Geddes Bandpass Subs and the Multi-sub approach - diyAudio
Go Back   Home > Forums > Commercial Sector > Manufacturers > GedLee

GedLee Home of the renown Geddes Loudspeakers

Reply
 
Thread Tools Search this Thread
Old 19th February 2010, 03:37 PM   #1
pjpoes is offline pjpoes  United States
diyAudio Member
 
Join Date: Jan 2005
Location: New York
Send a message via AIM to pjpoes
Default Geddes Bandpass Subs and the Multi-sub approach

My hope in starting this thread is to have a dedicated space within the Gedlee section on Dr. Gedde's own bandpass designed subs he is selling, as well as the multi-sub approach that has been talked about, and his room analysis and tuning.

I know I am not the first to receive Dr. Gedde's sub's, but I believe mine were among the first of his redesign. I have two of his bandpass subs which each have 3 flared ports placed strategically around the sub. Based on earlier inquiries of the 4 port design, I believe these three were likely also placed to maximize their effect based on the pressure zones within the enclosure.

The rest of my setup consists of 3 Abbey's for my front channels, 2 DIY surrounds based on an M&K tripole design I copied, and two more ultra low frequency subs. Amplification is quite excessive and more than enough to now overpower the room. Each of the broad band subs is recieving well in excess of 300 watts rms at very low distortion levels, and the power supply for each is capable of 1200 watts rms, and 2000 in peak output, meaning the could receive many times that if needed. It's far more power than they even need, as their efficiency and design gets more than loud enough on 1-200 watts. The ULF subs each are getting around 1000 watts from a pair of those Dayton plate amps. Obviously in the event that I ever turned things up enough to actually see those kind of numbers, I would likely trip a breaker real quick. The main speakers are each receiving 300 watts, and the surrounds get 200 watts. I also have two buttkickers hooked up to my LFE output.

Ok as for arrangement, I have each ULF in a corner placement, per Dr. Geddes recommendations in the past. While all the subs receive the same LFE+main feed for low frequencies, they each are tuned separately for cut-off. I have each of the ULF's starting around 50-60hz or so, with a 3rd order roll off, and the Broad bands are getting a single starting around 125hz on down with a 2nd order roll-off. I've not had time to take measurements properly, but I don't think the numeric values are accurate to the actual acoustic output. Take them for what they are. The Broad Band subs are placed each on sidewall positions, roughly opposite each other, but not in an orderly symmetric fashion. My room is far from symmetrical, which further adds to the odd arrangement. All the front main speakers are run full range, and the surrounds are cut-off at 110hz 2nd order.

When i first set everything up, the first thing I noticed in adding the new Geddes subs was that they had considerably more efficiency than my other subs, and between the extra 2 drivers and greater sensitivity, I needed to really turn things down. I initially had a lot of trouble getting a good balance of low frequencies utilizing the tools I had at my disposal. My BB sub amps didn't have gain controls of any sort, and I wasn't yet using the Behringer DCX-2496, this is mandatory. I simply could not balance things out without the ability to not only control each subs volume, but it's crossover and slope. While Dr. Geddes has suggested that adding additional subs, even sub-optimally is better than one, I would suggest that if not done properly, it will be far worse than what you started with. In addition, I don't think it's very easy to setup correctly, especially without measurement equipment. For those who have tried it and gone back to corner stacking of subs or just having one sub, I think you probably have not given this approach a fair shake, it really isn't that easy to do, and takes a lot of careful planning and patients.

Another problem I ran into initially was placement issues. I was finding that by having the ULF subs placed symmetrically in the room in opposite corners, I was getting a constructive interference around 50hz and destruction around 25hz and below. I tried simply switching the phase, but that actually had no impact on the destructive interference at all, only the upper part. What I had to do was move one of the subs slightly out of the corner, change where it's woofer was pointing (no idea why that made a difference), and adjust it's crossover. The other thing I had to do was remove the high pass filters built into the subamps (I didn't actually remove them, I set the value to a very low frequency, around 5hz for one and 10hz for the other). Each of these steps slowly restored things to more normal operation, hence why I think all of them contributed at some level.

