AudioNervosa
Systemic Development => Speakers => Topic started by: Werd on July 02, 2015, 02:55:00 PM
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Been using a dual sub configuration lately. I've got one Nola sub straight right in front of me. I am using the second along the wall behind me and over my left shoulder.
I am beginning to understand the problems with subs and placements. I think I might have solved a problem in my room using dual subs. In fact it sounds pretty snazzy.
The natural tendency is to put both subs on the ground. This is false and bad practice IMO. I've never been able to make it work. Subs on the ground are used for ambience. The problem with placing subs on the ground is it creates to many nodes by moving bass along the ground. Even if they are are used out of phase.
Another reason for using a sub is to enhance resolution in the bottom end. The way to do that is by applying a subwoofer lifted off the ground. A good height is the distance between the woofers on your mains. Or the distance between the midrange and woofer. The thing that isn't discussed ever with subs is Subs can be used to beam a signal much like a midrange and tweeter. When you take the sub off the ground it begins to be come less ambient and takes on more beam signal. If this wasn't possible there would be no bass resolution in the sweet spot from your mains. Mains woofers can direct bass and so can subs off the ground.
So I got a sub lifted in the sweet spot and I got a sub behind
me creating ambient room info.
So subs off the ground for resolution and subs on the ground for ambience. That's in my room anyways. :thumb:
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Also if I am using only one sub and I want ambient info I would use it behind me only. If the sub is going anywhere near the sweet spot it comes off the ground for resolution. Trying to retrieve ambient room info with a sub near the mains in front of me is really hard to do. Sub placement on the front wall is for resolution only and off the ground. That's been my findings. :thumb:
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I too raised my subwoofer off the floor with a sub stand that I built from 2" thick oak. I placed the stand next to my main Lorelei with the down firing woofer 30" off the floor, just below my midwoofer. The sub blends in nicely and just disappears.
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Exactly :thumb:
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My downfiring REL Strata III sub is turned upside down, so it is now upfiring and I have a heavy board on those upturned feet with some weight on top of it. It is placed right in line with the speakers, away from the wall.
Much better resolution and easier to blend, though it had to be turned up a wee bit now.
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so mine is pretty much on the floor. How high should I raise it.
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so mine is pretty much on the floor. How high should I raise it.
A little higher than the bottom driver on your mains.
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Actually I would go as high as you can go. Make sure the woofer is facing you.
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So as it turns out two subs and their placement pretty much dictates where the placement of the mains. IME in my room not vice veraa.
Getting the front sub off the floor for resolution requires the front sub to be off from centre from a point down from the middle of the wall, which then requires a main speaker on each side of the sub. This puts the sweetspot not in the center perfectly in the room but in a corner. While still having the mains run parallel with the walls . Basically using a corner of a room with the rest of the room open behind and to the left (in my situation) wide open.
What this does is sets up the use for the second sub placed on the floor behind my left shoulder along the side wall. It makes the room sound larger from a length and width persepective which i really like for ambience.
Putting a your speakers equally distanced with a 2/3 room placement (concert bowl) really screws with the ambience of the rear sub and reaolution of the forward sub. I will never do that again placing speakers with equal distance from side walls using subs.
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Si Senor I have been recommending that for years. To be precise get the center line of the woofer off the floor to 20% of floor to ceiling height. Meaning if clg. is 8 ft then 20% of that is the height.
Got the recco from ASC and NEVER turned back. Killer.
charles
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You don't want the front sub (resolution) to be at the same height as the bass drivers on your mains. If you can avoid it.
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Moving the sub from the floor/wall boundary to the somewhere on the wall itself changes the room boundary loading on the sub and may also change the room gain and the pattern of standing waves in the room resulting in a perceived improvement in the quality of the bass.
A frequency below 125Hz that was over emphasized at the listening position could be reduced in amplitude and the bass would seem to have better definition when the sub is relocated.
You have changed the sub-woofer loading from roughly a Quarter Space loading to approximately a Half Space condition.
See link, https://www.trueaudio.com/st_spcs1.htm
More useful information
https://www.trueaudio.com/st_index.htm
Here is the approach I use implemented with a sub in the rear and a sub on the side wall. Digital delay is provided by a QSC DSP 30 Processor
See Link, http://vbn.aau.dk/files/12831869/AC-phd.pdf
Scotty
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I put your method and it's science on my book marks. :thumb:
The problem i have with the science (and its good) they don't identify the problem and what it means to the system sound. They don't understand the problems. I have yet to read a paper that acknowledges putting subs on the floor is used to enhance the ambience. Which basically means loading the room. Putting subs on the floor to improve resolution is a failed concept imo. No subs should be on the floor in front of you. I am absolutley certain of that. I have yet to read that anywhere. So unless there is anecdotes to clarify -using subs on the floor and this is the science how to do that. This is only half information. They need to explain what loading does and how it creates a room and enhances ambient effects of the soundstage to the person listening to it.That is never said.
