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post #181 of 1029 Old 03-21-2013, 11:31 AM
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FOH, we're probably not far off in agreement then. I must have misunderstood you. I just don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law. Same take on it as Bosso as far as I know.
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post #182 of 1029 Old 03-21-2013, 12:34 PM
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"...that the "leading edge" of the spherical wave has zero interaction with the 6 boundaries because it hasn't reached the longest dimension yet."

re-read the papers bosso. there is no wave as you are thinking about it in the near field. that is where you are getting all caught up. the air slogs back and forth. no wave. just pressure.

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post #183 of 1029 Old 03-21-2013, 12:35 PM
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"I just don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law."

its not just below the room 2x room dimension point. this is a misunderstanding. it is in the near field, even if you are in anechoic space.

with low frequencies, the near field grows to the size of the room. that's all that is happening.

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post #184 of 1029 Old 03-21-2013, 01:00 PM
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Quote:
Originally Posted by LTD02 View Post

even if you are in anechoic space.



with low frequencies, the near field grows to the size of the room.

These don't go together...

You're contradicting yourself. You post the linkwitz link, which discusses the 2x the longest dimension. You've posted about ideal gas law in the other thread. You've discussed the hemholtz resonator effect. Are you now saying the paper isn't about supporting your previous ideas, it's a totally different theory, this nearfield idea. Maybe I've misunderstood but you've changed your position at least three times. It's hard to discuss this with you when you're hand waving like this.

So if we're in the nearfield, and like you said, you don't get the SPL drop with distance. If I could demonstrate an SPL drop with distance in my room, would you then agree this theory doesn't work?
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post #185 of 1029 Old 03-21-2013, 01:19 PM
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Quote:
Originally Posted by tuxedocivic View Post

FOH, we're probably not far off in agreement then. I must have misunderstood you. I just don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law. Same take on it as Bosso as far as I know.

Tell me, wrt Carps measurments, what you're taking exception to. Honestly, I've not a clue.


"don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law"

I don't understand.

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post #186 of 1029 Old 03-21-2013, 01:44 PM
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I, like Bosso afaik, think that room gain is a function of constructive wave patterns only, and not described by pressure fluctuations by deltaVroom. I don't have a problem with Carps measurements.
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post #187 of 1029 Old 03-21-2013, 01:52 PM
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"Maybe I've misunderstood but you've changed your position at least three times."

it's all the same effect. that is why it may seem confusing.

ideal gas law applies, but only at about 1/2 lambda. that is the linkwitz example. no waves. just pressure.

plane wave tube is the same. applies below 1/2 lambda. that is the section of the paper on compactness and incompressibility that i linked. no waves. just pressure.

spherical wave is the same again. that is the part bb snipped kr << 1. again no waves. just pressure.

all of these things are describing the hydrodynamic near field physics.

when the hydrodynamic near field is the the size of your room, you no longer have waves. just pressure.

doesn't matter if it is headphones, car, small room, large room, or outdoors. calculation is the same. iirc linkwitz shows inside box, inside car, inside room. same math.

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post #188 of 1029 Old 03-21-2013, 02:08 PM
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Quote:
Originally Posted by LTD02 View Post

"I just don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law."

its not just below the room 2x room dimension point. this is a misunderstanding. it is in the near field, even if you are in anechoic space.

with low frequencies, the near field grows to the size of the room. that's all that is happening.

How far you are in anechoic space from a point source is irrelevant. In such a case, the lower the frequency, the longer it will take for the compression/rarefaction phenomenon to wash over you. Otherwise, there is zero difference from any frequency in the band we are discussing.

What happens in the < room*2 is as FOH understands it... the room can no longer support a standing wave. The wave does not change to liquid, nor does the ambient pressure of the room change in cycles per second, both of which are, although fun to talk about, impossible in reality.

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Originally Posted by FOH View Post

Tell me, wrt Carps measurments, what you're taking exception to. Honestly, I've not a clue.


"don't agree that PVG is involved below the 2x room dimension in accordance with the ideal gas law"

I don't understand.