The BB subs from Dr. Geddes seemed fine where I initially placed them, and that is where they have sat. While they aren't huge, nor are they as heavy as other subs, they are big enough I don't want to move them much. I've considered mounting them on wheels to allow moving them more easily, as well as give them a little character, but haven't bothered yet. It may be that Bandpass designs are less picky about location, or as I suspect, I picked good locations to start.

Ok so how did it impact the LF response of my room. First thing is that i've never had, in any of my home theater rooms, bass that I considered truly prodigious. You need to keep in mind that I've been using 2 subs in my room for years now, with very high excursion 12" drivers and tons of amplifier power. It wasn't so much that the bass wasn't there, it measured well, in the right parts of the room I could hear it, but I could never optimize it to my listening position. Even with external eq, the problem was that it required too much boosting to be as flat at my listening position. Apparently 2/3's back in the center of a room is a common place for destructive interference. I've set all my rooms up roughly the same, with the speakers within the first 1/3 of the room, my couch at the back of the 2nd 3rd, etc. By adding these two more subs, I am now able to have a much broader area of good prodigious bass. As I said earlier, it was way too much at first. Basically what I'm trying to say is that it wasn't just that the response became flatter at my listening position, it was the psychological effect that had on my listening. Suddenly my room sounded bigger and fuller in a way I simply couldn't achieve before.

As for how well it flattened out the room. I don't think it's a magic bullet, but I think it can create a larger sweet spot for bass. I still have area's of my room where there is clearly a huge difference in bass as compared with my primary seats. The seat located nearest a 45 degree corner has the smoothest bass in the room, always has, and it still does. At the opposite end of my room, nearest an opening to my kitchen, I now have a seat with a sizable suckout in the bass response. I've also noticed that simply changing the height of my head considerably, sitting on the floor vs on a chair vs standing up, the bass changes, with it being greatest nearest the floor. I took measurements, and sure enough, there is a few db difference, on average, across the bass as you progressively go higher. I would guess it's because I'm getting farther from a boundary. This was an important consideration since not all my chairs are the same height. In most dedicated theaters with risers, it would be even more different, and I would imagine even more consideration might be needed.

Remaining for me is taking advantage of Dr. Geddes room tuning. I've not yet recorded the test tones and sent them to him for analysis. I'm very busy with my thesis right now, and playing with my toys has to take 2nd chair for now. I'm hoping to get back into proper setup over the summer again. Also, I will be adding a lot more acoustic treatment to my room in the near future. I have found a dealer with a large lot of previous generation RPG acoustic treatments and will be buying up a bunch of them. It will allow me to add a bunch of bass dampening through the Modex Corner trap, as well as some diffussion/absorption with the Abfussor's. I've been working slowly on my own attempt at the dampened plate absorber such as RPG's Modex Plate, and hope to finish it in the summer. I will also finish up some more absorption for the front of the room. All of this should add considerably more lf dampening, more midband absorption up front, and a mixture of absorption at lf and diffussion at high frequencies in the rear of the room. I believe that is what most people consider ideal for a room.
  Reply With Quote
Old 20th February 2010, 10:57 PM   #2
diyAudio Member
 
Join Date: Oct 2008
Sounds like the beginning of an interesting thread. I built a pair of bandpass subs using the 12TBX100s myself, and have been pretty happy with the way they turned out (link here). I'm curious to learn how the response of Earl's subs might compare (have you tried measuring them with all of the other sources turned off?).

I was originally planning on adding a third, lower tuned sub, but am not sure how much it will really add at this point (I'm still toying with the idea, though). Your experience would probably be relevant in that regard.

