Bandpass subwoofers and Geddes' design

**PaulyT** · February 22nd, 2010 08:05

TT and I were wondering about the design of the bandpass subwoofer that Geddes offers on his website (the image below is also a link to his site). So I figured I'd start a thread, maybe invite Dr.G to comment if there's sufficient interest.

First though, I admit I don't know anything about what a bandpass sub is, what the advantages and disadvantages are of this design. I've googled around a bit but there's not a lot out there about these things...? Seems these are commonly used in car audio, but not so much in home audio? If anyone knows of good reference material about bandpass design, let me know and I'll update the list here.

Here's what I've got so far:

Questions I have for DrG about his specific design, based on the (fairly sparse

) description on his website:

What do you mean by "passive subwoofer"? Does that mean it requires an separate amp?
What is the frequency response of this sub?
Why bandpass as opposed to other designs?
Why multiple ports, in that particular asymmetric configuration?
What is the size+weight of this sub?
I notice you can't get it as a kit. Why? If someone were to order one, how long is the build queue?

Anyway, I thought this might make for an interesting discussion. Comments welcome, of course!

**Maximo** · February 22nd, 2010 08:08

Dude that's one wierd subwoofer. Yes I would like to know more.

**IrritateGuy** · February 22nd, 2010 08:19

Bandpass subs are based on enclosure resonances which vastly increase the efficiency of the driver.

The main advantage is that high sensitivity which could result in a much smaller enclosure playing much louder than traditional designs.

There are two main drawbacks to bandpass subs:
- The operating range tends to be limited and typically much narrower than a traditional sub.
- The phase shifts are much more dramatic with typically lead to serious group delay.

I have designed and built many bandpass subwoofers over the years, and totally gave up on them in the mid-90s due to my inability to make one that sounded completely natural and perfectly blend with the main speakers.

Hopefuly Dr. Geddes has done something truly amazing to make something more natural and beautiful sounding.

**Gedlee** · February 22nd, 2010 15:23

Paul

First, thats an old design. The ports ended up being too small, so I changed it. I need to update the photo.

The response given above was basically correct, a bandpass does put a resonance chamber on the front of a closed box loudspeaker and this does increase the efficiency. It is a fairly narrow band design, which is fine for a sub, but I don't give any importance to group delay at LFs in small rooms.

They are hard to get working correctly, I will agree with that, but I have had a lot of experience with them so I can seem to make them work.

They are "passive" in that the amps are external. I have not found plate amps to be reliable and they are too limited in the controls. I recommend a DCX2496 and some rack amps for the LF subs.

I also don't worry too much about "frequency response" of a sub because the room changes it completely anyways. How the subs work in the room at the choosen location is what matters and this is far more determined by the room and placement than by any free field measurement of the subs. In the end the sub gets EQ'd all over the place anyways. What IS required is a great deal of linear output capability and bandpass does this when a robust driver is used. I measure this as a near field measurement of the output, but not anything free field.

I used multiple ports because there was no place to put one. Large areas require long lengths, but a small area has too much turbulence, noise and distortion. So I needed to devide the poart into smaller pieces that would fit.

The sub is about 18 x 16 x 14 inches and weighs about 45 lbs.

Such a simpledesign does make sense as a kit. There is no way for me to make a decent profit.

My queue is pretty long at the moment, about three months.

Earl Geddes

**Gedlee** · February 22nd, 2010 16:27

Originally Posted by PaulyT

Thanks!

By freq response I meant more about upper and lower limits - e.g. what is the band that's passed? Do you use this with conventional bass management (crossover)? If the band pass cutoff slope is steep, it seems you'd be pretty restricted in where the cutoff would be and how you blend it with the main speakers. Or do you just set the crossover lower than the upper limit of the sub?

Is there a reason you preferred the bandpass design over other types of enclosures? Is it just efficiency, or is there a benefit to distortion characteristics, etc.?

On paper, the designs usually cover about 1-2 octaves, so thats about 40-100 Hz. But when I measure them in-room they tend to be much wider than that. I don't use a "crossover" for the subs, but I do use a LP on them, but again this is room and setup dependent. Everything is done in the DCX based on real room measurements. The mains are seldom HP'd in the DCX.

Bandpass are generally very small for their efficiency. Unlike a ported sub, where the LF point is at the box resonance, in a bandpass the box resonance is the center of the bandpass and the LF point is as much as an octave below this. Thats a big saving in size.

**PaulyT** · February 22nd, 2010 16:48

Thanks!

By freq response I meant more about upper and lower limits - e.g. what is the band that's passed? Do you use this with conventional bass management (crossover)? If the band pass cutoff slope is steep, it seems you'd be pretty restricted in where the cutoff would be and how you blend it with the main speakers. Or do you just set the crossover lower than the upper limit of the sub?

Is there a reason you preferred the bandpass design over other types of enclosures? Is it just efficiency, or is there a benefit to distortion characteristics, etc.?