So this is what happens. People will do what they read and they will likely include a forward sub and then they are back to square one. Because that sub on the floor in front is the bane of all hi-fi. It wrecks the resolution of the soundstage and it cancels ambient nodes behind you. And that is if they do it right. They are better off fucking it up and getting lucky with a sub placement that at least gives you somewhat decent rear room loading.
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There is one thing that i've noticed regarding room treatments and sub placements papers. etc. There is inheritant need to fix a bad room with subs and room treatments. That is audio sales and what they have done to audio playback to make a pitch. My personal imo is to embrace the nodes and room peculiars. Its those things that get the toe tapping. You start cancelling nodes and using defusers you have lost what is great about the room. You have lost the beats. It becomes more sterile.
That is my personal opinion and it is not necessarily correct. People have their own way of identifying to their systems and that is cool. I have no problem with that. But if people come over and your system doesn't make them want to dance or groove, then there is a problem. You should have excellent sweetspot resolution with a good groove that makes people want to party.
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Another interesting thing i have noticed. It also seperates the 2 goals apart. That being resolution and ambience in subs. When dialing in cross overs use the highest crossover point on your sub for resolution then start moving back down to a lower crossover point. I have found if the sub is off the floor anything over 100 to 200 hz cross over is where you will probably end up.
If you are using a sub for anbience start at the lowest (probably around 40hz) and then move up to get a good sensation of the room. Mine is much off 40hz. This is IME.
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Werd, have you tried putting the sub in the rear 180 degrees out of phase with the sub in the front?
Scotty
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Werd, have you tried putting the sub in the rear 180 degrees out of phase with the sub in the front?
Scotty
Yes, i had it that way when i was running both L and R channels into the front (on the floor) and using the L - line-out on the front sub feeding the back sub. It still was bloated in the soundstage. It wasnt until i got the idea to put the sub on my amp stand. Using Modern Jazz Quartets "Softly, In as a Morning Sunrise" - took about 10 secs of walking bass to realize that was it. The sub walked with the mains. I knew that was it. You can't get that with bass moving along the floor. I tried for years. You need the driver in the soundstage and not the floor.
Right now with my sub on the stand i am phased into the left main with the sub and trying to get rear sub phased into the right main driver. I think i got it as good as i am going to get it with what i can do in the back of the sub.
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You might try using the summed input on both front and rear subs. If there are any phase differences in the bass frequencies between the left and the right channels you will have incomplete cancellation between the front and rear subs output. Also without the capability to delay the rear subs output relative to the front sub the rear sub has to be further away from the listening position than the front sub or the phase cancellation will not occur at the correct point in space.
I have the rear sub in my system setup to cancel out the bass wave from the front about 3ft. before it impacts the rear wall behind me. The side sub to my right is canceling out the bass wave from the right channel so to speak about 1ft. before it hits the side wall.
My remaining problem is that the both subs quit before the front mains. The front speakers are 3db down at 16Hz and the subs are quitting in the mid 30Hz. region. The one in the rear has to be re tuned so that it is flat to 20Hz. The right sub can't be altered and might be good to 32Hz. I will have to build or buy a sub for that location that is flat to 20Hz to optimize the system. It is still quite effective on 75% of the music I play with better bass definition and much better imaging and a better recreation of the spatial qualities in the recording. I might be getting 65% of what could be achieved with this approach due to my L shaped listening space and the frequency response limitations of the subs.
The key to making this work lies in being able to use a processor to properly delay the rear subs output.
Scotty
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You might try using the summed input on both front and rear subs. If there are any phase differences in the bass frequencies between the left and the right channels you will have incomplete cancellation between the front and rear subs output. Also without the capability to delay the rear subs output relative to the front sub the rear sub has to be further away from the listening position than the front sub or the phase cancellation will not occur at the correct point in space.
I have the rear sub in my system setup to cancel out the bass wave from the front about 3ft. before it impacts the rear wall behind me. The side sub to my right is canceling out the bass wave from the right channel so to speak about 1ft. before it hits the side wall.
My remaining problem is that the both subs quit before the front mains. The front speakers are 3db down at 16Hz and the subs are quitting in the mid 30Hz. region. The one in the rear has to be re tuned so that it is flat to 20Hz. The right sub can't be altered and might be good to 32Hz. I will have to build or buy a sub for that location that is flat to 20Hz to optimize the system. It is still quite effective on 75% of the music I play with better bass definition and much better imaging and a better recreation of the spatial qualities in the recording. I might be getting 65% of what could be achieved with this approach due to my L shaped listening space and the frequency response limitations of the subs.
