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Calculating room modes for an irregular shape
Old 16th December 2010
  #31
SAC
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Quote:
Originally Posted by recordinghopkins View Post
I am contemplating building some resonators to correct the dip at ~60Hz,

Just a head's up.

You don't use traps, porous or resonant, to correct nulls.
Old 16th December 2010
  #32
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Quote:
Originally Posted by SAC View Post
Just a head's up.

You don't use traps, porous or resonant, to correct nulls.
Thanks for the heads up, will you be a little more specific and explain yourself to make your comment more useful?

How is it exactly that trapping and resonators do not help to minimize nulls that occur as a result of reflected energy?
Old 16th December 2010
  #33
SAC
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A modal null is not the same as comb filtering or a null caused by the summation (superposition) of reflected specular energy. Such a null caused by the superposition of specular energy is in fact the result of spatial polar lobing caused by superposition that appears as null or comb filter in the frequency response.

The modal null is not comb filtering, but a region of low pressure in a spatially reinforced LF standing wave pertinent to frequencies with wavelengths greater in magnitude than the dimensions of the bounded space which are not reflected specularly.
Old 16th December 2010
  #34
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Ok, thanks for the clarification. SO what you are saying is that nothing can be done about modal nulls, other than changing the room geometry, or changing the listening position to avoid them?
If that's the case, tell me this, how can I differentiate a modal null from boundary interference so as to know what actually can be corrected with treatment?
Old 16th December 2010
  #35
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after a little math (based on 29ft x 11.5ft x 8ft), it looks like the axial mode nearest to the ~63Hz dip in my measurement is at 70.6 Hz, and that's a floor/ceiling mode. Are you suggesting that no amount of trapping on the ceiling will minimize the null created?
Old 16th December 2010
  #36
SAC
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Much of the confusion here stems, I suspect, from an unawareness of the fundamental nature of the physics of LF resonant wave behavior.

May I suggest you examine the structure of a standing wave which is defined as regions of high pressure and alternating regions of low pressure.
The null of a standing wave is not created by the destructive interference of multiple superposed out of phase signals! It is the reinforced fundamental wave behavior of a frequency whose energy is augmented by room geometry. Energy at the particular wavelength constructively sums and reinforces the resonance magnitude - and this wave behavior is defined by areas/regions of high pressure (anti-nodes) and low pressure (modes or nulls). A null exhibits both low pressure and essentially zero velocity. It is not anomalous but a fundamental part of the behavior. It is anomalous only insofar in that occurs where we would rather not have it occur.

And then you might want to examine the mechanics of specular reflections in which destructive interference results in polar lobing and regions not simply of low pressure, but in regions of active cancellation. You cannot simply interchange the behavioral characteristics.

If low frequency acoustic energy with wavelengths larger than the boundary dimensions result in standing waves, please tell me how you intend to modify the fundamental nature of of standing waves consisting of areas of low and high pressure?

And then please suggest a method whereby an absorber can increase the pressure in a low pressure region. And I do not mean simply redistribute said energy, but fundamentally redefine the behavior of a standing wave.

Just as equalization cannot recreate energy at a point in space where 2 superposed 180 degree out of phase signals result in cancellation, an absorber does not increase the pressure in a low pressure zone caused by a resonant standing wave.

You can partially mitigate high pressure regions with absorbers and/or active methods (see the Bag End e-trap for an example) and reduce said distributed effects - you do not eliminate them. And room geometry can augment this by spreading the distribution of the modal behavior such that multiple nulls and/or peaks are not coincident at the same spot - thus moderating their total effects. But geometry (except by transforming the small acoustical space into a large acoustical space!) can at best only moderate the modal behavior. It does not eliminate the fundamental behavior.
Old 16th December 2010
  #37
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I have come across many of your posts browsing GS, and while you are clearly educated, I find your posts to often be somewhat overworded for those of us who are not learned acousticians. I very much appreciate your insight, and will attempt to gain a greater understanding on the topic and unlock it's secrets by following your suggestions as I decipher the encrypted magical text you have presented to me in your last post. I did ask for it, though.

