Questions for the pros re: bandpass

[JoshK]

I was playing around with Unibox using data I have for the TC2+ 12" DVC drivers I have on hand. I became curious about bandpass designs after reading how Dr. Geddes likes and uses them and the experience of Patrick Bateman on another thread.

I modeled up my drivers and got the following response with a single tuned bandpass (see chart). This is pretty good response, no? I was thinking this could possibly make an ideal bassbin for my Modula MTMs. I was thinking they could serve also as stands like most bassbins and have the port firing downwards to the floor.

I see that I will hit xmax at 27hz with full power, but I don't think they will likely see that much abuse.

I have some questions:

1) Do you guys have experience with bandpass designs? Did you like them? Did they sound good in your experience? Have you ever ran them up to 100hz?

2) They look to my naked eyes to roll off at 12db/octave on either side of bandpass. I eyeballed 23hz and 77hz for F3's. How would you cross these to the Modula's?

3) Does this look like a good bassbin design to you?

4) What volume displacement do you assume for a 12" sub? If the volumes of the two chambers are sensitive to actual volume, I assume it matters to get the driver volume displacement correct. How would you measure the volume?

[John_E_Janowitz]

Hi josh,

One thing you'll want to do when looking at the bandpass is model the woofer with the BL decreased to 70% of the rest value. See how things change as that is what it will look like at higher excursions. Also you will need to find a way to model with inductance changes. As the VC in that driver moves forward and past the tip of the pole more it will become more air core. Drop Le to say 40% of the normal value and see how that changes. Then increase it by 40% and see how it models on the inward stroke. Compare the two, as that will be the difference in response from the outward and inward stroke. The bandpass will be much more affected by these issues than a vented enclosure. Lay the two plots over each other and post them up.

Then finally take the total volume you plan to use for the bandpass and model up a plain old vented enclosure of the same volume. See if you are really gaining much benefit from the complexity of the bandpass over the vented.

John

[JoshK]

Thanks John. I'll play around with such a scenario analysis and see what I see. This isn't a serious project yet, it was more a matter of fooling around to see what I can learn. The 3 way OB Lambda filled speaker is a more serious project at this point.

[1Michael]

I have built this Dual bandpass http://www.decware.com/whorn.htm
it pounds pretty darn hard for a couple of 10's and it clearly gains in efficiency according to the chart. But it Has to be crossed lower than 80Hz or it has a conipption fit.
I believe JCD has also built this or a variant thereof.