Are three inexpensive subs better than one expensive sub-Dr. Earl Geddes Approach - Page 71 - AVS Forum
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post #2101 of 2123 Old 11-03-2014, 07:22 AM
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Originally Posted by sdurani View Post
Randy, don't know if you had seen these measurements that Todd posted here at AVS in June. They are a clear example of why Harman pursues minimizing spatial variance.

One sub in one corner measured from 16 seats (blue traces) with average (red trace).


Four subs in four corners measure from 16 seats (blue traces) with average (red trace).


Note the improvement in frequency response is minor but the improvement in seat-to-seat consistency is substantial. Which would you rather EQ?
Thx Sanjay.
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post #2102 of 2123 Old 11-03-2014, 08:25 AM
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Originally Posted by acribeiro View Post
So does Dr. Geddes, or am I getting something wrong?
From Earl's post earlier in the thread.
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Originally Posted by gedlee View Post
The idea is to use multple subs located around the room to smooth the spatial and spectral response at the seating location.

IF you want the smoothest bass, then you must use multiple sources, no single source can compete.

This is done top get as smooth a response as possible.
No mention of minimizing mean spatial variance, no mention of multiple seating locations, just repeated mentions of smoother bass. By comparison, Welti never mentions smoother bass or improved frequency response in his research papers, just seat to seat consistency.
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Originally Posted by acribeiro View Post
I think the improvement in frequency response is not minor, specially regarding bass cancellations.
Cancellation at which frequencies? The peaks & dips in the first graph are in the second graph.

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post #2103 of 2123 Old 11-03-2014, 08:51 AM
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Originally Posted by sdurani View Post
From Earl's post earlier in the thread. No mention of minimizing mean spatial variance, no mention of multiple seating locations, just repeated mentions of smoother bass. By comparison, Welti never mentions smoother bass or improved frequency response in his research papers, just seat to seat consistency.
I will let him answer that, but I think it is just a matter of words. You can sound more or less technical but say the same. This is not a scientific forum. He also said here that his thesis is about bass in small rooms, but the words "bass" and "room" are not even mentioned in the title of his thesis: "An Analysis of the Low Frequency Sound Field in Non-Rectangular Enclosures Using the Finite Element Method". Does it sound more appealing to you?

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Originally Posted by sdurani View Post
Cancellation at which frequencies? The peaks & dips in the first graph are in the second graph.
Really? I thought it was obvious, see attachment (this is a quick and dirty mspaint job, the black markings are about the same on both pics). Note that I did not mention the peaks (they do not matter, as long as they are the same everywhere), but they also benefit from multiple subwoofers.
EDIT: I see now that the scale should be somewhat shifted to compensate for the SPL difference between 1 and 4 subwoofers (quick and dirty), but the argument still applies.
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post #2104 of 2123 Old 11-03-2014, 09:07 AM
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When using multiple subs both the spatial and the spectral responses are smoothed. The smoothing is greater for the spatial variance simply because it has the greater variance to begin with. My Thesis dates to 1981 and it was not about using multiple subs.


The fact is that I was using multiple subs prior to Todd Welti's publication, which was noted in my letter to the editor of JAES right after Todd's paper was printed. Todd studied regular placement of the subs and listener locations while I had already looked at more random placement and locations. Our results were almost the same and our conclusions for the need to use multiple subs were identical, but I was able to point out that fewer subs could be used if they were not located in symmetrical arraignments. Symmetry of source locations and listener locations reduces the degree of independence of each sample. Two random points are less correlated than two symmetrical points. Two corners are almost completely correlated for example.

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post #2105 of 2123 Old 11-03-2014, 09:23 AM
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Originally Posted by acribeiro View Post
I think it is just a matter of words. You can sound more or less technical but say the same.
Words mean something. Saying it less technically (smoothest bass at the seating location vs seat-to-seat consistency at multiple seating locations) still doesn't make it the same thing.
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This is not a scientific forum.
Do you know what the S in AVS stands for?
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I thought it was obvious, see attachment (this is a quick and dirty mspaint job, the black markings are about the same on both pics).
I was looking at the average, which has minor improvement in frequency response. You're pointing to individual seats, which conformed more closely to the average (the whole point of Welti's approach).