The key to making this work lies in being able to use a processor to properly delay the rear subs output.
Scotty
My goal is to try and phase in the rear with the right channel. In practice i end up adjusting back into the forward sub. I have a bad habit of reaching behind my sub looking for levels and adjusting phase by accident and i never look at the adjustment. So it's easier to phase in with the forward sub. Here is a tip, If you are trying to ohase in two subs. Lift one sub off the ground. :lol: The phase characteristics of the two subs are far easier to point out. If we lived closer i would let you use my Solid tech amp stand on one on your subs. That thing is as good for subs as is amps.
I can see how a delay would tighten up the room. Out of curiosity how do your subs measure for freq response with out the QSC in the signal path? Are you running balanced or unbalanced?
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I purchased some non-audiophile approved ICs and re-terminated the ends with XLRs, everything is unbalanced in my system. I grounded pin 3 at the input of the DSP 30 and I didn't connect pin 3 on the output.
While the QSC DSP 30 can be used as a parametric equalizer to boost or cut any frequency, I am using it for delay and have set up a 24/oct. Bessel filter at 125Hz to augment the 12dB/oct. filters built into the plate amps. I have set the knobs fully clockwise on both plate amps giving a filter pole of around 160Hz.
The subs are only being used to cancel out the bass wavefront headed towards their locations before they hit the wall and a reflection of the bass wavefront occurs which is the cause standing wave zones and bass response irregularities. Because the bass energy that reaches the subs locations has already been reduced by absorption by and transmission through the other four walls of the room I can operate these subs at a much lower SPL than the front speakers and still get the job done. You really can't hear that the subs are playing until you are standing right on top of them.
The link I gave you describes how this approach works.
With the current setup I have well defined bass in the front of the room and at the listening position, the standing wave zones at the rear and side walls of the room have disappeared.
I basically have active room treatment below 125Hz., without needing to resort passive room treatments for bass response problems.
Scotty
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Very cool Scotty
How do you mount the drivers on the walls, in cabinets or in wall? Sealed? Height?
How do you decide how many subs are needed for a given front/rear wall size?
Is there a spot in the rear where you can stand and hear the big bass cancellation dip, or does the location vary with freq?
I'm doing a new room this winter, thinking about trying this.
Thanks
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I purchased some non-audiophile approved ICs and re-terminated the ends with XLRs, everything is unbalanced in my system. I grounded pin 3 at the input of the DSP 30 and I didn't connect pin 3 on the output.
While the QSC DSP 30 can be used as a parametric equalizer to boost or cut any frequency, I am using it for delay and have set up a 24/oct. Bessel filter at 125Hz to augment the 12dB/oct. filters built into the plate amps. I have set the knobs fully clockwise on both plate amps giving a filter pole of around 160Hz.
The subs are only being used to cancel out the bass wavefront headed towards their locations before they hit the wall and a reflection of the bass wavefront occurs which is the cause standing wave zones and bass response irregularities. Because the bass energy that reaches the subs locations has already been reduced by absorption by and transmission through the other four walls of the room I can operate these subs at a much lower SPL than the front speakers and still get the job done. You really can't hear that the subs are playing until you are standing right on top of them.
The link I gave you describes how this approach works.
With the current setup I have well defined bass in the front of the room and at the listening position, the standing wave zones at the rear and side walls of the room have disappeared.
I basically have active room treatment below 125Hz., without needing to resort passive room treatments for bass response problems.
Scotty
Are you using those Tetron GS's. :lol:. Now i know why you aren't using forward subs. You are basically doing what i am doing but you've decided to cancel the nodes. Where i want to load the rear. That is the only difference. The only disadvantage i can see with the Reimer's are the top woofers come forward into the room pretty far. Where i can sit my sub a little closer to the front wall. Maybe not a disadvantage but i can see why you don't want to load the back of the room.
I use a couple of 244 panels standing on the floor perpendicular to the front wall. They stop the bass pretty good. Well that and the raised sub. So i have the sweetspot resolution with nice ambient loading in the rear. There isn't much sub travel towards the back of the room so i can load it up and i really don't hear the nasty buildup or boom anywhere.
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Hi Rich, every thing I am doing is based on the information contained in Adrian Celestinos' doctoral thesis.
http://vbn.aau.dk/files/12831869/AC-phd.pdf
The bass produced by the stereo system escapes the room through all six sides of the cube assuming an above ground structure. It may also be absorbed by the room's contents. As a result of this there is no need for the rear woofers SPL output to match that produced the front speakers. The only requirement is that the bass extension of the rear sub-woofers must equal that of the front system. Theoretically, for the front system to produce a planer bass wavefront, many woofers must be recess mounted in the front wall and like wise the cancellation of planer wave before it hits the rear wall theoretically requires another wall full of woofers.