In the mean time, I look forward to hearing the rest of the community's insight as well. Judging from your response, it looks like I am stuck with what I have unless I mix sitting on the floor, but I would hate to close the case before others weigh in.

Old 16th December 2010
  #38
SAC
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Quote:
Originally Posted by recordinghopkins View Post
...encrypted magical text ...

And yet, in your entire post, you fail to ask for the clarification of even one point, or even one confusing term, regarding this (and for that matter, in any other post.)

In that case, you might want to stick with post 31. ...But then you also objected to that very practical oriented post which leaves out any reference to the wave physics you declare too obtuse...

Quote:
Originally Posted by recordinghopkins View Post
How is it exactly that trapping and resonators do not help to minimize nulls that occur as a result of reflected energy?
"Exactly"??? OK.

They don't minimize nulls.

If you don't like the "null" thingie, rebuild the room with different dimensions or change your location. And if it helps, try imagining what might be implied by the "magical encrypted" word:"null", ...and think about what might be missing that you persist in assuming you might absorb...

You see, for those you need more than magic encrypted words, you need magical absorbers.

Good luck.
Old 16th December 2010
  #39
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Hey there, no need to get your feelings hurt. All I was saying is that to those that dont know the science of acoustics, an acoustic engineer, much like a mastering engineer, is thought to be in some secret society that knows magical and wondrous things that us common knob turners have no clue about.


Quote:
If you don't like the null thingie

Are you kidding me? Really?

Man, you are obviously smart, but you are a lousy teacher. I hate to be so succinct, but I gave you respect and acknowledged your education and thanked you for your insight, but when I don't understand your response, you talk down to me and scoff at the fact that I don't know the intricacies of wave physics.


Here's what I was thinking: nulls and summation points result as a wave is reflected off of a boundary and combines with the source wholly or partially out of phase. My thought is, if you can trap the wave near the boundary, then it will minimize the effect. There are no modes if there are effectively no boundaries, right? Now clearly, it would take an impractical amount of absorption to trap 60Hz, which is why I was thinking resonators might do the job more effectively. If that is wrong, then a clear simple explanation will go a lot farther than talking over my head. You finally achieved this in your last post, but I could have done without the sarcasm.

However, my initial question of calculating modes for an irregular room has really only been addressed by the very first response (that software looks pretty cool, btw). The remainder of the thread has been mostly the GS acoustic wizards debating on whether or not it is even worth it to try longhand calculation when the room is not square or rectangular. I asked about the math to calculate an irregular room's modes, and I am still interested in knowing what that is, no matter how complex or a waste of time it may be. I just need a place to start in order to do my own research and study.
Old 16th December 2010
  #40
SAC
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I am not sure what restating an erroneous concept in the name of greater clarity accomplishes at this point. Your thinking assumed erroneous elements. We tried to point that out. Modal behavior is not the same as specular behavior. You had plenty of opportunity to ask for clarification earlier of various concepts and descriptions and failed to do so.

And even FEM/BEM have limitations in calculating the effects of complex coupled spaces! (dare we ask where one will obtain the complex acoustical impedance of all of the boundary elements?) And that is the case for knowledgeable acoustical engineers! EASE and CATT-A come closest for the purpose of modeling yet un-built spaces, but even they still have significant limitations in this regards! Thus why modeling is useful for proposals and preliminary conceptual purposes, it is NOT sufficient to predict real world behavior. And for this very reason, measurements are fundamental, necessary and conclusive.

Absorbers do not correct nulls.

I am sorry that our various attempts at explanation were inadequate, as you found neither correct simple answers nor additional details provided in response to your demands for more detail adequate, as such information involved reference to simple physical concepts.


Good luck.
Old 16th December 2010
  #41
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Quote:
Originally Posted by SAC View Post
Your thinking assumed erroneous elements. We tried to point that out.
Who is we? I assume you mean you, as nobody else has joined the conversation since post 28.