Let's use my room because I know it better than Carp's or anyone else's and we can trust the results for accuracy in the region we're discussing:



Same room different sub location, all else equal. My magic frequency is approximately 20 Hz, below which there should be no difference in FR regardless of placement of the point source. This is clearly not the case. Not only is there a difference, the difference is higher above a point and lower below that point, so it can't be argued that the FRs are similar enough to reconcile with the theory that "everywhere in the room the pressure changes...". That's simply impossible. The ambient pressure in a room cannot change in cycles per second especially to the point where it rattles the pictures off the walls.

LTD is relying on a paper that theorizes mostly impossible situations inside of an infinitely rigid pipe. It's like teaching the trig class that there are 3 dimensions; a) length (draw a line on the board), b) length and height (draw a rectangle on the board) and c) length height and depth (draw a cube on the board. The fact is that those 3 exist only in theory. If each of the examples drawn on the board did not all have length height and depth, they would be invisible.
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post #189 of 1029 Old 03-21-2013, 02:17 PM
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"...the longer it will take for the compression/rarefaction phenomenon to wash over you."

what you are describing is how it works in the far field. you are failing to consider that the far field and the near field have two different physics. i am describing the hydrodynamic near field.

"It's like teaching the trig class that there are 3 dimensions..."

:-) more like teaching acoustics class that there are three fields: 1. hydrodynamic near field, 2. geometric near field, 3. far field.

different behavior in each field. pressure vessel gain is in topic 1.

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post #190 of 1029 Old 03-21-2013, 02:30 PM
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whats the name of that demo disc your using?

i see blueray LFE-MENU.
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post #191 of 1029 Old 03-21-2013, 02:41 PM
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It is the scuba steve bluray demo disc
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post #192 of 1029 Old 03-21-2013, 02:45 PM
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BTW, here are two measurments with no EQ and the same exact setups. The difference is I moved the mic from LP to just in front of the LP(3 feet closer and 2 feet lower) in a dead center cup holder.



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post #193 of 1029 Old 03-21-2013, 02:57 PM
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http://www.modulararts.com/

Thanks for posting this! ModularArts is actually here in Seattle and they sent me the pricing. Its not too bad for some of them, pretty reasonable. The blocks are a bit pricey though.

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post #194 of 1029 Old 03-21-2013, 05:21 PM
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Yes! That would be awesome. Was debating on making a trip out that way to pick up some more subs in June, so that might push me over the edge. I can just finish the boxes to the point of having them up and running for the time being and do the final finishing later.

I'll try to give you a holler over the next few days.
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post #195 of 1029 Old 03-21-2013, 06:16 PM
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Some revealing stuff....

http://www.falstad.com/ripple/

Play with it for a while. You can encompass the point source in a smaller 'room' to see what would happen.

Very instructive.

JSS
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post #196 of 1029 Old 03-21-2013, 07:24 PM
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i believe that the ripple simulator is modeling only the first part of the wave equation, which is how waves behave in the far field. the near field is different.

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post #197 of 1029 Old 03-22-2013, 03:37 AM - Thread Starter
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Send a message via Skype™ to popalock

Hey, thanks for posting this up! Sorry if I initally missed it, I'm trying to soak in all of this PVG/slog/gas/fluid dynamics talk. Great stuff, but very technical for sure. I actually researched and reached out to a few of those companies before I posted up those pics. Not cheap...lol

Hopefully the hookup beast has will be more reasonable. I'll let you guys know what I find out.

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Tell me, what is the approx distance from the your ears @the LP, and the back boundary?

Thanks

When my seat is reclined ATM I am about 7-8" from the wall. When I have the rear subs installed, I'm going to almost have the subs pressed up against my seat backings, so I will be about 14" from the rear wall/boundry. I'm going to try to catch up on the rest of your post and respond accordingly.

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Where have you been! You get your quads up and running yet? Feel free to stop by anytime to check out my updates. If you wait a few more weeks I should have the rest of the rig at least functional.