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post #2106 of 2123 Old 11-03-2014, 01:22 PM
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Originally Posted by sdurani View Post
Words mean something. Saying it less technically (smoothest bass at the seating location vs seat-to-seat consistency at multiple seating locations) still doesn't make it the same thing.
To quote properly, "spatial response at the seating location". I do not know what that shone location is not really "spatial". Yes, I did see it, and I just assumed an "s" was missing. Furthermore, I did read every line of Dr. Geddes in this thread and it was always clear for me that he optimizes more that one seat (and you might be surprised, but I do not agould mean, spatial response at ree with several of his statements here ). I recall a discussion about averaging before and after applying the metric, which makes little sense for one point in the room. Please do not make me look for it You can see he speaks about "listener locationS" above. I hope that clarifies it.
Interestingly enough (and that was one of my discussions with him in the last few posts), even if you optimize only one location, you can benefit of multiple subwoofers: same frequency response from one or from several subwoofers at one single listening position (point) does not sound the same.
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Do you know what the S in AVS stands for?
Yes, sure. With "scientific forum" I was rather thinking about PRL or something in that line (not just "internet forum". Sloppy wording. Words again :-D Some discussions here go really deep into technical aspects, but I would not see it as "science" or "scientifical". Words CAN mean something. I can call myself professor, but that does not make me one of them. And do not get me wrong, this is a great place for lots of information. The "problem" is that anybody can post anything and somehow "every view is valid" (my experience in one Spanish forum was quite disappointing in this regard). I believe Dr. Geddes also commented something on this lines earlier on this thread.
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Originally Posted by sdurani View Post
I was looking at the average, which has minor improvement in frequency response. You're pointing to individual seats, which conformed more closely to the average (the whole point of Welti's approach).
Even the average is better regarding cancellations, but the point is to optimize multiple seats, and if you have strong cancellations in single seats, you cannot correct them. His metric is defined to minimize seat-to-seat differences IIRC (in order to use one EQ and thus correct them all at the same time), so the presence of less cancellations and actually a more even response is an added plus that appears naturally.

Oh, I was just trying to learn something and ended up in the middle of the heated discussion

@ Dr. Geddes: the reference to your thesis was just an example for wording :-)

This is my view up to now, maybe somebody will show me I am wrong. No problem! Dr. Geddes did it already some lines above and it has been really helpful.
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post #2107 of 2123 Old 11-03-2014, 03:05 PM
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Originally Posted by acribeiro View Post
Interestingly enough (and that was one of my discussions with him in the last few posts), even if you optimize only one location, you can benefit of multiple subwoofers: same frequency response from one or from several subwoofers at one single listening position (point) does not sound the same.
That's the difference with the Harman/Welti approach: the notion of optimizing "only one location" never comes up because they're only concerned with minimizing variations between multiple locations. That variation (MSV) IS their metric. When the thing you're optimizing for is seat to seat consistency, there is no such thing as optimizing for a single seat as there can be with the Geddes approach.
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Even the average is better regarding cancellations, but the point is to optimize multiple seats, and if you have strong cancellations in single seats, you cannot correct them.
My point was that the improvement for the average is minor (same peaks and dips still there) compared to the improvement in consistency. The first thing most people notice is not that the peaks and dips went away (because they didn't) but that every seat now has the same peaks and dips.