Quote:
Absorbers do not correct nulls.
You have said this a few times now, but that contradicts everything I have read on the subject. For instance:

Quoting Ethan:
"When low frequencies are attenuated in a room, the cause is always canceling reflections. All that is needed to allow low frequency waves to sound properly and with a uniform frequency response is to remove or at least reduce the reflections."
AND
"With each increase in wall density, reflections will cause cancellations within the room at ever-lower frequencies as the walls become massive enough to reflect the waves.
Therefore, it is reflections that cause acoustic interference, standing waves, and resonances, and those are what reduce the level of low frequencies that are produced in a room. When the reflections are reduced by applying bass traps, the frequency response within the room improves. And if all reflections were able to be removed, the response would be exactly as flat as if the walls did not exist at all."
Old 16th December 2010
  #42
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Hey Hopkins.

I feel your pain. I've been reading this forum pretty religiously for a few years now, and this is a concept that is just now starting to get teased out. This thread comes to mind:

I need a few things clearing up...

Though I'm no more confident having read it
Old 16th December 2010
  #43
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Quote:
Originally Posted by SAC View Post
A modal null is not the same as comb filtering or a null caused by the summation (superposition) of reflected specular energy.
Still trying to understand this...
How is a modal null different from a standing wave? I assume that you are talking about a standing wave when you mention reflected specular energy....
Old 16th December 2010
  #44
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My Ah HA! moment has finally arrived.

Thanks johndykstra.


Quote:
Originally Posted by Dange View Post
Below the Schroeder frequency, the room mode resonances cause increases in level, the peaks. The 'nulls' are due to a lack of modal support, not any cancellation effects.
Now the question is how to determine the difference when you do find a null...
Old 16th December 2010
  #45
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Thanks for this:
Quote:
Originally Posted by SAC View Post
May I suggest you examine the structure of a standing wave
It led me to THIS, after deconstructing the url. Lots of great stuff.
Old 16th December 2010
  #46
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Quote:
Originally Posted by SAC View Post
Much of the confusion here stems, I suspect, from an unawareness of the fundamental nature of the physics of LF resonant wave behavior.

May I suggest you examine the structure of a standing wave which is defined as regions of high pressure and alternating regions of low pressure.
The null of a standing wave is not created by the destructive interference of multiple superposed out of phase signals! It is the reinforced fundamental wave behavior of a frequency whose energy is augmented by room geometry. Energy at the particular wavelength constructively sums and reinforces the resonance magnitude - and this wave behavior is defined by areas/regions of high pressure (anti-nodes) and low pressure (modes or nulls). A null exhibits both low pressure and essentially zero velocity. It is not anomalous but a fundamental part of the behavior. It is anomalous only insofar in that occurs where we would rather not have it occur.

And then you might want to examine the mechanics of specular reflections in which destructive interference results in polar lobing and regions not simply of low pressure, but in regions of active cancellation. You cannot simply interchange the behavioral characteristics.

If low frequency acoustic energy with wavelengths larger than the boundary dimensions result in standing waves, please tell me how you intend to modify the fundamental nature of of standing waves consisting of areas of low and high pressure?

And then please suggest a method whereby an absorber can increase the pressure in a low pressure region. And I do not mean simply redistribute said energy, but fundamentally redefine the behavior of a standing wave.

Just as equalization cannot recreate energy at a point in space where 2 superposed 180 degree out of phase signals result in cancellation, an absorber does not increase the pressure in a low pressure zone caused by a resonant standing wave.

You can partially mitigate high pressure regions with absorbers and/or active methods (see the Bag End e-trap for an example) and reduce said distributed effects - you do not eliminate them. And room geometry can augment this by spreading the distribution of the modal behavior such that multiple nulls and/or peaks are not coincident at the same spot - thus moderating their total effects. But geometry (except by transforming the small acoustical space into a large acoustical space!) can at best only moderate the modal behavior. It does not eliminate the fundamental behavior.

Well there is the partial answer that acoustical treatment does not change err eliminate modal behavior.. especially the last paragraph.

So we are left to finding the best position .

>EDIT<
Jumped the gun. We can only change the modal behavior by modifying the boundaries so as to avoid peaks/nulls at the same spot?
Old 16th December 2010
  #47
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Let me see if I can make some sense of this.