 

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post #198 of 1029 Old 03-22-2013, 06:50 AM
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To begin with, unfortunately I'm going to have precious little ability over the next few days to participate in any discussions. Then I transition from precious little to none whatsoever for the subsequent week or longer. We'll see, sucks.

frown.gif



Quote:
Originally Posted by bossobass View Post

This is a no-brainer. If the "critical frequency" is simply derived from the rooms longest dimension, why does that not jive with the actual responses posted by members here?

Twice the longest dimension in my room works out to below 20 Hz. So, why does room gain begin at nearly an octave above that, and why does everyone else's, regardless of the huge disparity in the "longest dimension" (the critical) part of the equation?

This makes it simpler to predict in-room performance when attempting to expand the BW of your system. This is why I can predict the in-room response of a sealed system given only (a) the signal chain roll off, (b) the systems naked response and (c) the construction method of the boundaries far better than anyone else can with the PVG critical frequency, sealed vs open stuff, and will be happy to prove that any time someone is interested.



If there's anything I've learned, it's small room acoustics is extremely complex and highly counter-intuititive. This I'm certain of, because all the smart dudes I read, that discuss such things (studio design types/engineers, acousticians, physics guys), constantly remind the lay people, working pro and enthusiast crowd, and they repeat it over and over.

Bosso, your "critical frequency" term, ... are you using that in place of the first axial mode (1, 0, 0)?



Some critical distinctions;
The acoustic gains elicited by PVG (or whatever term one wishs to call the results of that phenomenon), cannot begin within the modal range. Any acoustic gain enjoyed within that range is either boundary gain, or simple superposition effects.

Moving a LF source can easily excite stronger modal interaction, less modal interaction, stronger panel resonances, or less panel resonances.

The distances between two boundaries, determine the standing wave/modal freq therein. Anything less than a fully rigid boundary, interacts with that distance in a less than ideally well defined manner.



I'm not at my intelectual best right now, and I may be wrong, or easily omitted something, etc. However all the above explains the behavior you're questioning. Prior to skipping the theory, and moving on to examining more graphs, if we could discuss the above physics, and where within the above lies the chink in the armor for your theory. If anyone has the measurement gear, the playback gear, to establish something in the above can be "thrown out on it's ear", it'd be Bosso.


Sincere thanks, as the exchange of ideas advancing the understanding is the foundation of what AVS represents to me.

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post #199 of 1029 Old 03-22-2013, 06:54 AM
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Quote:
Originally Posted by bossobass View Post





Same room different sub location, all else equal. My magic frequency is approximately 20 Hz, below which there should be no difference in FR regardless of placement of the point source. This is clearly not the case. Not only is there a difference, the difference is higher above a point and lower below that point, so it can't be argued that the FRs are similar enough to reconcile with the theory that "everywhere in the room the pressure changes...". That's simply impossible. The ambient pressure in a room cannot change in cycles per second especially to the point where it rattles the pictures off the walls.

I too have seen changes in FR below modal region when changing delay settings on some of the subs. So I figured this maybe phase cancellations. But IIRC you stack the subs so all your units would probably sum up well at ULF. Very intriguing.

The bigger differences above the "magic" frequency are easily explained though - those ARE driven by modal interactions.
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post #200 of 1029 Old 03-22-2013, 08:55 AM
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The subject of PVG is one in which the proponent throws out some Star Trek stuff about the room being pressurized by fluid and walks away as though you're the village idiot for calling BS.

Where's the evidence?

FOH,

I have discussed the above physics, from many angles, for many years, over thousands of posts in a dozen forums.

I agree with the magic frequency as it defines a point below which the room can no longer support a standing wave. The evidence supports it and it's a theory with a sound basis.

It's what happens below that point that I question. From cross to that point, everything is explained as a result of reflections against boundaries. Then, magically, reflections are supposed to instantaneously cease entirely as the room becomes a pressure cooker.

If PVG as it's explained here by some members, holds true, then the shape of the room, the number of boundaries, the ratio of dimensions of those boundaries, where the subwoofer is placed and where one sits within those boundaries is irrelevant to the result, which is strictly a predictable phenomenon based on the rooms longest dimension.

