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#### JimP

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What does .5 at 50 Hz mean? Reduction by 50%??

#### Rod Gervais

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Quote:
 Originally Posted by JimP What does .5 at 50 Hz mean? Reduction by 50%??
Jim,

Absorption rates (measured in sabins) are based on the energy striking a 1' square surface - with 0.0= 100% reflection - and 1.0 = 100% absorption.

So a .50 would mean a reduction of 50% per square foot (of installed absorption) in a particular frequency, as a percentage of the total surface area of the room.

Rod

#### ---k---

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Quote:
 Originally Posted by Ethan Winer So I'm sorry if I get a little mad sometimes, but I think I'm entitled given the years these insults have been going on for, with no apology after I have proven the insulters wrong time and time again. --Ethan

#### Ethan Winer

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jim,

> What does .5 at 50 Hz mean? Reduction by 50%?

#### JohnPM

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Hello Ethan,

I'm afraid you are coming to a wrong conclusion based on the plots you showed in post #352. These don't show that EQ cannot reduce ringing (they successfully illustrate EQ doing just this at one frequency), unfortunately they show what happens when the EQ settings are incorrect. Perhaps this reinforces your point that it is far from easy to apply EQ correctly, though I am surprised that Terry used Smaart to adjust his EQ as from the typical plot he posted I've no idea how he could even get close.

As the ETF measurements are full range the impulse responses can be exported as pcm files and imported to my EQ app, with which it is a little easier to make measurements. Here is a plot showing the empty front center measurement, with the EQ filter response overlaid plus the predicted effect of the EQ (the lighter red trace). The EQ response is drawn inverted to make it easier to match the EQ to the peaks it should correct.

http://homepage.ntlworld.com/john.mu.../centre-eq.jpg

The only filter whose centre frequency closely matches a peak in the measured response is the one at 140.8Hz (filter 6), and as the plot shows it is a good match in bandwidth also. The actual frequency of this peak is approx 139.7Hz, so the filter is not an exact match, but it is within 1% so works reasonably well. The effect on decay at this frequency is shown in these spectral decay plots, the empty room is the upper plot, the response with EQ the lower:

http://homepage.ntlworld.com/john.mu...ecay-no-eq.jpg
http://homepage.ntlworld.com/john.mu...ay-with-eq.jpg

The plot with EQ is generated from the EQ-both speakers front center ETF data, which I trust is the correct file - there is an extra mode at 51Hz which was not present in the empty room response, which is a little odd.

The mismatched centre frequency of filter 6 causes a slight dip to the right of the mode's actual frequency. Despite this, the drops between the 80, 120, 160 and 200ms slices are 2.0, 3.4 and 4.7dB without EQ but 3.6, 6.7 and 11.0dB with EQ.

The plots also show why the 39Hz peak was not corrected by filter 2, as the peak is due to two modes at 37 and 40.5Hz rather than a single mode. The filter at 39.5Hz falls between the two modes and is too narrow, the 37Hz mode is unaffected though the 40.5Hz mode is partly reduced, the main effect is to introduce a dip at 39.5Hz. The 69Hz filter (#8) is 2% away from the mode's actual frequency of 70.6Hz which is too far to bring much benefit. I'm not sure what filter 7 (153Hz) is aimed at as the nearest mode is 159Hz, but I haven't looked at the other data sets so perhaps it addresses a peak elsewhere. A pity, as the 159Hz mode should respond very well to a properly matched filter. Filter 9 (205Hz) is 2% away from the 201Hz mode so again brings little benefit. It is also a little too broad, but modal resonances tend to have bandwidths of a few Hz (about 4Hz for that mode) and hence tend to be too high in Q once the frequency reaches 200Hz or more - most equalisers do not offer sufficiently narrow bandwidths to usefully treat modes beyond 150Hz or so.

I would like to point out that I am not posting this as an argument of EQ over room treatments, there is much that can be achieved with room treatment that is impossible with EQ. However your belief that EQ cannot address modal ringing is wrong, and I do not think you would wish to use incorrectly interpreted data to assert that.