If you have a "null" that doesn't drop below reference level (spl level of original source at distance), it's due to two peaks on either side... this is where we should aim our focus

If you have a null that drops below reference level and it is above the schroeder frequency of the room, it is due to SBIR or some other boundary interference. Course of action is to move listening position and or driver placement... or treat the cause of boundary interference.

If you have a null that drops below reference level, and is below the schoeder frequency, it is due to a low pressure region of modal behavior. Moving position is your only course of action.

How wrong am I?
Old 16th December 2010
  #48
SAC
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Pretty close.

One way to make all of this MUCH simpler is to stop living in the frequency domain! Let me explain.

Modal behavior dominates the 0-~300 Hz region. Generate waterfall plots for the room in general - stimulate one long corner (e.g. lower front right), and measure in the diagonal opposite corner (e.g. upper left rear). You will have the modes the room itself is capable of supporting - including the lowest mode as the long diagonal is the greatest dimension... (Measurement Note: This is ideally one with a non-directional source - often a 'dodec'.)

Then generate a 'local' modal map for the proposed topology of speakers and seating. You will note the peak frequencies and their maximal gain locations and note the nulls. (Measurement note: Nearfield modal measurements become skewed by local directional information! Use the local measurements simply to help with listener positioning by identifying and avoiding nulls and excessive peaks.)

Treat the room by reference to the room waterfall and location mapping at positions of maximal gain (anti-nodes) with absorption appropriately, and physically try to avoid the nulls.

Thus, by recognizing that modal behavior is the result of wavelengths larger than the dimensions of the boundaries upon which they are incident, it is relatively easy to estimate the approximate cutoff frequency. But as you will see, it is not important to determine the Schroeder 'critical' frequency.
By identifying the major modes below ~300 Hz, you will have addressed the fundamental modal frequencies.



Now, for all practical intents and purposes, once that is complete, forget the frequency domain.
You are now going to be looking in the time domain and using the ETC response.

Having addressed the LF modal issues and positioned yourself appropriately for maximal L/R symmetry, derive the ETC responses for left and right speakers in the listening position.From this you will determine the specular energy paths that exceed the appropriate gain levels with respect to time in accordance with the acoustical response model you have chosen as your room response template.

You don't have to worry about frequency as by definition the specular behavior operates above the Schroeder critical frequency and the modal behavior resides below. And as the waterfall and ETC responses conveniently focus on different behavioral perspectives, you will effectively deal with them both based upon using he appropriate tool for each function.




By 'living' in the frequency domain for all behaviors, not only can you not determine where one behavior is differentiated from the other (specular vs modal), but the frequency response fails to provide adequate isolation and identification of the specular gain, arrival time, and spatial and temporal energy distribution.


Conversely, by utilizing the ETC, you will easily be able to identify energy that functions specularly. The behavioral signature itself takes care of worrying which is which!

Use the proper tools and the issue becomes one of an academic nature.

Thus your job becomes one of selecting the appropriate/desired acoustical response model as templated by the ETC, and using the appropriate treatment to achieve the signature.
Old 16th December 2010
  #49
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sheeesh SAC! now anybody is going to be able to do this! you were supposed to keep this stuff secret

seriously though, this last one should probably be put into the room measurement stickie as the preamble...
Old 16th December 2010
  #50
SAC
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Covering some ground twice, and a few areas for the first time, here are a few aspects of the overall behavior. Each has in some form or fashion been mentioned or at lest inadvertently touched upon...

This is simply an attempt to clarify a few points distinguishing some of the wave behavior confusion discussed earlier.

As mentioned above, do not become obsessed with trying to distinguish modal from specular reflections. Oh, they are distinctly different in nature and behavior. But if the appropriate tools are employed to evaluate them, the distinction becomes rather academic and simple. And you can obsess over the logical descriptions and distinctions at you leisure.


As has been stated on multiple occasions, a modal standing wave behaves differently then a focused specular reflection.

The primary distinguishing factor is their size.

Specular reflections are reflected based upon their wavelength being equal to or smaller than the dimensions of an incident surface.

