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post #271 of 341 Old 03-20-2014, 12:42 AM
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Originally Posted by pitviper33 View Post


The idea is that the action of the rear array is the exact inverse of the pressure wave arriving from the front array. Therefore, because the resultant pressure is zero, there is no wave propagation from the rear array toward your ears. You should perceive nothing at 20ms in your example. It's the same kind of idea as noise canceling headphones. You can't detect the inverse signal they're playing, because its energy has been completely cancelled by the incoming wave.

Hi Pit,

 

Thanks for the explanation; I understood this was the theory but was interested in the practical application - just how effective is this theoretical cancellation compared with a 'total' absorber.  I am lucky enough to be able to spare the metre-thick space I would lose with an SBA so was keen to learn which system is actually superior, regardles of cost and complexity,

 

Cheers, Carl.

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post #272 of 341 Old 03-20-2014, 02:17 AM
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^
DBA is IF the plane wave generated by the array isn't too much distorted by the room. But there are lots of other considerations (like overall reverberation time) that make a room sound great, so other solutions might be preferable when looking at the big picture.

Markus

"In science, contrary evidence causes one to question a theory. In religion, contrary evidence causes one to question the evidence." - Floyd Toole
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post #273 of 341 Old 03-20-2014, 05:23 AM
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Quote:
Originally Posted by pitviper33 View Post

The idea is that the action of the rear array is the exact inverse of the pressure wave arriving from the front array. Therefore, because the resultant pressure is zero, there is no wave propagation from the rear array toward your ears. You should perceive nothing at 20ms in your example. It's the same kind of idea as noise canceling headphones. You can't detect the inverse signal they're playing, because its energy has been completely cancelled by the incoming wave.

When you say that the rear wave is the exact inverse of the wave from the front array, are you saying that the subs in the front array are supposed to be out of phase with the rear array?

Also, if the point of adding a rear array to a setup with a front array is that there is no wave propagation from the rear wave and the resultant pressure is zero....can you explain in some detail what exactly you mean by this?
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post #274 of 341 Old 03-20-2014, 05:31 AM
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Also, I have another question with regards to constructing a front & rear array of subs.

Ok, so first off, I am trying to wrap my head around all of this. I know that the optimum place for each sub in a DBA and SBA would be for each sub to be mounted 1/4 of the distance between the floor, ceiling, and side walls on both the front and rear walls. My questions are:

1. Are the enclosures for these subs an IB? Or are they smaller sealed enclosures mounted to the wall in the 1/4 locations?

2. Are the SI18's or the Dayton HO18's suitable drivers for this design? If not, are there any suitable drivers that would be better for the similar sub-$250 dollar price range?

3. What kind of power would be best used for this? Let's say you have 4 drivers on both the front & rear walls. What would be a suitable amp for them ?

4. Last but not least, How would you do the setup & EQ with these 8 subs? (Ie: four on each front & rear wall)

I would appreciate anyone that would be willing to take the time to explain the answers to these questions!
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post #275 of 341 Old 03-20-2014, 06:30 AM
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Originally Posted by Klinkenborg View Post

Hi Pit,

Thanks for the explanation; I understood this was the theory but was interested in the practical application - just how effective is this theoretical cancellation compared with a 'total' absorber.  I am lucky enough to be able to spare the metre-thick space I would lose with an SBA so was keen to learn which system is actually superior, regardles of cost and complexity,

Cheers, Carl.

Disclaimer: Please note that I have not actually built one of these things and have no hands-on experience with this concept. I have a strong background in physics and engineering, and my understanding only theoretical at this point. If someone has built one of these systems and has seen different results than what is being discussed, please speak up! Data trumps theory every time.