I am interested to see how your room optimization goes. I found that I had to make extensive use of parametric notch filters to even out the response (across the 20-200 Hz range) in my room. There is still some ugly stuff in 200-350 Hz region that I can't really address with the limited PEQ filters available in my receiver (although it still sounds good). I suspect your setup will probably do better there, with three Abbeys instead of two.
  Reply With Quote
Old 21st February 2010, 01:14 AM   #3
pjpoes is offline pjpoes  United States
diyAudio Member
 
Join Date: Jan 2005
Location: New York
Send a message via AIM to pjpoes
Your sub's look very nice. Good use of sonotube. I've actually thought about ways to implement a bandpass into a sonotube enclosure like that. Do you think having the port so close to the bottom of the chamber creates any issues?

I do have measurements of Dr. Geddes bandpass subs without anything else. I will post them at some point later. They won't be free air measurements, It's still far too cold to take measurements outside, so I have to take measurements inside my room.

Without knowing more about your room's dimensions, angles, and arrangement, I can offer no help on that bass issue you have. However that seems to be right at the limit of the transition frequency, if not above it. In that case I would think that a better solution than the PEQ and multi-subs is going to be room treatment. Have you tried moving the speakers around at all? Adding absorption and dampening in that range is much easier than the places I need to add absorption and dampening, as the air velocity is still high enough that a large velocity absorber is relatively effective. I would also consider a large broad band diffuser/absorber such as the Abfussor from RPG. Good luck with the optimization of your room.
  Reply With Quote
Old 21st February 2010, 01:29 AM   #4
diyAudio Member
 
Join Date: Jul 2008
How does the placement of multiple subs affect the sound outisde of the main listening area? The reason I ask is that the kids rooms are above my listening area (ranch style house). I would love to even the bass out in my listening space, but I am concerned about adding more bass to do that. My gut feeling is that I could actually even the bass out at my listening position, but have less overall bass in the room by carefully positioning multiple subs as you have done. Right now, I seem to be getting more bass in adjacent rooms than I am in the listening position. Does anyone have any experience with this issue?

Thanks,
Markus
  Reply With Quote
Old 21st February 2010, 03:08 AM   #5
diyAudio Member
 
Join Date: Oct 2008
Quote:
Originally Posted by pjpoes View Post
Your sub's look very nice. Good use of sonotube. I've actually thought about ways to implement a bandpass into a sonotube enclosure like that. Do you think having the port so close to the bottom of the chamber creates any issues?

I do have measurements of Dr. Geddes bandpass subs without anything else. I will post them at some point later. They won't be free air measurements, It's still far too cold to take measurements outside, so I have to take measurements inside my room.

Without knowing more about your room's dimensions, angles, and arrangement, I can offer no help on that bass issue you have. However that seems to be right at the limit of the transition frequency, if not above it. In that case I would think that a better solution than the PEQ and multi-subs is going to be room treatment. Have you tried moving the speakers around at all? Adding absorption and dampening in that range is much easier than the places I need to add absorption and dampening, as the air velocity is still high enough that a large velocity absorber is relatively effective. I would also consider a large broad band diffuser/absorber such as the Abfussor from RPG. Good luck with the optimization of your room.
Thanks. The response of the subs goes a bit deeper than the model says it should, but I actually expected that based on some results posted by others (I wanted to accentuate the low end as much as possible). I haven't experienced any negatives yet - no obvious distortion or port noises, for example. I did move the subs around a bit initially, but was happy with the arrangement I ended up with in terms of how they integrated. Some info on my room can be found in this link. The location of the subs is shown in this updated flooplan. I have thought a bit about room treatment, but so far am just relying on typical furnishings to do the job.
  Reply With Quote
Old 21st February 2010, 11:27 PM   #6
pjpoes is offline pjpoes  United States
diyAudio Member
 
Join Date: Jan 2005
Location: New York
Send a message via AIM to pjpoes
I think Dr. Geddes discussion on sub placement would suggest that your placement of the sub in the front is a wasted location, since the front speakers can be run as full range, and thus act as the front lf sources. However, I'm sure your room is like mine, and ideal placement isn't always feasible within the room.

The speaker moving around was actually the front main speakers I was wondering about. Because the bass problem is so high in the bass range, I was thinking that it's mostly being contributed by the main speakers, not the subs.

You do have a tough room, again, much like mine. I'm not an acoustician, and the little I know would not be enough to make suggestions on how to improve your room. With diffussors and absorbers, then need to be placed in the right location. 200-300hz is high enough that I don't think typical corner bass absorber mounting is adequate. I think that is high enough, and your room seems long enough, that the wave could bounce around. I think very thick velocity absorbers are probably a good solution. I also think it's worth a shot, but unlike normal 1" or 2" absorbers which can be made to look like a picture, you need something bigger, like 4" or 6" thick. These wouldn't look like a picture anymore.
  Reply With Quote
Old 22nd February 2010, 03:41 AM   #7
diyAudio Member
 
Join Date: Oct 2008
Quote:
Originally Posted by pjpoes View Post
I think Dr. Geddes discussion on sub placement would suggest that your placement of the sub in the front is a wasted location, since the front speakers can be run as full range, and thus act as the front lf sources. However, I'm sure your room is like mine, and ideal placement isn't always feasible within the room.
Possibly, but I was able to get a flat response with the front sub there, and that's all that really counts in the end. I thought it made sense to have the subs located asymmetrically with respect to the room boundaries.

Quote:
Originally Posted by pjpoes View Post
The speaker moving around was actually the front main speakers I was wondering about. Because the bass problem is so high in the bass range, I was thinking that it's mostly being contributed by the main speakers, not the subs.
Right. I'm not sure what the origin of that bumpiness is (it is hard to do a realistic theoretical modal analysis because of the cathedral ceiling). I must be getting some additive effects of several higher order modes.

Quote:
Originally Posted by pjpoes View Post
You do have a tough room, again, much like mine. I'm not an acoustician, and the little I know would not be enough to make suggestions on how to improve your room. With diffussors and absorbers, then need to be placed in the right location. 200-300hz is high enough that I don't think typical corner bass absorber mounting is adequate. I think that is high enough, and your room seems long enough, that the wave could bounce around. I think very thick velocity absorbers are probably a good solution. I also think it's worth a shot, but unlike normal 1" or 2" absorbers which can be made to look like a picture, you need something bigger, like 4" or 6" thick. These wouldn't look like a picture anymore.
I may experiment with adding some absorption to the front wall. The nice thing about notch filters is that they are very precise, affecting only the frequencies at which they are targeted - and they don't make any dust!. I am tempted to try using the extra channels in my sub amp for the mains together with the DCX to provide the additional filters needed just as an experiment to see if I can smooth things out that way (and to see if that sounds better).

- Doug
  Reply With Quote
Old 22nd February 2010, 04:26 AM   #8
freddi is offline freddi  United States
diyAudio Member
 
freddi's Avatar
 
Join Date: Aug 2005
four small vents are shown on Dr. Geddes bandpass at his website - superficially it looks like it could "chuff" and compress but he must have done it for a reason. What is that composite vent doing under higher cone excursions? Is there any LC network in that model?
  Reply With Quote
Old 22nd February 2010, 09:46 AM   #9
diyAudio Member
 
Join Date: Jun 2004
Location: Praha
Smaller vents allow higher air speed because of Reynolds number Re is directly proportional to diameter and velocity. If you use four half diameter ports you can allow double velocity (within margins).
__________________
Tomas
  Reply With Quote
Old 22nd February 2010, 05:50 PM   #10
freddi is offline freddi  United States
diyAudio Member
 
freddi's Avatar
 
Join Date: Aug 2005
thanks Tomas - how would you distribute four vents in a typical Karlson coupler rear chamber? - where would you put them? tuning is generally pretty high and the front aperture is the final vent
http://img406.imageshack.us/img406/7577/insert1lc8.jpg
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 04:35 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2