Modal standing waves have wavelengths larger than the dimensions of a boundary surface.
This’ break point’ is referred to as the Schroeder ‘critical frequency’. But please do not go trying to calculate or determine this! It is neither necessary no clear cut, as you will generally have a region of overlapping modal and specular behavior as you will have various boundary dimensions resulting in some energy being reflected specularly while some energy is reflected in a modal manner. Again – don’t worry, as the measurement methods will sort all of this out very elegantly as mentioned above.

With reference to the earlier reference to Ethan’s example, the only way to remove the standing waves in a small acoustic space is to effectively remove the boundary - thus effectively requiring a 100% efficient absorber covering all of the LF frequencies! Now, as fascinating as this imagined concept is, does anyone here posit that absorbing 100% of the LF is a realistic viable option - especially considering that most would kill for a reliable LF absorber capable of decreasing the gain at an anti-node (peak) by just 6 dB SPL??

Thus, in a real world room, you will NOT remove ALL of the returned energy!

The following erroneous notion has been mentioned several times, and it seems a source of at least part the confusion.


Quote:
Originally Posted by recordinghopkins View Post
Still trying to understand this...
How is a modal null different from a standing wave? I assume that you are talking about a standing wave when you mention reflected specular energy....

In that statement we are trying to differentiate a standing wave from modal behavior and associate a standing wave with specular reflections. This has gotten the notions confused. If you want to associate labels, associate the standing wave with modal behavior.


Thus we are still confusing specular behavior with modal behavior. And modal behavior is not the result of two specular reflections superposing (summing). Modal behavior is the result of the same incident energy that is reflected and returned that reinforces (itself) and both the magnitude and the null of the resultant wave.

Not to get too far afield on a tangent with this element, but one that will be mentioned as it is fundamental to the behavior describes - One will also note that boundaries featuring greater acoustical impedance than the transmission medium return the reflected incident wave energy in phase, thus resulting in the constructive reinforcement of the resultant (incident + reflected) waveform. And I know of no practical effective room boundary featuring a lesser complex acoustical impedance capable of delaying or inverting the phase of the reflected energy propagation sufficiently to disrupt said reinforcement. Perhaps when we begin manufacturing walls of closed cell aerogel or of sparse matrix carbon nanotube Buckyballs…. So in a practical bounded enclosure, we are ‘stuck’ with this condition…

Jumping over to specular behavior for a moment to present a contrast, the energy content of mid and high frequency wavelengths is a fraction of that of low frequency wavelengths. There is good reason why low frequency waves, such as one might experience with thunder or ELF - extremely low frequency communications which can be perceived far away from the actual source while relatively short low energy content specular energy has already been dissipated by friction.

So, with specular reflections, we are able to control the resultant polar lobing that appears as comb filtering in a frequency response because we are more able to intercept and absorb the more limited (focused) spatial distribution of the relatively low energy content of the constituent mid-high frequency shorter wavelength energy of which specular reflections are comprised.


With low frequencies the difficulty of doing this is increased substantially.


And I know folks don’t want to hear about this, but our conceptualization of this phenomena is also confused by our wave models. We so often imagine wave behavior in the form of a transverse wave (you know, what appears to be a sine or cosine wave). But a bounded modal space the air column more effectively exhibits a behavior more consistent with a longitudinal wave featuring periodic regions of low and high pressure.


Oh, and other aspect that many forget. Sound has size. Oh, we talk of wavelengths all the time, but how often do folks actually think of the relative size of the wavelengths in the real world? Specular energy is specular precisely because the wavelengths are smaller than the incident boundaries. They are focused, meaning they do not occupy the entire room, instead being limited to smaller ray like distributions. And as a result, not only are the effects of superposition position related, but due to their relatively small size, they are able to be effective controlled in the room by the application of absorption, diffusion, or redirection/controlled reflection.


The modal standing waves, on the other hand, are not small localized waves. They are large relative to the room boundaries and to the room itself. They fill the room. Thus you have areas of relative high and low pressure distribution, but you do not in a small space have regions of modal behavior and regions lacking it. It dominates the room.

