought worked best. ...]

That's a valid philosophy. Where bass is concerned, the response of the speaker alone becomes less important than the overall response in its intended location. "Commercial" speakers have to sound acceptable in a range of rooms. With DIY we have the opportunity to consider the speaker and room as a system, and it is no harder to build a speaker specifically designed for a room (and personal listening tastes) than it is to build a "compromise" speaker.

[sreten]

Hmmmm......

I see a certain very poor isobaric design has hijacked this thread,
very poor show. I sick of reading about how great one man's
poorly designed subwoofer is in his opinion, its boring.

rgds, sreten.

[ODougbo]

Originally Posted by Don Hills

I've said before that mounting drivers using a tunnel is a sub-optimal method. The "tunnel" graph illustrates why. It's from the model I did of your MCM 55-2421 design. Note that the "outer" (visible) driver (red line) will hit its excursion limits well before the inner driver (black line). The green line shows the theoretical response for drivers with no space between them. Mounting the drivers face-to-face improves the performance considerably ("clamshell" graph).

Don, the graphs are convincing enough, although a couple of questions:

Would a parallel (4ohm) connection help keep the two drivers in sync (vs. series)?

Would a good heavy/powerful stereo amp keep them “together” in parallel mode?

What about a 6 ½” driver? I would have to think a smaller cone would have fewer problems than 10’s and up.

The tunnel is minimal size and air tight (the tunnel could be made hour glass shape).

The new box will be done soon, added a 1/4” masoite to the front so I could flare the port holes.

[ODougbo]

Well here it is.....

It's okay, too much low end. All the bad characteristics of a tunnel are showing with this experiment.

OR maybe I expected too much from the “toy” drivers.

The cones are moving hard and not a lot of output, well there is very low bass and not much else, I moved the crossover up to 100hz, that helped some.

The MCM box is about 4x as “powerful”; and this not a total disaster, not going to break it apart, not yet anyway. It will be fine for a computer station, maybe the car.

[Don Hills]

Originally Posted by ODougbo

Would a parallel (4ohm) connection help keep the two drivers in sync (vs. series)?

The model I used was parallel connected already. I just now tried a series connection, it looks even worse. You need to avoid series connection unless you can ensure that both drivers are subject to the same load.

Originally Posted by ODougbo

Would a good heavy/powerful stereo amp keep them “together” in parallel mode?

The model assumes a perfect amplifier. Any real world setup would perform worse.

Originally Posted by ODougbo

What about a 6 ½” driver? I would have to think a smaller cone would have fewer problems than 10’s and up.

It's nothing to do with absolute cone size - it's the relationship between the drivers' characteristics and the volume of the inter-driver space.

Originally Posted by ODougbo

The tunnel is minimal size and air tight (the tunnel could be made hour glass shape).

Unless you are specifically after the look of a conventional speaker (visible cone facing out), I would recommend face-to-face mounting. To avoid the "outer" driver protruding, you could build the enclosure as you do now, but mount both drivers on the inner end of the tunnel. Or just recess the end of the baffle where the drivers are mounted. You could make a very tidy square section grille to fit into the recess and provide a flush front look.

Note that the "different excursion" problem mainly applies to vented enclosures. It does occur with sealed enclosures but to a lesser degree. The excursion differential is more or less proportional to the ratio of the volumes of the main enclosure and the inter-driver volume. With a vented enclosure, the rear enclosure can be thought of as very "small" at resonance - the enclosure pressure is almost equal to the pressure exerted by the driver, which is why the driver excursion falls to a minimum at resonance. That's why even a small inter-driver volume becomes significant at resonance of a vented enclosure.

[Don Hills]

Originally Posted by sreten

Hmmmm......

I see a certain very poor isobaric design has hijacked this thread,
very poor show. I sick of reading about how great one man's
poorly designed subwoofer is in his opinion, its boring.

rgds, sreten.

On the contrary, if you look at the opening post, this thread is all about that certain isobaric design. You are welcome to express your opinion of it, which you have. If you find it boring, DIYAudio is a large board. I'm sure there are many other threads you will find interesting.