Celestinos' work applies the idea to real rooms in a practical manner. Experimental data shows that two woofers located in the front at the floor/wall boundary and equidistant from the corners will generate a close approximation of a planer wavefront at frequencies below 125Hz. Two subs placed on the rear wall in the same locations as the subs in the front of the room and operated 180 degrees out of phase with the appropriate delay will be enough to create a matching opposing wavefront and efficiently cancel out the bass wavefront before it impacts the rear wall and reflects causing standing waves. In fact according to Celestinos measurements plus and minus 3dB throughout the listening room is possible. This approach can be applied to any room whether rectangular or a perfect cube. The exception is the L shaped room, which of course is what I have to deal with, and he discusses this situation at the end of his paper.
What is not discussed is the effect on the imaging and the improvement in the size of acoustic space created by the reduction in standing wave zones between the listener and the front speaker system. When the standing waves are not present, the recreated acoustic space expands towards the listener to the point that complete envelopment occurs, image precision improves as does depth layering and separation in space of instruments in the sound stage.
I first heard of the idea of using an 180 degree out of phase sub-woofer in the rear of the room to cancel out standing waves from my friend Stan Warren over 25years ago. His idea involved a sub-woofer with phase correcting servo control like the one he designed for RH Labs sub-woofers and delay provided by an analogue bucket brigade line, which was the only available technology at the time, to implement the necessary delay of the signal to the rear woofer.
His explanation for the perceived improvement in sound staging was that the bass wavelengths act as a carrier wave for the the mid and high frequencies. When the complex phase relationships that exist in the Mid/Hf signal that is carried by the bass are degraded or destroyed by the presence standing waves in the room, then the spaciousness and precision of the imaging inherent program material will also be lost.
This can be demonstrated quite dramatically by simply turning off the out of phase woofers in the rear of the room. The re-created space and 3 dimensional image will collapse towards the front of the room and be trapped on the front wall.
When Stan approached lawyer friend with the idea of patenting this concept he was told to put his plans in a paper bag and place them on a top shelf of his closet. The reason being that he could not afford to defend the patent in court when infringing products appeared based on his published patent.
When I told Stan about Celistinos' work he said that as simple an idea as it was, it was remarkable that someone didn't discover and put the concept into to the test before now, especially with the ready availability of inexpensive digital chips to implement the delay with.
Even one woofer in the rear with delayed output is sufficient to realize a marked improvement in spaciousness and bass accuracy even if it does not result in + or - 3dB bass response in the entire listening room.
Paper C which starts on page 50 of Celestinos' work discusses mesurements of the equalization system in the real rooms.
Paper D starting on page 70 outlines his Controlled Acoustically Bass System(CABS), A method to achieve uniform sound field distribution at low frequencies in rectangular rooms.
This is where the rubber meets the road. How CABS is implemented and the measured results in two real world rooms are presented. The science underlying the approach easily understandable as explained in this part of the thesis. The graphs are very valuable.
Scotty
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I glanced through the link you provided earlier, but have not read it through yet. Thanks for the summary Scotty.
The room I am considering moving to will have slanted ceilings on the side walls parallel to the listening axis. There is a flat center section, of same width as the side ceilings, like an octagon. Will these slanted walls prevent success of CABS method? Must the room be a traditional box shape with parallel and equal size floor/ceiling?
I'll read Castelinos paper today. Thanks. I have seen other articles about CABS method. I will try to find my links to them.
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Rich, aside from the design of ceiling what do the rooms dimensions consist of?
Scotty
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11' - 6" wide
15' - 7" long
8' - 0" tall
Tim Welti comparing CABS (DBA) to Swarm:
http://www.whatsbestforum.com/showthread.php?3320-Comparison-of-Double-Bass-Array-to-Sound-Field-Management-Overview
What I'd love to do is infinite baffle CABS. But the room might be too small for that. I'd need a false wall in the front, and build double walls in the back to put the IB drivers on front and back walls and channel their exhaust to the side volumes. The center ceiling and side walls are attic space, so it would be far easier to put the IBs there, but can't do the cancellation.
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I"m no expert but I can't see any reason that CABS would not be very effective in that room. You have a rectangular floor plan irregardless of the shape of the ceiling. You will probably find RPG type diffusers mounted on the angled portion of the ceiling helpful. That angle may re-direct sound from the speakers back down into the listening area more effectively than a flat ceiling with a regular ceiling/wall boundary angle of 90degrees. The reflections may occur closer in time to the direct radiated sound from the loudspeaker. This would be entirely dependent on the directivity of the loudspeaker. An REW measurement might show a different comb-filter pattern in a room like this compared to a conventionally shaped room.