And as far as absorbing a null… how do you absorb that which is effectively 'absent', meaning the particle velocity is effectively 'zero' and the pressure is low - rarified. Technically, neither porous velocity traps nor tuned resonant absorbers are effective here! It is akin to trying to absorb a shadow. The null is a function dependent upon the relation of the dimensions of the room and the wavelength of the energy that is returned. And at the risk of creating more misconceptions rather than increased understanding (hey, can it get worse?? ;-), modal behavior is a LF behavior akin to mid-high frequency flutter echo that occurs between parallel surfaces, only modes have big honkin' wavelengths (and where small surface angle irregularities are not critical due precisely to the relative size of the wavelengths involved).

Oh, and let's jump over for a moment and consider the compounding influence of coupled spaces in the suggested room posited in the thread...At its simplest, the room functions, as you will, as a large tuned pipe With closed ends terminated in anti-nodes and open ends terminated in nodes. It is this perspective by which the myriad traditional modal calculators function. And due to ideal geometric and simplistic bounded acoustical impedance assumptions, they are estimates at best.

In the worst case, you have a series of irregularly bounded spaces that are coupled in a complex manner. And this complex topology effectively functions as multiple enclosures of the various component sizes as well as in all of the various combinations and permutations of the various component and summed space. Such calculations are complex (to say the least) in the simplest of configurations where the boundaries are assumed to be 'regular' and of constant impedance. But the difficulty scales exponentially with the complexity of the assemblage of bounded spaces - compounded still further by the real world acoustical impedances of the boundary surfaces which function as frequency and time dependent absorbers and reflectors varying with the angle of incidence. And not only are the reinforced frequencies more difficult to calculate, but the energy density distribution becomes much more complex.

Suffice it to say that reliable prediction of such complex behavior is beyond any practical desktop solution currently available. We can obtain a rough idea that tends to eliminate inadvertently creating really bad spaces, but such modeling is not capable of producing a model sufficient to replace real world proof of performance measurement and verification. (In other words, we can make them look really good for the purpose of making sales and marketing proposals! ;-)


Both resonant tuned absorbers and velocity based porous absorbers function best in regions high pressure and high velocity, respectively. And neither condition exists at the null. Thus, unlike baseball where you “hit ‘em where they ain’t”, in the treatment of modes, you “hit ‘em where they are” based upon how absorbers work.


We have a bounded space. And we have modal behavior. Our tools are relatively ineffective in the nulls. But we can mitigate the ‘peakiness’ by absorbing the peaks and minimizing the reinforced resonant energy. And if we are designing a space, we can chose dimensions that most effectively distribute the modes - spreading them out as much as possible. But as far as the nulls in an existing space, the simplest and most effective solution is to literally avoid them. Move. Assuming no other adjustments, adjust your seating arrangement forward or backward (assuming a left right symmetry) such that you are in between the null and the peak.

Then treat the room modal behavior as suggested above.

And then you can begin obsessing over the specular energy and obtain practice making and interpreting and discovering just how treatment moderates specular reflections. And that is quickly summarized above.
Old 16th December 2010
  #51
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Lightbulb

Quote:
Originally Posted by recordinghopkins View Post
You have said this a few times now, but that contradicts everything I have read on the subject. Quoting Ethan:
I love SAC to death, but he's just wrong on this. All peaks and all nulls are caused by acoustic interference, which is the same as comb filtering. Peaks and nulls occur outdoors against a single reflecting boundary:

Bass Waves in the Control Room

So adding more boundaries adds more "poles" (electrical term) into the equation, but the basic principle of peaks and nulls is boundary interference. Here's the proof:

A room that is 100 percent bass trapped has neither peaks nor nulls. Think of an anechoic chamber at frequencies where the chamber is truly anechoic. No peaks. No nulls. The rigid outer shell dimensions of the chamber are irrelevant because the 100 percent absorption cures both the peaks and nulls.