Regards,

#### bpape

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Making my point exactly John. To get the EQ EXACTLY correct - even for 1 seat, takes serious EQ and lots of setup and analysis. EQ is bandied about by some (not anyone particular or anyone in this thread) as a much 'easier' way to deal with frequency response. What shows above and before is that not done EXACTLY correct can cause as many problems as it helps. All that AND the fact that this is all for just one single location. When you move to the 2nd, 3rd, 4th, etc. seats in a real room, there is no way you can match exactly the inverse of all of the responses because the response is different at each seat.

#### Rod Gervais

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Quote:
 Originally Posted by bpape Making my point exactly John. To get the EQ EXACTLY correct - even for 1 seat, takes serious EQ and lots of setup and analysis. EQ is bandied about by some (not anyone particular or anyone in this thread) as a much 'easier' way to deal with frequency response. What shows above and before is that not done EXACTLY correct can cause as many problems as it helps. All that AND the fact that this is all for just one single location. When you move to the 2nd, 3rd, 4th, etc. seats in a real room, there is no way you can match exactly the inverse of all of the responses because the response is different at each seat.
Not only is the response different at each seat - but for any response that's modal - it's physically impossible to chage the settings without screwing up what we already worked to achieve. In fact - even if there were problems with different frequencies than the initial center seat - any adjustments made on ANY frequency will affect that seating location as well.

And, introducing a new set of filters with a totally different set of speakers (intended for seat 2) is impossible without (again) screwing up that first seat we already adjusted for.

Guys, I have been quietly working through this in my head ever since becoming involved in the thread......... and I have to tell you - although I know in theory how this works - and I do believe that it's possible - I think from a practical point of view - it doesn't work. And from the impractical view - you aren't going to get there without a lot of equipment much more sophisticated than anything put out by Behringer for a couple of hundred bucks........

Now if you go back to computer generated solutions - you could actually analyze the signal at each listening position - compare that analysis with the original signals - generate a series of frequency pulses either in sysnc or out of sysnc in a localized manner - to affect ony what was heard in each individual seating area - correcting for any modal or non-modal phenomenon - probably including flutter echo,comb filtering, stereo imaging, etc.

However - I have to believe, seeing as I have 2 broithers who are computer scientists - one who's worked with the Cray computers inside of NSA (and was rated one of the top 100 super-computer scientists in the world by the NSA)- and having discussed this with them - that we could probably treat a room a lot cheaper than we could deal with it otherwise...........

Sincerely,

Rod

#### myfipie

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Quote:
 Originally Posted by Rod Gervais Not only is the response different at each seat - but for any response that's modal - it's physically impossible to chage the settings without screwing up what we already worked to achieve. In fact - even if there were problems with different frequencies than the initial center seat - any adjustments made on ANY frequency will affect that seating location as well. And, introducing a new set of filters with a totally different set of speakers (intended for seat 2) is impossible without (again) screwing up that first seat we already adjusted for. Guys, I have been quietly working through this in my head ever since becoming involved in the thread......... and I have to tell you - although I know in theory how this works - and I do believe that it's possible - I think from a practical point of view - it doesn't work. And from the impractical view - you aren't going to get there without a lot of equipment much more sophisticated than anything put out by Behringer for a couple of hundred bucks........ Now if you go back to computer generated solutions - you could actually analyze the signal at each listening position - compare that analysis with the original signals - generate a series of frequency pulses either in sysnc or out of sysnc in a localized manner - to affect ony what was heard in each individual seating area - correcting for any modal or non-modal phenomenon - probably including flutter echo,comb filtering, stereo imaging, etc. However - I have to believe, seeing as I have 2 broithers who are computer scientists - one who's worked with the Cray computers inside of NSA (and was rated one of the top 100 super-computer scientists in the world by the NSA)- and having discussed this with them - that we could probably treat a room a lot cheaper than we could deal with it otherwise........... Sincerely, Rod
Rod I must say this is the best post so far on this thread... It really comes down to one thing... Enjoy the sound!!!!!!!!

Glenn

#### Savant

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I've scanned this thread and, in general, I'm very glad the room treatment versus corrective EQ thing is being addressed at such length.

Having said that, I'd like to risk stirring things up in light of the last series few posts...
Quote:
 Originally Posted by Ethan Winer This started three years ago with non-modal peaks and nulls (basic comb filtering) that a bunch of ivory tower types insisted cannot exist at low frequencies.
No one ever insisted that comb-filtering could not exist at low frequencies. The argument came down to semantics. What is described by you as "non-modal peaks and nulls" is the well-documented phenomenon known as "speaker-boundary interference" (which I believe you also prefer to call "listener-boundary interference" - again, semantics).