I suspect that the SBA with a large absorber at the back of the room would create superior results. Here's why.
1. The concept of the planar wave pitch-catch relies on the idea that the wave arriving at the back of the room is an exact match to the wave that originated at the front of the room, transformed only with a delay and a slight attenuation. This ignores any frequency specific absorption that may have occurred in the walls, the equipment, the furniture, or the people in the room. Conversely, a passive absorber at the back of the room has no expectations for what the waves it receives should look like. Whatever they are, it absorbs them.
2. Think about how this planar wave is formed. We start with an array of discrete sources, each emitting an approximately spherical wave. As we get farther from the front wall those spherical wave-fronts combine to form an approximately planar wave travelling through the room. Thanks to the wavelengths involved, we can be reasonably assured that this happens before that wave-front reaches our ears. But now think about how this looks at the back of the room. We have a planar wave coming in, and it somehow needs to break into pieces to be absorbed by an array of discrete sinks. Each rear driver is properly timed to create a low pressure zone to "suck up" its fair share of that incoming pressure pulse. For me, this splitting up of the planar wave is much harder to visualize than what's happening at the front of the room. It should work. But it's certainly a much more complicated way to absorb a pressure wave than a passive absorber would be, and it surely can't happen perfectly.

An open-backed room is the theoretical idea. It can generate no reflections. In my mind, the SBA seems closest to that theoretical ideal. The DBA is an approximation of it with the primary advantage being space savings.

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post #276 of 341 Old 03-20-2014, 06:30 AM
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Quote:
Originally Posted by Martycool007 View Post

When you say that the rear wave is the exact inverse of the wave from the front array, are you saying that the subs in the front array are supposed to be out of phase with the rear array?

Also, if the point of adding a rear array to a setup with a front array is that there is no wave propagation from the rear wave and the resultant pressure is zero....can you explain in some detail what exactly you mean by this?

Maybe this helps:



Adapted from http://de.wikipedia.org/wiki/Double_Bass_Array

Markus

"In science, contrary evidence causes one to question a theory. In religion, contrary evidence causes one to question the evidence." - Floyd Toole
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post #277 of 341 Old 03-20-2014, 06:39 AM
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Quote:
Originally Posted by pitviper33 View Post

Disclaimer: Please note that I have not actually built one of these things and have no hands-on experience with this concept. I have a strong background in physics and engineering, and my understanding only theoretical at this point. If someone has built one of these systems and has seen different results than what is being discussed, please speak up! Data trumps theory every time.

I suspect that the SBA with a large absorber at the back of the room would create superior results. Here's why.
1. The concept of the planar wave pitch-catch relies on the idea that the wave arriving at the back of the room is an exact match to the wave that originated at the front of the room, transformed only with a delay and a slight attenuation. This ignores any frequency specific absorption that may have occurred in the walls, the equipment, the furniture, or the people in the room. Conversely, a passive absorber at the back of the room has no expectations for what the waves it receives should look like. Whatever they are, it absorbs them.
2. Think about how this planar wave is formed. We start with an array of discrete sources, each emitting an approximately spherical wave. As we get farther from the front wall those spherical wave-fronts combine to form an approximately planar wave travelling through the room. Thanks to the wavelengths involved, we can be reasonably assured that this happens before that wave-front reaches our ears. But now think about how this looks at the back of the room. We have a planar wave coming in, and it somehow needs to break into pieces to be absorbed by an array of discrete sinks. Each rear driver is properly timed to create a low pressure zone to "suck up" its fair share of that incoming pressure pulse. For me, this splitting up of the planar wave is much harder to visualize than what's happening at the front of the room. It should work. But it's certainly a much more complicated way to absorb a pressure wave than a passive absorber would be, and it surely can't happen perfectly.

An open-backed room is the theoretical idea. It can generate no reflections. In my mind, the SBA seems closest to that theoretical ideal. The DBA is an approximation of it with the primary advantage being space savings.

Have you ever tried to absorb 20Hz with passive means? It's virtually impossible. DBA works reasonably well even under suboptimal conditions. But in the end it's not DBA vs. SBA. It depends on the specific room and goal.