So, show the evidence. (Money talks, BS walks)
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post #201 of 1029 Old 03-22-2013, 09:16 AM
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"If PVG as it's explained here by some members, holds true, then the shape of the room, the number of boundaries, the ratio of dimensions of those boundaries, where the subwoofer is placed and where one sits within those boundaries is irrelevant to the result, which is strictly a predictable phenomenon based on the rooms longest dimension."

that may be overshooting the argument a little. just as the wave theory of sound for far field effects makes predictions that get all garbled in the modal region in actual rooms, it is reasonable to assume the predictions made by pressure vessel gain (hydrodynamic near field) theory may also be imperfect descriptions of actual rooms. i'm not sure that has really ever be in question.

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post #202 of 1029 Old 03-22-2013, 11:29 AM
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Quote:
Originally Posted by LTD02 View Post

"If PVG as it's explained here by some members, holds true, then the shape of the room, the number of boundaries, the ratio of dimensions of those boundaries, where the subwoofer is placed and where one sits within those boundaries is irrelevant to the result, which is strictly a predictable phenomenon based on the rooms longest dimension."

that may be overshooting the argument a little. just as the wave theory of sound for far field effects makes predictions that get all garbled in the modal region in actual rooms, it is reasonable to assume the predictions made by pressure vessel gain (hydrodynamic near field) theory may also be imperfect descriptions of actual rooms. i'm not sure that has really ever be in question.

So what would make the theory falsifiable? How do you explain the differences in the sub-modal region Bosso observed?
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post #203 of 1029 Old 03-22-2013, 11:36 AM
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Quote:
Originally Posted by tuxedocivic View Post

I, like Bosso afaik, think that room gain is a function of constructive wave patterns only, and not described by pressure fluctuations by deltaVroom..

Here is what I do not get about this hypothesis. If there are no standing waves in the sub-modal region, what is there to construct?
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post #204 of 1029 Old 03-22-2013, 02:10 PM
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Quote:
Originally Posted by FOH View Post

The acoustic gains elicited by PVG (or whatever term one wishs to call the results of that phenomenon), cannot begin within the modal range. Any acoustic gain enjoyed within that range is either boundary gain, or simple superposition effects.

Moving a LF source can easily excite stronger modal interaction, less modal interaction, stronger panel resonances, or less panel resonances.

The distances between two boundaries, determine the standing wave/modal freq therein. Anything less than a fully rigid boundary, interacts with that distance in a less than ideally well defined manner.

Good summary...There are no simple answers there are many complex interactions all happening at once and/or transitioning into each other.

I think we can all agree that below the frequency corresponding to the longest dimension of the space we no longer have modal distance related effects and something else occurs whereby the frequencies below that point get reinforced more uniformly and heavily. Note that modal issues stop below a certain point but that leaves a whole plethora of other things that can still affect the energy in the space below that point. The structural makeup and transmission losses of the room boundaries (perhaps even from one corner to the next, brick walls in one versus wood in another for example) differences in shape, distance, size and positioning of the subwoofer from the LP and even the furnishings close range can all cause change in the response delivered to the LP below the modal region. Generally the differences are smaller than what you would see above the modal region but they are still there. For example when LTD02 mentioned large openings such as hall ways and ducts acting as giant Helmholtz resonators and messing with the response down low he was generally scoffed at but I am convinced this behavior does occur. I believe Tom Danley has commented on seeing effects of this as well. How much of an effect it has in the real world on most systems is debateable but it could have some effect depending on the circumstances. I am convinced that walls and floors can have sympathetic diaphragmatic movement that can cause resonance issues and cancellation of energy in the very deep bass as well.

I have not taken the time to read some of the papers LTD02 has linked which he seems to believe explains the behavior below the modal region in typical rooms. Regardless of whether PVG is misunderstood or misapplied typically or whether some of these theories are true or not is interesting but at the end of the day will only amount to a fraction of the puzzle of what occurs to the energy emmitted from a speaker into a room in the very low bass or otherwise.



Here is my actual room gain seen at the main LP in my old room which LP was almost dead center of the room BTW. If I recall correctly this room was about 10x17x25'. This is the response of a sealed sub measured outside at 1m and the same sub placed into the back left corner and the front right both of which were nearly equidistant and about 4m from the LP give or take. 4200cubes or so. Wood construction over crawlspace. Really lossy and prone to vibrating. Black is the outdoor sealed sub test, red is the front right sub position. Blue is the back left.