Scotty
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Agreed on all points. 90 deg corner reflections are very damaging to midrange tone, but I'm glad to hear that you think the CABS might work.
In this relatively small room 1300 cu ft, do you think I'd benefit from additional woofers wall mounted up higher, or do you think a pair of floor subs front and back is adequate?
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From Paper D, CABS does not need wall mounted woofers to give reasonable results based on the measurements done in the two rooms referenced in the paper. Not installing real 20Hz capable subs in the walls gets you away from structural resonances due to the wall vibrating.
You should be golden with four subs. If 10in. subs would reach low enough to meet your criteria, you would have more than enough bass energy in a room with of that cubic footage.
Scotty
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OK, thanks much Scotty!
I have some reading to do!
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Here are links to QSC DSP30 processors for sale on ebay.
http://www.ebay.com/itm/QSC-Audio-DSP-30-Computer-Configurable-Digital-Signal-Processor-/171904588902?hash=item28064fdc66
http://www.ebay.com/itm/QSC-DSP-30-Audio-Signal-Processor-Compressor-Limiter-Crossover-Parametric-EQ-/262009621088?hash=item3d00fd8a60
http://www.ebay.com/itm/QSC-DSP-30-Computer-Configurable-Digital-Signal-Processor-/321838983041?hash=item4aef192781
This model has been discontinued by QSC and alternative processors are more expensive.
There are also any number of inexpensive prosound digital amps that have configurable digital delay built into them as part of their feature set.
What you are looking for is delay capability between 1millisec and 30millisecs. This gives you the ability to put the sub-woofer immediately behind the listening position or up to around 28ft. behind you. The DSP 30 has sub millisec delay capability and up almost 1000ft. of delay for use in large venues.
Shameless plug for HAL's MS-3 Music Server and dspMusik DSP DAC. The dspMusik DSP DAC has the necessary programmable delay capability.
I use a HAL MS-2 Music Server in my system to good effect.
Scotty
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Here is from the Cabs paper.
"The advantages of CABS are:
• More even sound level distribution below 100 Hz is achieved throughout the room. • The effect of the room resonances in the reproduced sound is decreased considerably. • Only simple signal processing is needed"
End quote
The problem with Cabs appears to be in the first advantage. The advantage of more even sound distribution below 100hz is not a desirable effect at the source when using subwoofers. Only because that statement exists in a pure theoretical environment and not practically. It never ends up like that. If it did cabs would function in a room with any dimension. The system that works in every environment and room dimension is sub placements - raised- for resolution (basically over 100hz) and as the waves move out subs for ambience. Subs on the floor to radiate the room boundaries. Phase or out of phase what ever floats your boat.
Think of sitting in front of a bass drum. The kick gives you a sensation above the 100hz range. Raised subs will form the sound of the kick (resolution) and will also give you a threshold of low frequency slam. Since raising subs doesnt eliminate ambience but only reduces it. As the wave radiates out, the room becomes the major factor for ambience.
What becomes apparent in the sweetspot is the resolution of the raised sub and the loading of the sub at room boundaries or the ambience adjustable by sub volume. Cabs is taking a problem easier solved and making it more difficult and it only works in certain environments. Though Its got good info about how low frequencies move around the room.
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If you are using a standing wave zone that is located on top of your listening position to compensate for program material that may be lacking in bass impact then you wouldn't necessarily want an effective solution to standing wave problems.
CABS actually does work with any room having six walls regardless of their dimensions, it even works to a lesser degree in the an L shaped which is covered in Chapter E starting on page 97. This is the shape of room that I have to contend with.
Scotty
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Hey Scotty, what do you think about these driver placements on my front/rear walls?
They all follow the CABS placement rules pretty closely... except for the 3 driver version, where the top driver is centered between where the two top drivers would be if the room was cuboid, but the distance between the 3 drivers is not constant like it should.
I'm not sure if distance to the wall is more important than driver spacing, and with my wall shape one of them has to give. However, the one with 2 drivers per wall seems to fit all the rules despite the slant ceiling.