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post #2108 of 2123 Old 11-03-2014, 03:59 PM
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Originally Posted by sdurani View Post
That's the difference with the Harman/Welti approach: the notion of optimizing "only one location" never comes up because they're only concerned with minimizing variations between multiple locations.
Let me reformulate: when you optimize several locations, you can benefit of multiple subwoofers as a single listener: same frequency response from one or from several subwoofers at one single listening position (point) does not sound the same.
I think this is very relevant, but you seem to overlook it and just state the obvious.
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Originally Posted by sdurani View Post
That variation (MSV) IS their metric. When the thing you're optimizing for is seat to seat consistency, there is no such thing as optimizing for a single seat as there can be with the Geddes approach.
I know the metric, I have read the paper and even discussed with Todd Welti about it few years ago as mentioned before.
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My point was that the improvement for the average is minor (same peaks and dips still there) compared to the improvement in consistency. The first thing most people notice is not that the peaks and dips went away (because they didn't) but that every seat now has the same peaks and dips.
I think I showed you that dips went away to a high degree in the blue lines, and this is a very relevant thing for the listeners, together with the seat-to-seat consistency. "Improvement for the average" per se is irrelevant (and not included in the metric). You will never hear the average, you will hear one of the blue curves, depending on where you sit. If the dips did not go away, but be the same everywhere, you would have the single-location dips of the first graph in the average of the second graph (caused by the seat-to-seat consistency). In that case, you could not correct those and the sound would not be so good.

We agreed above that the frequency response alone is not the full story regarding bass performance, so I feel there must be a better merit function to define "good bass". Dr. Geddes claims (please correct me if I got you wrong) that it does not matter much, as he gets the same good bass with three subwoofers and way less "number crunching". Maybe the "LF envelopment" of Griesinger could be included, but I have no idea how he puts that into an equation...
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post #2109 of 2123 Old 11-03-2014, 04:30 PM
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I know the metric, I have read the paper and even discussed with Todd Welti about it few years ago as mentioned before.
Then you know that the papers don't mention smooth bass or improved frequency response.
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We agreed above that the frequency response alone is not the full story regarding bass performance, so I feel there must be a better merit function to define "good bass".
That's the point I can't seem to get across. The Harman papers never mention good bass, just the same bass in all seats. Making the bass sound good comes in the next step (global equalization).

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post #2110 of 2123 Old 11-03-2014, 04:40 PM
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Hey guys based on the info I reading, would it be better to get three bic F-12's over a svs pb-1000?
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post #2111 of 2123 Old 11-03-2014, 04:57 PM
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Originally Posted by sdurani View Post
Then you know that the papers don't mention smooth bass or improved frequency response. That's the point I can't seem to get across. The Harman papers never mention good bass, just the same bass in all seats. Making the bass sound good comes in the next step (global equalization).
Are you still dealing with the wording? Call it X I do not care about wording, it was some years ago and I kept the concepts in my head, not the exact wording. You can say the same in several ways. Even with equations
Maybe you should look beyond the Harman papers and avoid mixing models with reality
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post #2112 of 2123 Old 11-03-2014, 05:01 PM
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Then you know that the papers don't mention smooth bass or improved frequency response. That's the point I can't seem to get across. The Harman papers never mention good bass, just the same bass in all seats. Making the bass sound good comes in the next step (global equalization).
So what?


Whether someone is talking about spatial variance improvement or frequency response improvement at a single point is irrelevant as far as multiple subs are concerned. That is because they both benefit, both are improved. So what if one group ONLY talks about spatial improvement while another talks about improving the spatial and the spectral response at either a single location or more than one location. The answer is still the same - use multiple subs. Making a big argument out of a non-issue is not very interesting to readers, but mostly just a waste of time.

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post #2113 of 2123 Old 11-03-2014, 05:01 PM
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Are you still dealing with the wording?
Do you believe that the only difference between Welti's and Geddes' approach is wording?

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post #2114 of 2123 Old 11-03-2014, 05:05 PM
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Hey guys based on the info I reading, would it be better to get three bic F-12's over a svs pb-1000?
It depends If you want to feel what I call "the earthquake effect", the PB1000 should manage it better. If you do not mind missing bass below 25Hz, the three subwoofers will deliver a more even response. No idea about max. output of the bics, but I would guess they will beat one PB1000 based on their specs.