Here is the same for my current room which is smaller at about 3,150 cubes. Again similar layout with the main LP almost dead center of the room. This time the room is longer but narrower and shorter 8.5x14.5x25.5. The construction is now cement slab with 3 walls of brick 2 of which are under ground level. Much more solid and less "live" than the old. Again the black trace is a sealed sub measured at 1m outdoors and compared to the response shape from the same sub inside at 2 different positions with the mic at the LP. The red trace is with the sub in the front left position and the blue is with the sub in the back right position. EDIT: Note that I have not actually done the work to get the actual amount of gain occuring versus a 1m halfspace measurement so this is a guesstimate on my part for this room. However I suspect this is in the ballpark.





To my eyes there are some things going on below the modal region that are not readily explained by a simple PVG theory.


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post #205 of 1029 Old 03-22-2013, 02:27 PM
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Ricci,

is there any way you can post the same overlays but with in-room curves (heavily) soothed out?
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post #206 of 1029 Old 03-22-2013, 02:47 PM
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Quote:
Originally Posted by zheka View Post

Ricci,

is there any way you can post the same overlays but with in-room curves (heavily) soothed out?

Yeah, play some Barry White songs and measure them. biggrin.gif

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post #207 of 1029 Old 03-22-2013, 02:49 PM
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LOL.
i do need some heavy soothing
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post #208 of 1029 Old 03-22-2013, 05:20 PM
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Where have you been! You get your quads up and running yet? Feel free to stop by anytime to check out my updates. If you wait a few more weeks I should have the rest of the rig at least functional.

I've only used 2 of the subs so far in a dual opposed box, placed centered behind my screen. I'm very happy with them for now, however I plan to build the other 2 into singles for the front corners. I originally had an issue where I didn't think the subs add enough and at times, my mains actually had more punch, but that feeling is completely gone. When I turn off the subs, the mains are very lacking in comparison.

I would love to see your creation, it looks very menacing. I'll wait until you have it all up and running, however I imagine it is amazing. I mean your 2 other subs blew my mind with their insane output.

I've seen you mention a few times about getting another amp, I may consider getting one if you wanted to split the shipping. I'm even considering getting 2 more subs, although I don't know why.
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post #209 of 1029 Old 03-22-2013, 06:34 PM
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Please, no misunderstanding, my tone here is entirely amiable.

Bosso, you're moving too fast for me*. This bottom octave range is fascinating, and to that end ... great contributions such as many of yours over time, have really inspired me to learn more and more. But you made bold statements "tossing theory out on it's ear", focused pointedly on Carps graphs, posed questions about specific behaivior that doesn't fit the known physics, claiming your theory does explain it, soliciting answers otherwise. I answered them, *now you want to move on without further discussion? .. without even finishing your contention about Carps graph.

I've been around long enough to know how you, Bosso, would have reacted if a contributor did that to you.

I'm far from the sharpest dude around here, but I do attempt to always maintain a positive manner of comportment. No ******** Jedi Mind Tricks here, like Sweet Brown says,.. there's no time. But yeah, I'd like to discuss both your room, or any room all in order, but we've not yet finished with your initial Carp example, and the very thought provoking position you've taken.


You, using Carps room, measurements and graphs, because they aligned perfectly with your theory, etc. You stating "in Carp's thread about my theory for room gain. I've tossed PVG out on it's ear and focussed on actual data", let's focus on those contentions prior to moving on. Like I said, I'll be glad to move to your room's examples too, but I took exception to your new theory, you posed a few questions and I answered them. I'd appreciate wrapping that up first. If not I understand, I'm in no position to steer this thread.

One of us wasn't correct. You stated you didn't want any of this feel good BS,...where everybodys right, etc. I aggree.


Also, you claim this theory allows you to accurately predict a room gain profile down into the bottom octaves without inclusion of a room's dimensions.

I disagree. There's no way, unless there's more you've yet to disclose. If so, please do.