--Ethan

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Old 16th December 2010
  #52
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SAC:
Those last couple of posts were very well written, and I had almost no trouble following you. Please know that I do indeed respect your wealth of knowledge, I was just a little irritated with your first comment as it felt like a bit of a jab at me. I felt like you were kind of trolling by calling me out but not supporting your view. When asked for an explanation, it seemed like you went to the opposite extreme and overcomplicated just for spite. Whether that is true or not, I appreciate your continued effort to help me learn a thing or two. Please just be aware that some of us that read this forum have neither the knowledge nor the vocabulary to easily follow all of your posts. If you have found my posts offensive, I am sorry. It is not my intention to stir up arguments on here. The last thing I want is to get the boot, I love Gearslutz.
By the way, what is your real name?
-Greg Hopkins
Old 16th December 2010
  #53
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In the modal region, the 'nulls' for the most part, are simply a lack of peak reinforcement. That said, it is still possible though to have nulls in that region which are not modal in nature. If we can tame the peaks which cause the apparent nulls, then we are by definition making them less obtrusive.

To simply say though that one cannot ever treat any nulls via absorption is, in my opinion, incorrect. It's done all the time to address nulls which are not modal in nature (think SBIR, cancellations off rear wall based on seating distance, etc.)

I would agree that in the lower modal regions, the best method in an already built room is to avoid sitting in places where apparent nulls exist due to high peak reinforcement. One can also 'cheat' in the higher modal area at times by stimulating a dip in response via monitor placement/SBIR to offset an offensive peak at the seating position - provided this does not cause other anomalies.

Many ways to skin a cat.

I would also agree that once you're above 300Hz (for the sake of a random number...) you're then primarily dealing in domains other than frequency even though they may present themselves as frequency response problems.

Bryan
Old 16th December 2010
  #54
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One can easily see why I might be so confused while learning outside of a classroom when I gather contradictory information from different sources.

Quote:
Originally Posted by Ethan Winer View Post
I love SAC to death, but he's just wrong on this. All peaks and all nulls are caused by acoustic interference, which is the same as comb filtering. Peaks and nulls occur outdoors against a single reflecting boundary:

Bass Waves in the Control Room

So adding more boundaries adds more "poles" (electrical term) into the equation, but the basic principle of peaks and nulls is boundary interference. Here's the proof:

A room that is 100 percent bass trapped has neither peaks nor nulls. Think of an anechoic chamber at frequencies where the chamber is truly anechoic. No peaks. No nulls. The rigid outer shell dimensions of the chamber are irrelevant because the 100 percent absorption cures both the peaks and nulls.

--Ethan

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Old 16th December 2010
  #55
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Lightbulb

Here are some images that should help to clarify. The first is a screen cap of my ModeCalc program showing the precise dimensions of the RealTraps lab room and the predicted mode frequencies. ModeCalc has a graphical display too of course, but this is the text output screen. The second image is a graph showing what was actually measured in the room with no bass traps. For this test we put the microphone and speaker in opposite tri-corners of the room, to measure only the modes with minimal contribution from non-modal peaks and nulls. The third image is the same test but with ten bass traps in the room. Gearslutz wouldn't let me attach the photos of the microphone and speaker placement here, since I already posted them in this thread:

https://www.gearslutz.com/board/5763525-post89.html
https://www.gearslutz.com/board/5764143-post91.html

These images make two important points. First, modes as predicted will never match what's actually measured unless the room is bounded by cement or some other perfectly rigid and reflecting surface on all six sides. This room has standard drywall for all four walls and the ceiling, and a solid cement floor. But there's another important point here that relates to using tuned absorbers to target very low frequencies, as amish mentioned earlier. Note that all but one of the mode frequencies shifted downward slightly when ten traps were added to the room. Ten traps is not unreasonable for a room this size, and neither is 20 traps which would probably shift the mode frequencies even lower. So if a room is treated with lots of broadband absorption and also tuned absorption, you better determine the tuned frequencies by measuring - and do that after all the broadband traps are in place. heh

--Ethan

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Attached Thumbnails
Calculating room modes for an irregular shape-modecalc-lab.gif   Calculating room modes for an irregular shape-lab-no-traps.gif   Calculating room modes for an irregular shape-lab-10-traps.gif  
Old 16th December 2010
  #56
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Quote:
Originally Posted by bpape View Post
In the modal region, the 'nulls' for the most part, are simply a lack of peak reinforcement.
That's a common misconception but it's not really true Bryan. Peaks are typically 6 dB or less due to wave reinforcement, but nulls can be infinitely deep. So the dB span between a peak and the "non-supported" response should be 4 to 8 dB at most. Clearly, the nulls we see every day in graphs from REW and ETF etc are far deeper than that. In my "no traps" blue graph above, the span between the peak at 82 Hz and the null at 74.87 Hz to its left is 25 dB. So this proves those are legitimate nulls, not just the lack of a peak.