Markus

"In science, contrary evidence causes one to question a theory. In religion, contrary evidence causes one to question the evidence." - Floyd Toole
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post #278 of 341 Old 03-20-2014, 06:56 AM
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Have you ever tried to absorb 20Hz with passive means? It's virtually impossible. DBA works reasonably well even under suboptimal conditions. But in the end it's not DBA vs. SBA. It depends on the specific room and goal.
Good point. My statement was probably a little too general. Both designs have advantages, and space savings isn't the only one for DBA.

On the subject of low frequency absorption: Of course you're right. It's extremely hard to passively absorb giant wavelengths. But below some frequency, do we really care? For frequencies below the first room mode, reflections off the back wall can no longer create standing waves. In that frequency region, where it's exceptionally hard to generate strong output anyway, less energy absorption might be viewed as a good thing. I recognize that's deviating from the DBA ideal, but why carry the concept beyond where it's needed?

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post #279 of 341 Old 03-20-2014, 07:40 AM - Thread Starter
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Originally Posted by pitviper33 View Post

I suspect that the SBA with a large absorber at the back of the room would create superior results.

That depends. As you can see in the first post my old DBA worked flawlessly. But I have seen measurements of other people's DBA's that were never that good. I guess the reasons were asymmetry and absorption in the room, as you described.
Recently, in one specific case there was a baffle wall at the front which behave different to the naked back wall. A DBA worked well when the primary array was on the back wall. But it didn't work well when the primary array was on the front. This seemed strange, but was explainable.

Here is the documentation of my recent SBA + absorber. It is in german, but I think the measurents tells their own tale. With 55 cm rockwool + vapor trap + 10 cm air gap there is not much left of the 1st order room mode at 27 Hz. There still is a wide peak, but nearly no decay. The peak means that SPLmax is higher than with an equivalent DBA. This is a big advantage in home theater. On the other side the wasted space for the absorber can not be neglected.
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post #280 of 341 Old 03-20-2014, 08:00 AM
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Can anyone tell me this....

In either a SBA or a DBA, do the drivers mount to an enclosure, ( ie sealed or ported), or are they mounted in an Infinite Baffle like setup? I currently have two additional HO18's that I could build into a SBA which would total 4 drivers, and could mount each one at 1/4 distance from floor, ceiling, left wall & right wall. What type of enclosure should be used for the drivers?
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post #281 of 341 Old 03-20-2014, 08:30 AM
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That depends. As you can see in the first post my old DBA worked flawlessly. But I have seen measurements of other people's DBA's that were never that good. I guess the reasons were asymmetry and absorption in the room, as you described.
Recently, in one specific case there was a baffle wall at the front which behave different to the naked back wall. A DBA worked well when the primary array was on the back wall. But it didn't work well when the primary array was on the front. This seemed strange, but was explainable.

Here is the documentation of my recent SBA + absorber. It is in german, but I think the measurents tells their own tale. With 55 cm rockwool + vapor trap + 10 cm air gap there is not much left of the 1st order room mode at 27 Hz. There still is a wide peak, but nearly no decay. The peak means that SPLmax is higher than with an equivalent DBA. This is a big advantage in home theater. On the other side the wasted space for the absorber can not be neglected.
Strangely, that URL seems to be blocked here at work. I'll be sure to check it out later.
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Originally Posted by Martycool007 View Post

Can anyone tell me this....

In either a SBA or a DBA, do the drivers mount to an enclosure, ( ie sealed or ported), or are they mounted in an Infinite Baffle like setup? I currently have two additional HO18's that I could build into a SBA which would total 4 drivers, and could mount each one at 1/4 distance from floor, ceiling, left wall & right wall. What type of enclosure should be used for the drivers?
The concept is driver enclosure independent. That decision should be made based on the normal factors: frequency response, enclosure size, power demands, ease of install, et cetera. If you were building a DBA, there would be some implementation benefits to having similar natural frequency and phase responses between front and back arrays. In other words, it'd be easiest if they were the same.