Quote:
Originally Posted by Ricci View Post

Note that modal issues stop below a certain point but that leaves a whole plethora of other things that can still affect the energy in the space below that point. The structural makeup and transmission losses of the room boundaries (perhaps even from one corner to the next, brick walls in one versus wood in another for example) differences in shape, distance, size and positioning of the subwoofer from the LP and even the furnishings close range can all cause change in the response delivered to the LP below the modal region. Generally the differences are smaller than what you would see above the modal region but they are still there. For example when LTD02 mentioned large openings such as hall ways and ducts acting as giant Helmholtz resonators and messing with the response down low he was generally scoffed at but I am convinced this behavior does occur. I believe Tom Danley has commented on seeing effects of this as well. How much of an effect it has in the real world on most systems is debateable but it could have some effect depending on the circumstances. I am convinced that walls and floors can have sympathetic diaphragmatic movement that can cause resonance issues and cancellation of energy in the very deep bass as well.

I agree, and mentioned all this and explained Carp's effects Bosso interpreted as unexplained. I explained and accounted for the change that he pointed out and insist don't occur within the current theory. If he's commented about my explanation to his theory and Carps example, I've not seen it.


Quote:
Originally Posted by Ricci View Post

I have not taken the time to read some of the papers LTD02 has linked which he seems to believe explains the behavior below the modal region in typical rooms.

I haven't either.

Quote:
Originally Posted by Ricci View Post

To my eyes there are some things going on below the modal region that are not readily explained by a simple PVG theory.

emphasis mine. I don't know what you mean by simple PVG theory.

That segment of my post you quoted, is accepted, widely understood, and none of this falls outside those aspects of the known physics.

"The acoustic gains elicited by PVG (or whatever term one wishs to call the results of that phenomenon), cannot begin within the modal range. Any acoustic gain enjoyed within that range is either boundary gain, or simple superposition effects. Moving a LF source can easily excite stronger modal interaction, less modal interaction, stronger panel resonances, or less panel resonances. The distances between two boundaries, determine the standing wave/modal freq therein. Anything less than a fully rigid boundary, interacts with that distance in a less than ideally well defined manner."


I'm guessing over time Bosso had witnessed other odd behavioral examples similar to the Carp example he cited. However, nobody commented upon me explaining what could easily account for the odd behavior Bosso's cited. It is interesting, absolutely. But inexplicable? No.


About the "sympathetic diaphragmatic movement", I contacted you long ago about this, as I saw it in some of your measurements. I recognized it, as I've seen it myself, in my room. Jordan, aka Krypt**, also discussed this with me and yeah, these boundary impedances are absolutely a factor. Any energy moving such a panel is detracting from the SPL experienced at the LP.

And perhaps most importantly, yes, re-locating the LF source exterts a varying amount of energy, via selective mode cancelation/excitation. The entire point behind multi-subs being the path to bass bliss; changing the modal interactions. Put 'em all up front, they're going to drive that length mode maximally. Spread them out, lessens the effect.



Is this thing on? smile.gif I think most just gloss right over these long, bordering on self indulgent posts. But I read every word of the mega posts, so I know some cull all they can.




**may he rest in piece, I loved the guy, we PM'd all the time, and I wish I could share all the variety of things we discussed, what an amazingly inquisitive dude, he's truly missed. In Nascar, there's cookie cutter 1.5 mile tracks. Well, I suppose there's cookie cutter AVS'ers too. Jordan, was anything but.



Sincere and amiable thanks

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post #210 of 1029 Old 03-22-2013, 08:46 PM
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Most of the "issue" here is in previous discussions posters (not just LTD02) have said that ULF room gain was a result of physically pressurizing the room. And when the room way and window measurements were shown, the it was said that it was because of a hemholtz resonator. And those posters argued like it was black and white. You both (Ricci and FOH) both are taking a much more reasonable position that its complex. Bottom line is in any room, a sound wave is a sound wave. There's no point where the wave turns into a room pressure fluctuation. The gain is a result of the waves piling on top of each other because there's no where for them to go. They can still be deconstructive, but are far more constructive.
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