Please don't advise the one with 14 drivers per wall. ;) I only did that because I like playing with CAD, although it would be fun to hear it. :D
I think the CABS concept of wall reflections is key to making compromises like this work, but I'm still trying to wrap my head around that. The reflection idea suggests that there will be sound bouncing back and forth across the side walls and floor/ceiling while traveling toward the rear, but I'm not sure why that is significant. It seems to me that the only thing that matters is having the radius of individual wavefronts large enough to they are flat enough to merge into one relatively flat wave. So that the bumpiness of the wave front due to multiple drivers is of small enough deviation to be small error during the cancellation. Due to wall reflections and arbitrary room length the phase bumps in the wavefront will not stay in the same location relative to the pitching and catching walls throughout the trip. So the wave has to be flat enough to allow the dips to migrate randomly across the planar wave and still be flat enough to be cancelled with decent efficiency. Make sense to you?
So then what happens if there is only one driver in the center of each wall? How do you calculate Fc? Would CABS even work? The wavefront will be hemispherical coming from one driver, so to flatten it sufficiently to be cancellable at the rear the 1/2 wavelength should be largest wall dimension? In my case Fc = 49Hz. But my fundamental room modes are 36, 49, 70. So CABS with Fc = 49Hz can't prevent the floor ceiling mode at 70Hz. So I need at least 2 drivers per wall.
Considering the small window in the center of the front wall and the need for a entry door on the rear wall, (and my driver and amp budget) I think my best option is 2 drivers per wall.
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If you are using a standing wave zone that is located on top of your listening position to compensate for program material that may be lacking in bass impact then you wouldn't necessarily want an effective solution to standing wave problems.
CABS actually does work with any room having six walls regardless of their dimensions, it even works to a lesser degree in the an L shaped which is covered in Chapter E starting on page 97. This is the shape of room that I have to contend with.
Scotty
What you are saying is buy the right speaker for the room. That will solve the bass problems.
The entire cab model including examples and math are all based in the time domain. Multiple woofers vertical mounted (raised) on your mains operate in the time domain. They operate like this because of floor bounce. If you have a main speaker with at least 2 woofers and a sub that operates at a different height than the mains. There will be no standing waves in the sweetspot. Unless you are right up against a wall.
Then who knows. You can correct any malfunction by using subs out of phase in the back. But the entire paper is based on 4 subs all operating at the same height in a rectangular room. Its all there to read. The reason that the time domain problem exists is because all the drivers are at the same height on the floor! To eliminate this problem put the drivers at different heights especially the subs. The more i read that paper the more it becomes clear they reversed the engineered the problem. They look at a rectangular room with 4 subs and what it looks like with no time sucks or humps. Then they reversed engineered the math back to the problem. That helps nobody because it isn't realistic. They even claim a major drawback is furniture. It diminishes the effect.
Anyways i like the delay on subs but you won't need it if you have a raised forward sub.
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I don't know if you've had any experiences with wave tanks in high school or college physics classes but a planer wavefront can simulated in a wave tank. Any frequency propagated in the wave tank that has the a wavelength with a close harmonic relationship to the length of the tank will nicely demonstrate the concept of standing waves. It will be seen that there are no reflections from the sidewalls of the tank at all. A planar wave can also be simulated in your common garden variety bathtub by moving your palm to and fro until your find the frequency that causes all of the water in the tube to move as single mass.
If you propagate a mostly planer wavefront in the bass frequencies from the front of the room the sidewall reflections are sufficiently suppressed to the point that you can deal the standing waves that are function of the rooms' longitudinal dimension by generating anti-phase counter planer wave from the back of the room.
I think your 2 drivers per wall solution is the most economical approach that will work in your room. If the subs are not mounted in the wall you could play with their height off the floor to achieve the best results.
The operational wavelengths are so long relative to the box mounted woofers location away from the plane of the wall that I think that any negative from this is out weighed by the placement flexibility gained.
From Figure 45 on page 91 its obvious the having the ability to relocate the sub-woofers vertically and horizontally to fine tune the cumulative spectral decay results is a useful option to have. REW RT 60 measurements would allow one to zero in on the optimum vertical locations for the subs on both front and rear walls.
Even with a considerably compromised room shape and lacking the ability to put the subs in the best location in either
the horizontal or vertical planes on the walls I am still very happy with limited results I have in this situation. The bass is much less audible at the end of the hallway where the master bedroom is situated and is a godsend for late night listening sessions.
The slick thing is that one can boost the bass via EQ in the playback software when more impact is desired and the corresponding boost will show up in the rear channels to cancel it out with less likelihood unplanned boominess.
Scotty
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werd, if you were to download the free REW program and take RT60 measurements you will find ample evidence that the standing wave problem exists in any untreated average listening room regardless of where the woofers are located in the vertical plane. It looks like the CABS approach can generate RT60 measurement results similar to that of a well treated room.
This is the reason I am enthusiastic about it, as complete acoustic treatment of my living room to solve the bass settling time problems it has are both physically and financially out of the question.
While the Swarm approach can yield similar results, I already have the equivalent of 4 subs in the front of the room and no way to afford or room to place the multiple small subs in my room that a using Swarm would require.