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post #282 of 341 Old 03-20-2014, 11:28 AM - Thread Starter
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Strangely, that URL seems to be blocked here at work. I'll be sure to check it out later.

*lol*
The domain name contains the word "hardcore". My brother is a DJ and does Hardcore (Techno). I uploaded the PDF on his site. Your web filter is now thinking that it is a porn site... biggrin.gif
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post #283 of 341 Old 03-20-2014, 11:37 AM
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*lol*
The domain name contains the word "hardcore". My brother is a DJ and does Hardcore (Techno). I uploaded the PDF on his site. Your web filter is now thinking that it is a porn site... biggrin.gif
If I get fired for trying to load it... mad.gif

Need to hire a newly unemployed engineer with an interest in acoustics? I'm willing to travel to Germany. tongue.gif

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post #284 of 341 Old 03-20-2014, 12:23 PM
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How about using passive radiators as a backwall array?


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My quess is that it would resonate at room modes and be bad because of it but i want to hear it from someone with authority smile.gif


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post #286 of 341 Old 03-20-2014, 01:41 PM
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How about using passive radiators as a backwall array?


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PR have a resonant frequency, the DBA tracks the signal ideally ... somewhat of a pitch and catch scenario. With that in mind, instead of the reflective surface returning the ball, the ball merely drops, as all energy has been removed.

In essence, it removes the rear wall. The energy encounters the listener ... and is subsequently nulled at the rear wall.

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post #288 of 341 Old 03-21-2014, 06:09 AM
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Pitviper33- thanks for the response!

So basically what you are saying, is that with a SBA or a DBA, you can use either sealed or vented enclosures, as long as they are the same? I would guess that most folks do these SBA's or DBA's with sealed enclosures with 18" drivers, right? How would you manage to use a ported enclosure mounted up the wall 1/4 distance from the floor & ceiling, as well as 1/4 from the side walls?

Let's say I want to do a SBA with '4 of these Dayton HO18's in either a sealed or ported configuration. What would be the optimal enclosure size and how would they need to be mounted?
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post #289 of 341 Old 03-21-2014, 07:27 AM
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Pitviper33- thanks for the response!

So basically what you are saying, is that with a SBA or a DBA, you can use either sealed or vented enclosures, as long as they are the same? I would guess that most folks do these SBA's or DBA's with sealed enclosures with 18" drivers, right? How would you manage to use a ported enclosure mounted up the wall 1/4 distance from the floor & ceiling, as well as 1/4 from the side walls?

Let's say I want to do a SBA with '4 of these Dayton HO18's in either a sealed or ported configuration. What would be the optimal enclosure size and how would they need to be mounted?
I'd expect you to build a full height baffle wall to elevate the higher sources. I'm not sure there's been enough of these designs built to be able to say that "most" people do any particular driver alignment or installation style. But if I were building one, it would be very hard to convince me to build anything other than an infinite baffle setup. Once you're getting to this level of investment of resources, planning, and effort, why settle for anything with more distortion? If you're building this type of array, you're committing to a large number of drivers and a large scale construction anyway, so the primary detractors to IB are eliminated. But of course, there can be plenty of reasons to arrive at a different conclusion than mine. Those reasons could include incompatible parts acquired at a steal or inappropriate back space.

As for what configuration would be best for those drivers you already own, I don't know. I think you'd generate more responses from the members knowledgeable in that area by creating a thread dedicated to the question. The DBA concept being discussed here cares only about proper positioning of the acoustic centers of the multiple sources in the room. What those sources look like doesn't really affect the principles. Choices of driver alignment and tuning are the same as they would be in any other installation.

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I'd expect you to build a full height baffle wall to elevate the higher sources. I'm not sure there's been enough of these designs built to be able to say that "most" people do any particular driver alignment or installation style. But if I were building one, it would be very hard to convince me to build anything other than an infinite baffle setup. Once you're getting to this level of investment of resources, planning, and effort, why settle for anything with more distortion? If you're building this type of array, you're committing to a large number of drivers and a large scale construction anyway, so the primary detractors to IB are eliminated. But of course, there can be plenty of reasons to arrive at a different conclusion than mine. Those reasons could include incompatible parts acquired at a steal or inappropriate back space.