The argument of random excitation of room modes sounds very reasonable to me. On the other side, the "number crunching" can inlude this, provided the grid is small enough and all combinations are considered (which Welti does now). It would be interesting to know how the solution changes depending on the size and position of the optimized region.
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Do you believe that the only difference between Welti's and Geddes' approach is wording?
No, but it is not what you mentioned above (one vs. several positions) and both approaches are not that far from each other, as they both commented in this thread.
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post #2116 of 2123 Old 11-03-2014, 05:08 PM
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So what if one group ONLY talks about spatial improvement while another talks about improving the spatial and the spectral response at either a single location or more than one location.
You don't believe there is more than one approach to using multiple subs?
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The answer is still the same - use multiple subs.
You're knocking down a straw man, asthough anyone is arguing against using multiple subs.

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post #2117 of 2123 Old 11-03-2014, 05:10 PM
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No, but it is not what you mentioned above (one vs. several positions) and both approaches are not that far from each other, as they both commented in this thread.
What I mentioned? I posted a quote.

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post #2118 of 2123 Old 11-03-2014, 05:16 PM
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It depends If you want to feel what I call "the earthquake effect", the PB1000 should manage it better. If you do not mind missing bass below 25Hz, the three subwoofers will deliver a more even response. No idea about max. output of the bics, but I would guess they will beat one PB1000 based on their specs.


The argument of random excitation of room modes sounds very reasonable to me. On the other side, the "number crunching" can inlude this, provided the grid is small enough and all combinations are considered (which Welti does now). It would be interesting to know how the solution changes depending on the size and position of the optimized region.
Ok thanks man......
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post #2119 of 2123 Old 11-03-2014, 05:24 PM
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Ok thanks man......
Others may have a different view and this was a rather coarse recommendation. I would need to know your situation in detail (room size, placement possibilities, playback levels, AVR and speakers, eventually external EQ...) but that may be better dealt with in an own thread ;-)
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post #2120 of 2123 Old 11-03-2014, 05:42 PM
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Originally Posted by sdurani View Post
You don't believe there is more than one approach to using multiple subs?
I know that Todd's approach and mine are identical regardless of the wording being used. The only difference is that he looked only at symmetric arraignments in his paper and I had looked at both symmetric and non-symmetric.


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You're knocking down a straw man, asthough anyone is arguing against using multiple subs.
Then what are you arguing about that isn't just semantics?

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post #2121 of 2123 Old 11-03-2014, 05:59 PM
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Then what are you arguing about that isn't just semantics?
That your approach and Harman's aren't identical.

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post #2122 of 2123 Old 11-03-2014, 07:26 PM
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When this thread was revived back in June, I mentioned I was working on some freeware to do the optimization in an automatic way. It's been ready for private beta testing for a while now, but the person who was going to test it is really busy with work and other things. so I'm looking for one or two beta testers before making semi-final changes and releasing it for general use. It is Windows-only and is free for non-commercial use only, kind of like Holm. There is no commercial version, just the free one.

It handles per-subwoofer EQ, delay and gain to optimize response flatness at multiple listening positions. It requires that the user have an XLR mic (not USB) and use REW with a loopback connection or Holm with time locking for time-synchronized measurements. It works on FRD files exported by the user from the measurement software. Shoot me a PM for more details if interested.
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post #2123 of 2123 Old 11-03-2014, 08:05 PM
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Originally Posted by sdurani View Post
That your approach and Harman's aren't identical.
I'm certain Dr. G can answer for himself and this is both a long and old thread, though I believe he spoke to all of this over five years ago in this very thread.
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Originally Posted by gedlee
As I said, I've done a lot of rooms this way and have a lot of measurements, I just lack the time to display them. But there is an abundance of articles, papers and simulations (one on my website I believe) which show that multiple subs will yield the "best" response in an objective sense. On this point there can be no argument - everyone who has studied the situation agrees that multiple subs spatially located achieves the best overall response.

We all disagree on the details - where to put them, how to set them up, etc. - but no one disagrees on the premise that you need more than one. JBL would like to sell lots of expensive subs and a very expensive magic box. I don;t feel that is necessary - I have no doubt that the results will be great, only that its not the most cost effective solution. I like cost effective.

Are three inexpensive subs better than one expensive sub-Dr. Earl Geddes Approach

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Last edited by DocCasualty; 11-03-2014 at 09:37 PM. Reason: emphasis
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