--Ethan

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Old 16th December 2010
  #57
SAC
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Ethan, in asserting that what i said was wrong, where do you get the idea that we have asserted that modal behavior and comb filtering are not caused by superpositional 'interference'.

The fundamental difference is that specular anomalies such as comb filtering tend to be characterized by destructive interference, where as nodes are reinforced by constructive 'interference'. The same energy reflected back in phase characterized by widely distributed spatial energy as opposed to more localized specular energy.

In trying to explain the differences in how modes and specular energy behave - and they do manifest themselves in a different manner, it serves little purpose to tell folks who state that they have little or no knowledge of physics and are lost with basic acoustical physics terms.

So I have attempted to simply describe the distinguishing features between the two instead of simply telling them that it is all a unified theory differentiated solely by topological boundary conditions!

And that is after stressing the overly simplified nature of all modal calculators - including those featuring FEM/BEM modeling.

And as I would suggest that no one contemplate designing their room as an anechoic chamber, suggesting that absorption can in some way some the problem as is done in an anechoic chamber misses the practical point. We already have far too many doing this without knowing why.

Personally, if we want to debate this, this should all be stated mathematically, and the logical descriptions be damned.

But as usual. on the one hand we are chastised by folks complaining that we use a confusing terms and on the other hand we are chastised for not representing a rigorous mathematical proof. And whereas one group wants a simplified non-technical presentation in order to gain a better grasp on the very basic fundamental concepts after complaining that our descriptions were too complex, the other group dutifully arrives to complain that we have overly reduced the full range of possible contributing factors that can cause nulls! And to think the original issue that was asked in the 2nd half of the thread was simply to explain the distinction between modal and specular behavior and how these models can be practically used.

And as most people do not relate to all constructive superposition as being interference and instead commonly reserve interference to mean destructive interference, I have no problem simply stating that in-phase reflections amplify the magnitude of the resultant while maintaining the integrity of the basic waveform.

And we have intentionally limited all discussion of frequency peaks and nulls to room reinforced energy. I am not about to mix signal alignment issues due to misaligned driver/loudspeaker acoustical origins/centers into this discussion! One issue at a time! It is confused enough as it is. To state that other factors can come to play in a given situation should be obvious. And nearfield issues such as SBIR - note the limitations stated regarding the ALL TOO COMMON use of modal measurements made using directional sources with microphones effectively placed in the near field!!!! Of course this can create destructive interference. And as such it does not constitute error, but rather a mistake in process.

The thread was not a comprehensive list of factors that can potential corrupt modal analysis - but simply a few trying to get a grasp on what modal behavior is!

Besides, the ETC easily identifies and allows for the correction of those, and more, issues.
Old 16th December 2010
  #58
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recordinghopkins's Avatar
What I am gathering is that even computers have a hard time calculating modal resonances of a room with more than 6 dimensions, yes? Much less actually doing it with a handheld calculator and pencil.

I suppose I was hoping to learn some new formula or matrix that allows the user to input the results of F=s/W for each dimension of the room.
Old 16th December 2010
  #59
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Ethan.

IF (big if) the nulls you're seeing are truly modal in nature.

Bryan
Old 16th December 2010
  #60
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Ethan Winer's Avatar
 

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Quote:
Originally Posted by SAC View Post
Ethan, where do you get the idea that modal behavior and comb filtering are not caused by 'interference'.
Where did I say that? In fact, I always say the opposite because this is so poorly understood. All peaks and nulls are caused by acoustic interference.

--Ethan

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