As for what configuration would be best for those drivers you already own, I don't know. I think you'd generate more responses from the members knowledgeable in that area by creating a thread dedicated to the question. The DBA concept being discussed here cares only about proper positioning of the acoustic centers of the multiple sources in the room. What those sources look like doesn't really affect the principles. Choices of driver alignment and tuning are the same as they would be in any other installation.

I really hate to start a whole new thread, but, I may do so as I am considering doing a IB setup anyway. I just have one question that will determine my subwoofer plans...and that is....would a quad set of HO18's would well in an IB? I will start a new thread regarding this topic depending on the answer to this question, smile.gif
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post #291 of 341 Old 03-21-2014, 08:54 AM
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I really hate to start a whole new thread, but, I may do so as I am considering doing a IB setup anyway. I just have one question that will determine my subwoofer plans...and that is....would a quad set of HO18's would well in an IB? I will start a new thread regarding this topic depending on the answer to this question, smile.gif
They would work. But they aren't very optimized for IB. To use those is spending your money on a strong motor that you don't need. The ideal IB driver would spend less on the motor and more on excursion capability. If you were going IB, I would sell the HOs and replace them with IB318s for more than double the displacement at the same price.

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post #292 of 341 Old 03-21-2014, 09:37 AM
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^

Or try them since you already own them, ... then if they prove inappropriate, act accordingly.

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post #293 of 341 Old 03-24-2014, 08:02 AM - Thread Starter
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REW's room simulator now supports delays and inverting. This allows us to simulate a DBA. smile.gif

SBA + absorber:


DBA:
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post #294 of 341 Old 03-24-2014, 08:21 AM
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Simulation suggests that SBA works well when only one row needs to be optimized. DBA works for multiple rows:




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post #295 of 341 Old 03-24-2014, 08:29 AM
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Your simulation has minimal absorption. I noticed that as you add front and rear absorption, the response gets worse. The subs don't need a super low -3dB frequency to still play flat with a DBA. Its cool to move around the seating location and notice that the bass still stays almost the same:

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post #296 of 341 Old 03-24-2014, 08:42 AM
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I noticed that as you add front and rear absorption, the response gets worse.

Yes but real rooms have virtually no absorption at low frequencies.

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post #297 of 341 Old 03-24-2014, 09:07 AM - Thread Starter
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Yes but real rooms have virtually no absorption at low frequencies.

That's why I always state that a SBA only works well with a big absorber in the back smile.gif

Here are measurements of my two seat rows with a smoothing of 1/24 oct. Without the absorber the variance was much greater. I measured this a while ago.





Of course a DBA is still better in this regard. This is out of question. smile.gif
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By the way, here's a simulation for a small room (single couch) with a DBA to about 60Hz. The front sub can't be located at the correct height but it doesn't seem to matter much:




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post #299 of 341 Old 03-24-2014, 09:25 AM
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Quote:
Originally Posted by markus767 View Post

Yes but real rooms have virtually no absorption at low frequencies.
Does "real rooms" mean "common rooms" or "rooms designed for SBA"?

If you mean normal rooms, then I agree. If you mean that a room designed for SBA doesn't have much LF absorption, then I'd say the room you're describing (and the one you simulated) isn't a SBA design at all. It's just a big array of woofers.

edit: Actually, on second look, maybe you did have a bunch of absorption in that simulation. I'm not familiar enough with how REW's simulation controls work. I'll have to open it up when I get home.

I'm not into "thumbs upping" or "liking". Don't take it personally. Just assume that I found your post helpful. Unless it wasn't.
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post #300 of 341 Old 03-24-2014, 09:26 AM
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^
"Single Bass Array" doesn't say anything about aborption wink.gif

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