Quote:
 Then it was the density of rigid fiberglass versus its performance at very low frequencies.
This sort of performance depends on more than density. But if only density is considered, the results are well-documented elsewhere - some of it by you, if I'm not mistaken? I suppose that's kind of your point!

Quote:
 Next it was loudspeaker radiation versus frequency as relates to treating the entire front wall with thin absorption.
This depends on what frequency (or range) is being considered and how "thin" the absorption is. I have treated a front wall null quite well in the past with 2" thick acoustical foam. Other times, 3" or thicker was necessary.

Quote:
 Now these "trained acousticians" claimed EQ can cancel modal ringing, and I proved that wrong too.
The only note I would add here is that some "room correction" systems - Bag End and Meridian come to mind - are using electronic damping as opposed to EQ. The former can be quite effective. The latter is well-documented here and elsewhere as being limited in its effectiveness.

******

I'd also like to clarify some other points:

â€¢ The absorption coefficients that are typically published for acoustical materials are found using the reverberation chamber method. This method yields random incidence absorption coefficients, which are not percentages. Normal incidence absorption coefficients are percentages. The two are often confused in the literature. And, it would seem, here as well! A material that has a random incidence absorption coefficient of 1.22 is simply a better absorber relative to a material with a random incidence absorption coefficient of 0.67 for the same frequency band, all other factors being equal. The numbers should not, however, be treated as an indicator of the percentage of sound absorbed by the material.

â€¢ Along the same lines, the SPL decrease based on percentage sound absorbed for random incidence absorption coefficients is not as simple as described above. In fact, SPL reduction due to absorption cannot typically be calculated accurately in small rooms like studios since most positions in the room will be in the near-field of the loudspeaker (or other sound source). However, if the far-field can be reasonably assumed, the amount of SPL decrease can be estimated using standard formulae. In this case, the result is relative to what the initial absorption of the surface is without the absorber present. Thus, one application of a material with a 0.50 absorption coefficient could reduce SPL by less than 1 dB. Another application could yield a >10 dB decrease. In most real-world applications of absorbers, SPL is reduced by ~5-10 dB over the range of effectiveness of the absorber. For early reflection applications in studios, greater than 10 dB of reduction is not uncommon - even for absorbers with seemingly "low" absorption coefficients. However, this is beginning to mix concepts since the random incidence absorption coefficient is not at all indicative of an absorber's behavior when a single bounce from one early reflection is considered as opposed to randomly incident sound.

#### noah katz

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Ethan,

"I think I'm entitled given the years these insults have been going on for, with no apology after I have proven the insulters wrong time and time again."

Curious if I count as an "insulter", as I can't recall anyone behaving that badly.

Perhaps anyone who continues to disagree with your misconceptions is in that category?

I, as well as Terry, gave several reasoned statements that illustrate that slap echo is not intrinsically a modal phenomenon.

You haven't countered any of the reasoning, you simply make further arguments, that while not false, in no way support your point.

But you'll be glad to know that I won't bother engaging you in any further discussions.

#### bpape

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Great post Rod. Thanks for expounding further on what I was trying to say. It's just another one of those things where in theory it could work VERY well and in practice it just doesn't.

Before anybody gets their shorts in a wad here, I'm not saying that EQ doesn't work. I'm just saying that you can't get EVERYTHING with EQ only.

#### JimP

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Would it be accurate to say that you can't get everything with EQ any more than you can with accoustical treatments? That they're problems that neither can correct. The trick to to find out before you spend any money what can and can't be done??

#### Savant

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Jim,
Quote:
 Originally Posted by JimP Would it be accurate to say that you can't get everything with EQ any more than you can with accoustical treatments? That they're problems that neither can correct. The trick to to find out before you spend any money what can and can't be done??
From my experience, I would say that thorough acoustical treatment of a room can render something like EQ unnecessary. However, most theater rooms are not going to be able to control all the modal peaks below 50 Hz. Thus, something like electronic damping, or even a good digital EQ can help tweak the lowest octave or so.