Scotty
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Cool, thanks Scotty.
What does he mean by CABS .2.2 vs CABS .2.0?
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It looks like these labels apply to two different vertical locations on the subs, the second one looks like it give a little better cumulative spectral decay results, ie faster settling time/RT 60 numbers.
Scotty
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IMO most of the issues of sub placement is most subs are in a box. Open baffle is a huge advantage over seled or ported subs IMO.
Add servo drivers with a plate amp designed for such will change your sub opinions.
We recently showed at Capitol Audiofest using three per side servo open baffle subs with great success.
Using servo controlled drivers eliminates any box coloration from the back wave. All drivers are designed to act as one with a computer controlled coil on the drivers.
Tight articulated bass wit zero boom or or overhang. tone with harmonics. One is easily able to discern foot pedal action on organ as never heard before.
After hearing open baffle design done right it would be impossible for me to listen to a sub in a box.
charles
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OB subs will be my next venture if i ever get to it. :thumb:
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You should get to it , you will NOT be disappointed.
charles
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You should get to it , you will NOT be disappointed.
charles
Charles,
Which subs did you use?
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Inner Sound SI 300 servo subs, powered by Rythmic plate amps made to our spec.
The servo controlled GR Research drivers are a dipole OB design. Down to 16HZ at 3db.
Fast, accurate with tone , no bloat or boom. just clean articulated bass.
charles
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Inner Sound SI 300 servo subs, powered by Rythmic plate amps made to our spec.
The servo controlled GR Research drivers are a dipole OB design. Down to 16HZ at 3db.
Fast, accurate with tone , no bloat or boom. just clean articulated bass.
charles
~8K - Ouch
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Having heard the open baffle subs at CAF, I would say that a servoed switching AMP like the Rhythmic AMP is a must. High SPL output at bass frequencies requires a lot of cone area. The three 12in woofers per side were a good match for the hotel room and the resulting bass had excellent definition and extension. Acoustic bass and drums had a real life quality and impact that I have not encountered before.
Scotty
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@Scotty
No doubt about the servo sub. I own one lonely 12" sealed GR Research sub and I am exploring options to add open baffle sub in similar 2 or 3 woofer fashion. Will have to explore DIY options (get someone to cut the flat pack and I get to finish them off)
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shadowlight,
Ruben is building OB H-Frame MDF flat packs for sale for both the 8" and 12" servo subs for the GR-Research drivers and Rythmik amps. Not sure of price with shipping, but he will give you a quote. If you need the tools, he can supply a set as well for an upcharge. I did not have the clamps, so got him to get me a set for my builds, so I bought them.
The 3x12" OB H-Frames at CAF were driven by the Rythmik PEQ370 Class AB amps. That is the amp they were developed for. Same amps I have for the Super-V's. Brian makes bigger servo amps like the HX800.
I have built both the 3x8" H-Frames and the 1x12" modules and it was pretty easy the way Ruben marked them, with the tools and glue he supplied.
Hope that gives you something to investigate.
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Thx Rich. I will reach out to him when I am ready and will also look to see if local cabinet maker can help.
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A different article about DBA, translated by Google from German:
http://translate.google.com/translate?prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.poisonnuke.de%2Findex.php%3Faction%3DRaumakustik&sl=de&tl=en&history_state0=
This is interesting to me, because he says you need minimum 4 drivers per array to make a planar wave. Using only two at mid-height or two on the floor per array will make cylindrical wave with poor cancellation efficiency and will also create floor ceiling modes. But I can see how the effect could still be positive because while it may not avoid as many modes the wave is roughly planar at ear height because there is a cylindrical wave formed with two horizontally spaced drivers. And side to side wall modes are still avoided.
This picture is interesting.
(http://www.poisonnuke.de/Grafiken/SBA.JPG)
It shows how the individual spherical waves eventually combine to become planar waves. But this requires some distance. At zero distance, along the front wall, there are only standing waves. At several times the distance of the driver to the wall, and with help from side wall reflections (shown in red,) the spherical waves finally combine to become planar. This is true of front and rear arrays in DBA. I guess the planar formation is about 3x the distance of driver to wall, but planarity increases with distance as the waves naturally flatten with increasing radius.
It's important to note that waves do not have any replationship of affinity to each other. They don't know each other is there. They pass right through each other with no affect on each other. We observe the addition and subtraction at specific times and locations as modes, but there is no real interaction. It is the reflections from the sidewalls and floor ceiling that add more waves to the tank which add to the desired longitudinal wave to flatten out the observed planar wave faster than a single driver would by increasing wave radius alone. The planar wave in this case is not really a single wave, but a collection of spherical waves and wall reflections of spherical waves combining to make the appearance of a single wave.