IMO, EQ is a tweak that should only be used in rooms where the acoustical problems have been addressed as much as possible. In most of the HTs I've tuned and tweaked, it was debatable whether EQ would have helped that much for the lowest octave or so. In most, I've done without it as any attempt at implementing it would only either make the sound in general worse (subjectively), or it would fix a problem at one seat or area, but not affect anything or make things worse elsewhere in the room.

I haven't come across a room problem yet that wasn't completely treatable with well-thought-out acoustical treatments. But I can think of possible applications for systems like the Meridian and the Bag End I mentioned above.

#### Rod Gervais

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Quote:
 Originally Posted by JimP Would it be accurate to say that you can't get everything with EQ any more than you can with accoustical treatments? That they're problems that neither can correct. The trick to to find out before you spend any money what can and can't be done??
Jim,

No it would not be accurate to say that. I've built recording studios that (when completed) needed no treatments outside of what was in the original design.

A properly designed room - with treatments taken into consideration as a part of the design, can achieve everything that's needed - something that cannot be claimed with EQ. Leaving EQ to only deal with speaker issues - not room issues.

If I were to strip the walls of those studios to bare drywall - you could never replace those treatments through the use of EQ.

Thus - one approach can handle everything - and one approach cannot.

That having been said, I will grant you that there are rooms that exist that cannot be turned into a home theaters, with perfect sound, through the use of treatments - some rooms are just impossible from that point of view. But, even in those rooms, treatments alone will get you to places you never could with EQ alone.

Rod

#### Eyleron

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Can anyone come up with evidence demonstrating a response trough / peak and ringing that was left AFTER corner treatments, that was fixed with EQ? Especially EQ that fixed it for multiple seats?

If one has as the primary seating 3 seats next to eachother in a row, a length mode would affect each seat the same, wouldn't it? So EQing that problem would fix it for those 3 seats, which might be "good enough".

#### GGA

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"The only note I would add here is that some "room correction" systems - Bag End and Meridian come to mind - are using electronic damping as opposed to EQ. The former can be quite effective. The latter is well-documented here and elsewhere as being limited in its effectiveness"

Could someone please explain what "electronic damping" is? Thank you.

#### bpape

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Eyleron.

Your thinking is correct. Unfortunately, it's rarely that cut and dried that all of the issues are purely length related. If it was then that might work. Also, in many theaters there are more than one row in which case even that can't do everything.

The idea, as Rod pointed out, is to deal with the overall issues via appropriate placement of seating, speakers, sub, etc. first. Then deal with the room as much as possible via treatment. After that, if you still have issues that can't be resolved for whatever reason (spousal approval, lack of appropriate mounting positions, budget, whatever) via treatment - then you can look at the response at all of the seats and see where you can make changes that will help all of them.

#### Rod Gervais

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Quote:
 Originally Posted by Eyleron If one has as the primary seating 3 seats next to eachother in a row, a length mode would affect each seat the same, wouldn't it? So EQing that problem would fix it for those 3 seats, which might be "good enough".
Eyleron,

Picture, you have length - height, width - for axial modes - then any combined 4 surfaces for tangential modes, combined 6 surfaces for oblique modes - and we haven't even begun speaking to back wall interference, first reflections, etc.

So even if you could cure for one condition at a seating position as you suggest - it wouldn't be (i.e.: couldn't be) "good enough" if one is looking for anywhere near accurate sound reporduction.

Then again - "good enough" is a subjective term - as I mentioned very early on in the thread.......... for some people no treatments or eq is "good enough" - but for anyone with a discerning ear - it just doesn't work.

Rod

#### Ethan Winer

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John,

Thanks for your truly fabulous post. I do not disagree with any of it. Just to be clear, I am not arguing that an EQ could not be designed to counter ringing for one location in the room. I understand that in theory this is possible. But if an expert like Terry is unable to make it happen with a sophisticated EQ like the one he used, then it's just not practical as a solution for us mere mortals. More to the point, the precision counter-EQ needed to reduce ringing (not just help the raw LF response) will be valid for only one very tiny place in the room. Move even a few inches, or just turn your head, and I'm pretty sure all of the ringing will return. So even if there were such a thing as an auto-adjusting EQ that dials in the exact inverse response, it will help too small of an area to be useful for reducing ringing.

--Ethan

#### Ethan Winer

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Jeff,

> No one ever insisted that comb-filtering could not exist at low frequencies. electronic damping

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