I think this is why a true cuboid is so important to the planar wave phenomena. Angled ceiling will mess up the reflections to some degree. But I think this will have less affect at lower frequencies with more drivers in the array, since there will be more reflections also.
You want the listening seat and the cancel zone to be within the planar wave zone. There must be some distance between listening seat and the cancel zone where bass respone falls off. So a room needs to have enough length to allow the planar zone to be long enough.
I read in one description (maybe FoLLgoTT on avsforum? (http://www.avsforum.com/forum/155-diy-speakers-subs/837744-double-bass-array-dba-modern-bass-concept.html)) that the number of drivers in the array affects the high frequency extension of the array because it allows shorter wavelengths to form planar waves coherently. But in light of the need for minimum length of the planar zone, adding more drivers to the array also shortens the distance required to form a planar wave at any freq. So in small rooms, increasing the number of drivers in the array will also create longer "planar zone."
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There are certainly optimal conditions for achieving very high cancellation of standing waves in the listening room and then there are going to be situations, while sub-optimal, where it is still worth implementing a CABS array as it will still likely be quite effective and will have the added benefit of much improved imaging and flatter bass response. The wave from the front doesn't have to cancelled out until the last millisecond.
It's also hard to argue with Celestinos real world measurements.
I wonder if the other people playing around with this concept are aware of his thesis.
Scotty
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Yeah you're right Scotty. Thanks
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There are certainly optimal conditions for achieving very high cancellation of standing waves in the listening room and then there are going to be situations, while sub-optimal, where it is still worth implementing a CABS array as it will still likely be quite effective and will have the added benefit of much improved imaging and flatter bass response. The wave from the front doesn't have to cancelled out until the last millisecond.
It's also hard to argue with Celestinos real world measurements.
I wonder if the other people playing around with this concept are aware of his thesis.
Scotty
The problem with that Cabs paper. Like i said, I like it for the theoritcal numbering. The problem with it (people seem to overlook) are the drawbacks listed at the end of the paper. The one i have is the diminishing returns in a furnished room. That paper which uses a smorgasboard of numbers and figures to show a problem solved. When it comes to it's own stated drawback... Nothing. No examples or anything just a statement. As a work of science with the intent to apply practical methods would then apply and show work to the diminishing returns. Its not there so what does that tell you. Its a theoretical work that isn't meant for house hold use or the diminishing returns are so bad that it destroys its own paper. Which one is it?
I only say this because you mentioned it. That other paper won't load on my ipad :cry:
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Very low frequency wavelengths will not be diffracted by room furniture because furniture bits are too small to affect the long wavelengths. With only two subs per array, the bandwidth of the array will not be high into the upper bass range where furniture will be a problem, methinks. Try it and see...
It is a valuable resource because it gives practical advice and provides measurements proving the concept works. Of course one can find faults in anything.
It was his PhD dissertation afterall so you can't expect him to hype the problems. ;)
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Very low frequency wavelengths will not be diffracted by room furniture because furniture bits are too small to affect the long wavelengths. With only two subs per array, the bandwidth of the array will not be high into the upper bass range where furniture will be a problem, methinks. Try it and see...
It is a valuable resource because it gives practical advice and provides measurements proving the concept works. Of course one can find faults in anything.
It was his PhD dissertation afterall so you can't expect him to hype the problems. ;)
The applications on the paper into a bare rectangular room are excellent. You can't make me not like it... :thumb: :rofl:. It is a theoretical exercise and an excellent learning tool. Couches and heavy curtains do sink bass including other speakers. What they (furniture,etc) are good for is not allowing bass nodes sound audible back at the sweetspot. If you want the room to low freq. sound the same at different spots Like a a sitting chair in the corner then CABs is what you want. If you don't care about that and only care about the sweet spot then it might be chasing over kill and can be accomplished doing other things.
Mind you the cost of a decent sub stand is probably more expensive than a delay. :duh so maybe not :lol:
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IMO most of the issues of sub placement is most subs are in a box. Open baffle is a huge advantage over seled or ported subs IMO.
Add servo drivers with a plate amp designed for such will change your sub opinions.
We recently showed at Capitol Audiofest using three per side servo open baffle subs with great success.
Using servo controlled drivers eliminates any box coloration from the back wave. All drivers are designed to act as one with a computer controlled coil on the drivers.
Tight articulated bass wit zero boom or or overhang. tone with harmonics. One is easily able to discern foot pedal action on organ as never heard before.
After hearing open baffle design done right it would be impossible for me to listen to a sub in a box.
charles
i like how an enclosed subs work with OB mids and tweets. :thumb: