EQ Does Improve Bass Decay

EQ Does Improve Bass Decay​

I wanted to share some examples of how EQ does reduce ringing in the bass, or rather, improves bass decay. Why? Because it's minimum phase and it must. I'm including both simulations of the EQ effect, as generated in REW, as well as an actual measurement using DIRAC (which is still EQ, even if it's very special EQ). It shows that EQ does improve bass decay. This comes up a lot and I wanted to provide some solid results to make the point.

First, a few animations to show a comparison of the waterfall (what most are used to seeing). This is an actual speaker measurement but showing the simulated effect on the waterfall when EQ is applied to flatten the response.

Here is a similar animation showing what DIRAC did as compared to the No EQ result. This is not a simulation but a real-world measurement.

However this may only be somewhat interesting, as many might suggest that cutting the peak lowers the level of the peak, but decay is a slope not a level, so what effect does it have on the slope? Well, look for yourself. These are comparison of the impulse response filtered at 63hz (1/3 octave) which was around where a large peak in the untreated response showed up.

Look at the slope of that black line.

Here is the same information as above, but using just PEQ and not DIRAC.

And finally an overlay to show just how seriously it impacted decay. Look at the tail.

And again, same information as above, but using PEQ

EQ clearly improves bass decay and does so far better than reasonable bass traps could achieve at such low frequencies. That isn't to say that LF damping is not important in a room, but my view is you need both. Now keep in mind that in general EQ will not provide identical results (and thus identical improvement in bass decay) at different positions in the room. That is why multiple low frequency sources (i.e. subwoofers) and some bass trapping is still needed.

 

[ISharp]

EQ Does Improve Bass Decay​

I wanted to share some examples of how EQ does reduce ringing in the bass, or rather, improves bass decay. Why? Because it's minimum phase and it must. I'm including both simulations of the EQ effect, as generated in REW, as well as an actual measurement using DIRAC (which is still EQ, even if it's very special EQ). It shows that EQ does improve bass decay. This comes up a lot and I wanted to provide some solid results to make the point.

First, a few animations to show a comparison of the waterfall (what most are used to seeing). This is an actual speaker measurement but showing the simulated effect on the waterfall when EQ is applied to flatten the response.

View attachment 18730

Here is a similar animation showing what DIRAC did as compared to the No EQ result. This is not a simulation but a real-world measurement.

View attachment 18731

However this may only be somewhat interesting, as many might suggest that cutting the peak lowers the level of the peak, but decay is a slope not a level, so what effect does it have on the slope? Well, look for yourself. These are comparison of the impulse response filtered at 63hz (1/3 octave) which was around where a large peak in the untreated response showed up.

View attachment 18732
Look at the slope of that black line.

Here is the same information as above, but using just PEQ and not DIRAC.

View attachment 18733

And finally an overlay to show just how seriously it impacted decay. Look at the tail.

View attachment 18734

And again, same information as above, but using PEQ

View attachment 18735

EQ clearly improves bass decay and does so far better than reasonable bass traps could achieve at such low frequencies. That isn't to say that LF damping is not important in a room, but my view is you need both. Now keep in mind that in general EQ will not provide identical results (and thus identical improvement in bass decay) at different positions in the room. That is why multiple low frequency sources (i.e. subwoofers) and some bass trapping is still needed.

Hi, Matthew, my observations on this subject indicate the same conclusion. In one of my projects, acoustic optimization was not a key priority for the client. Nevertheless, the bass response with a mini dsp running Dirac improved the decay by reducing the duration by roughly 50% (800ms prior equalization vs 420ms after the alignment)

 

[Sledge]

Hi all,
in my oppinion sayin that EQ does improve decay is simply not truth as only speciffic strong modification of IR of the audio source is able to affect the decay, but usually in limited area in the room (in another area there would be even longer). The first gif showed that peak was reduced by EQ, but decay slope remains the same. For this observation normalized waterfall plot would be more practical....

It's just law of keeping the amount of energy (or what would be the english equivalent) - ince the room is excited by coustical energy, it is able to "store" the energy for a while. IIR EQ just reduce the initial amount of excitation signal but cannot affect the decay (only reduction in level makes the band less audible and subjectively dissappear earlier). DIRAC is able to affect IR of the source in that way, that in certain point convolution if the loudspeaker IR together with room IR results in faster decay. But actually the radiated energy will be a bit longer so the room will be excited with a more energy so in different area the decay must be even longer....

If somebody would have the oportunity, try to measure the response before and after dirac implemetation in the listening position AND in the corner of the room (where all modes has its node/maximum) and compare. I might be wrong, but I would expect that measurement in the corner will show even longer decay with dirac (of course it depends on the coupling/allingment with the listening position - some bands might be in phase so the decay reduction will be visible in the corner also)...

 

[DanDan]

Consider a 35Hz Length Mode. Narrow Eq to match the mode, -10dB 35Hz Q 5. The SPL at the speakers is reduced dramatically. The SPL at the opposite wall and corners is reduced dramatically. It is not increased anywhere.
In tandem with the level reduction there is an actual, albeit not big, increase in the rate of decay.
This has been described and proved ad infinitum elsewhere, but in short:- The speakers and room work together. When a strong mode occurs they are really synced tightly. An Eq applies tuned damping on the resonance. The shorter IR of the Eq/Speaker dampens the longer one of the Room/Speaker.

 

[Sledge]

There are two different phenomenons - increase of the SPL caused by untreated modal frequency, which can be reduced b EQ. But the other part - decay of the modal frequency, cannot be shortened by EQ. The IIR EQ only reduce the amount of energy radiated into the room, but the rate how fast the acoustical energy is transformed into heat, is totaly independent of the rloudspeaker as this is purely determined by the room parameters. Thanks to FIR processing you only can "invert" the reverberation in certain point in the room, but this is not somethin, that at least I would call "EQ" as this is very specific processing.... It is basically similar to eliminate reflections in the horn in the large PA loudspeakers...
Check the first picture here - the modal frequency arorund 120 Hz - the peak has been eliminated by EQ, but the decay slope remains the same.... The only examples where the decay rate has been changed, used DIRAC FIR optimization....

 

[AJ Soundfield]

(only reduction in level makes the band less audible and subjectively dissappear earlier).

Correct. Exactly what EQ does. As does using a gradient source. It's a sound power reduction. You may "see" different pressure maxima/minima at various spatial azimuth, but the power is reduced...and as you confirm, that is audible.

cheers,

 

[Jean Ibarz]

I tend to agree with Sledge.

Dandan, you say "In tandem with the level reduction there is an actual, albeit not big, increase in the rate of decay."

If IIR PEQ increases the rate of decay, why it is asserted the opposite ? (EQ Does Improve Bass Decay -we all understand Decay rate implicitly right ?-)

I also agree to get an amplitude normalized waterfall plot before EQ and after EQ, at different positions in a reasonable sweet spot in a room as a practical visualization of the data that support the initial assertion.

 

[DanDan]

Sledge:- "But the other part - decay of the modal frequency, cannot be shortened by EQ."

This is not correct. The rate of decay can be increased, the slope steeper, shortening the decay time.


Jean, I suspect a language thing. Increased rate of decay means a shorter decay.

 

[Sledge]

Sledge:- "But the other part - decay of the modal frequency, cannot be shortened by EQ."

This is not correct. The rate of decay can be increased, the slope steeper, shortening the decay time.
...

I feel here some misunderstanding - it was not said explicitely, but the measurement showed decay/reverberation in the room. Based on this, I was expecting that you all are all talking about improvement of the decay of the reverberation in the room, which cannot be reached by IIR, only FIR processing and only in the specific point in space when you XXXX- (Mod Edit) the IR of the loudspeaker in that way, that convolution of the IR of the loudspeaker and IR of the room will result in "optimalized decay" (and of course virtually shorter).
On the other hand decay of the loudspeaker itself, which is the matter of its IR, can be completely equalized by the EQ. But there is no doubt about this effect, at least between people aware of LTI systems etc...

 

[Jean Ibarz]

Well a lot of people here seems to be convinced by the theory. But why can't we have a pratical measurement with appropriate plots ? No one have this kind of measurements to be shared with mode resonances before/after EQing ?

 

[DanDan]

Sledge, please research the subject a lot more and desist from simply saying 'No it doesn't' or 'virtually shorter' The room cannot be separated from the loudspeaker driving it, there is physical feedback.
You are simply wrong. https://www.gearslutz.com/board/stu...mmended-software-filter-impulse-response.html
Mention of FIR and IIR is spurious distraction. Meyer pioneered this in Analogue. https://tcfurlong.com/wp-content/uploads/Meyer-Sound-CP-10-Rental-Manual.pdf

Jean, in the first linked thread you will find both theory and measurement of the phenomenon. Here is a fuller working. http://www.acousticfrontiers.com/201163hard-proof-that-equalization-kills-room-modes-html/

 

[DanDan]

Let's stick close to the content of Matthew's Original Post. Perhaps he will post the source .mdat so anyone here can use their preferred REW tools.
To all, the increase in Decay Rate is widely understood and measured. But let's face it, rarely large, as I said, at most 25% in a really optimum situation. But Eq reduces the absolute SPL at the problem frequency. On top of making it simply less audible due to prominence, the Decay tails are now push down into Fletcher Munson and Masking inaudibility. Add the 25% increase in the downward slope of the Decay, and we have a seriously cleaned up boom.

 

[Sledge]

Sledge, please research the subject a lot more and desist from simply saying 'No it doesn't' or 'virtually shorter' The room cannot be separated from the loudspeaker driving it, there is physical feedback.
You are simply wrong. https://www.gearslutz.com/board/stu...mmended-software-filter-impulse-response.html
Mention of FIR and IIR is spurious distraction. Meyer pioneered this in Analogue. https://tcfurlong.com/wp-content/uploads/Meyer-Sound-CP-10-Rental-Manual.pdf

Jean, in the first linked thread you will find both theory and measurement of the phenomenon. Here is a fuller working. http://www.acousticfrontiers.com/201163hard-proof-that-equalization-kills-room-modes-html/

Dandan,
I will try to study and practise more, but I cannot help myself in this - Matthew presented the statement on EQ made by DIRAC. The full name of this tool is Dirac Live, right? In its tech notes, they stated the followings:
"Minimum-phase and linear-phase room correction filters can’t physically optimize the acoustic impulse response in a room. At best, they can minimize problems caused by the application of a filter. Room-acoustic responses are non-minimum-phase, which is why Dirac Live uses mixed-phase correction."
link: source
I can miss the point, but it sounds to me like the opposite to what you are saying....

Another thing in which I am a bit sceptical, is the question of separation of loudspeaker and room. They are just two individual almost-LTI systems and at leats you can measure them separately. I can completely imagine, that there is a direct relation between loudspeaker and the room - if you put a loudspeaker in the room, depending on its placement, there might be a build-up in the transfer function caused by modal activity of the room. Depending on the bandwidth of the mode etc. it has the consequences on the transfer function, IR, ETC, etc. This part of the TF can be corrected by the EQ with the consequences on the decay rate. But similar work does movement of the loudspeaker outside the pertinent node of the modal activity....
But regardles of the loudspeaker EQ, the basic decay rate of the mode can be measured - e.g. by impulse source in the corner etc., and its lets call it natural decay rate is simply given by the rate of dissipation of the acoustical energy into heat. I can hardly imagine how an EQ can affect this....

Or I might be wrong with understanding of what do you mean - my english is not the best, especially the "conversation" related part. Maybe just the "improved rate" means the same as I have just described, but the description lacks some additional information that will explain the limits of the improvement and the relations to the rest of the physics directly involved....

Anyway, everybody is still learning

EDIT: PS: from the second waterfall from the initial post - even if there is no directly comparable level in all bands, note that the optimized waterfall has quite a signifficant increase in the late energy level, some modes/ringing are even new, like the marked 24 Hz, also 45 and 75 Hz (the second one band's level is almost comparable to untreated response, but now the decay almost stopped at approx 60 dB)...

 

[Jean Ibarz]

Going back to my request of being able to analyze a pratical example, can you tell me what EQ to use if I provide you an impulse response of a loudspeaker in my room ?

 

[DanDan]

Sledge, you are reading that particular piece of text wrongly, and it is badly written. They are Swedish. This is better http://diracdocs.com/on_room_correction.pdf
Moving a loudspeaker out of a modal hot spot will not reduce the rate of decay, only the level.
Please read the links provided

Jean, any Minimum Phase Eq, but to get a clear view I recommend using a room with an actually strong and isolated lowest mode.
Why do you want to repeat the tests seen in the links provided?

 

[DanDan]

Right, enough you guys. This ongoing misinterpretation and ' I don't think so' is unproductive and to be honest, boring.
If you wish to debate the hypothesis and tests in this thread, please go away and learn the subject. Come back if you think you have a credible case that the reality described is not happening. And wear a Mask.

 

[Matthew J Poes]

Dandan,
I will try to study and practise more, but I cannot help myself in this - Matthew presented the statement on EQ made by DIRAC. The full name of this tool is Dirac Live, right? In its tech notes, they stated the followings:
"Minimum-phase and linear-phase room correction filters can’t physically optimize the acoustic impulse response in a room. At best, they can minimize problems caused by the application of a filter. Room-acoustic responses are non-minimum-phase, which is why Dirac Live uses mixed-phase correction."
link: source
I can miss the point, but it sounds to me like the opposite to what you are saying....

Another thing in which I am a bit sceptical, is the question of separation of loudspeaker and room. They are just two individual almost-LTI systems and at leats you can measure them separately. I can completely imagine, that there is a direct relation between loudspeaker and the room - if you put a loudspeaker in the room, depending on its placement, there might be a build-up in the transfer function caused by modal activity of the room. Depending on the bandwidth of the mode etc. it has the consequences on the transfer function, IR, ETC, etc. This part of the TF can be corrected by the EQ with the consequences on the decay rate. But similar work does movement of the loudspeaker outside the pertinent node of the modal activity....
But regardles of the loudspeaker EQ, the basic decay rate of the mode can be measured - e.g. by impulse source in the corner etc., and its lets call it natural decay rate is simply given by the rate of dissipation of the acoustical energy into heat. I can hardly imagine how an EQ can affect this....

Or I might be wrong with understanding of what do you mean - my english is not the best, especially the "conversation" related part. Maybe just the "improved rate" means the same as I have just described, but the description lacks some additional information that will explain the limits of the improvement and the relations to the rest of the physics directly involved....

Anyway, everybody is still learning

EDIT: PS: from the second waterfall from the initial post - even if there is no directly comparable level in all bands, note that the optimized waterfall has quite a signifficant increase in the late energy level, some modes/ringing are even new, like the marked 24 Hz, also 45 and 75 Hz (the second one band's level is almost comparable to untreated response, but now the decay almost stopped at approx 60 dB)...

 

[Matthew J Poes]

Dandan,
I will try to study and practise more, but I cannot help myself in this - Matthew presented the statement on EQ made by DIRAC. The full name of this tool is Dirac Live, right? In its tech notes, they stated the followings:
"Minimum-phase and linear-phase room correction filters can’t physically optimize the acoustic impulse response in a room. At best, they can minimize problems caused by the application of a filter. Room-acoustic responses are non-minimum-phase, which is why Dirac Live uses mixed-phase correction."
link: source
I can miss the point, but it sounds to me like the opposite to what you are saying....

Another thing in which I am a bit sceptical, is the question of separation of loudspeaker and room. They are just two individual almost-LTI systems and at leats you can measure them separately. I can completely imagine, that there is a direct relation between loudspeaker and the room - if you put a loudspeaker in the room, depending on its placement, there might be a build-up in the transfer function caused by modal activity of the room. Depending on the bandwidth of the mode etc. it has the consequences on the transfer function, IR, ETC, etc. This part of the TF can be corrected by the EQ with the consequences on the decay rate. But similar work does movement of the loudspeaker outside the pertinent node of the modal activity....
But regardles of the loudspeaker EQ, the basic decay rate of the mode can be measured - e.g. by impulse source in the corner etc., and its lets call it natural decay rate is simply given by the rate of dissipation of the acoustical energy into heat. I can hardly imagine how an EQ can affect this....

Or I might be wrong with understanding of what do you mean - my english is not the best, especially the "conversation" related part. Maybe just the "improved rate" means the same as I have just described, but the description lacks some additional information that will explain the limits of the improvement and the relations to the rest of the physics directly involved....

Anyway, everybody is still learning

EDIT: PS: from the second waterfall from the initial post - even if there is no directly comparable level in all bands, note that the optimized waterfall has quite a signifficant increase in the late energy level, some modes/ringing are even new, like the marked 24 Hz, also 45 and 75 Hz (the second one band's level is almost comparable to untreated response, but now the decay almost stopped at approx 60 dB)...

a couple things.

you really need to educate yourself on the topic of acoustics before arguing with experts. You misunderstood what you read and provided it as proof.

to use a source form Dirac’s developer:

https://static1.squarespace.com/static/55c8a274e4b09cb562cd3ea0/t/5694b0c3d82d5e959756b637/1452585157921/on_room_correction.pdf

“that one of the most efficient mixed-phase equalizer implementations consists of a combination of IIR and FIR filters.”

which means that minimum phase or IIR filters should be a part of how a room is eqed. They are stating that as a whole, across the entire audio band, a system is mixed phase and IIR can’t correct the entire thing. But it’s an efficient way to correct part of it.

“(the combined room and subwoofer transfer function actually is well modeled by a minimum-phase system at such low frequencies.)“

here again you see my point as shown in my analysis. A room is well modeled meaning it generally is minimum phase at low frequencies and this can be inverted with only IIR filters. Since it IS minimum phase, it fully corrects both the amplitude and phase (this time) response errors.

nothing I’ve said or done in my little experiment is in contrast to the claims of Mathias in that paper. It is well accepted that rooms are typically minimum phase at low frequencies.

He also points out in that paper that a system can be safely eqed in the lower frequencies even if the system is mixed phase. The key is that you have to understand what the non-minimum phase behavior is and how it will react to the minimum phase inversion. For example, some rooms show a spatially robust Resonance within the listening area. This may be fine to eq with normal IIR filters. The inversion is likely introducing errors outside the measured spatial area, but If this area is at the extremes of the room, it’s of Little consequence.

finally, I must note that you do not seem to understand how to read a waterfall. Waterfalls have a noise floor limit and the style I used inevitably shows greater decay at ever lower frequencies because it fixes time even though the period of a wave increases at lower frequencies. In addition, noise floor in all rooms increases substantial at low frequencies. As such, at a certain point ridges that you see in the waterfall corrupt an accurate interpretation. What you saw as proof of non-minimum phase behavior is room noise. Nothing more.

I also want to note that leading experts in this topic reviewed the original post (which was not a forum post but part of a tech talk I wrote as staff for AV NIRVANA, and that other experts have reviewed it since. None came away concerned with what I did or said. Most complimented the thoroughness of my experiment to highlight what I did.

 

[Matthew J Poes]

Going back to my request of being able to analyze a pratical example, can you tell me what EQ to use if I provide you an impulse response of a loudspeaker in my room ?

The only Requirement to replicating what I did is that the system cannot be eqed ahead of time with all pass filters or FIR Eq. These both can create a mixed phase scenario at low frequencies that isn’t naturally there. At low frequencies the room and speaker are minimum phase but eq can be anything but if of the wrong type.

then when you eq the IR you must use either IIR or FIR minimum phase filters. A linear phase filter won’t work and will show what you have been claiming would happen.

you can simulate this using REW alone if you wish. Otherwise, if you want to actually apply the eq and measure the before and after, you will need to use an EQ software. I believe there are free ones listed on REW’s home page.

 

[Matthew J Poes]

Well a lot of people here seems to be convinced by the theory. But why can't we have a pratical measurement with appropriate plots ? No one have this kind of measurements to be shared with mode resonances before/after EQing ?

It’s exactly what I provided. You just don’t seem to see that. I’m really at a loss by this comment. A filtered impulse response about the area of a resonance before and after eq depicts the change in decay caused by the eq.

I also showed the results in a waterfall before and after. While some mis-read the noise as meaningful, it’s all there.

you are free to replicate this yourself. I’m not just convinced by theory. What I showed were real measurements in a real room with real speakers. Nothing was simulated or manipulated beyond the addition of eq. I even used two eq methods to prove the point.

 

[Matthew J Poes]

I feel here some misunderstanding - it was not said explicitely, but the measurement showed decay/reverberation in the room. Based on this, I was expecting that you all are all talking about improvement of the decay of the reverberation in the room, which cannot be reached by IIR, only FIR processing and only in the specific point in space when you XXXX- (Mod Edit) the IR of the loudspeaker in that way, that convolution of the IR of the loudspeaker and IR of the room will result in "optimalized decay" (and of course virtually shorter).
On the other hand decay of the loudspeaker itself, which is the matter of its IR, can be completely equalized by the EQ. But there is no doubt about this effect, at least between people aware of LTI systems etc...

hopefully from other post you understand that this statement is incorrect. Not only proven by the data I presented but discussed in theory by Mathias in the Dirac paper. Himself a proponent of mixed phase filtering. What you are saying is only true of a room and speaker above the transition zone where it is typically non-minimum phase. This entire article was dealing with room modes which are minimum phase.

 

[Matthew J Poes]

Correct. Exactly what EQ does. As does using a gradient source. It's a sound power reduction. You may "see" different pressure maxima/minima at various spatial azimuth, but the power is reduced...and as you confirm, that is audible.

cheers,

AJ I hope you Know what I showed here was fact not opinion. I’m not totally sure what you are agreeing with but most of what he has been saying is incorrect. It’s assuming rooms are non-minimum phase at low frequencies. That the all pass of the room remains detached at low frequencies. That is absolutely Not true in the vast majority of rooms. When IIR EQ is used to reduce the amplitude it also Effects phase and in this example increase the rate of decay causing a reduction in ringing at the ridge. Which my data clearly shows.

 

[AJ Soundfield]

I’m not totally sure what you are agreeing with but most of what he has been saying is incorrect.

I quoted the snippet of his post I'm in agreement with (he may have inadvertently stated), the rest being incorrect.
The snippet that appears to concur with your facts. Yes Matt, I know the difference between facts/opinion lol
EQ reduces audibility of excess modal excitement. I've used it for decades. Along with less excitement to begin with.
Go green, waste less power as heat.

cheers

 

[Matthew J Poes]

I quoted the snippet of his post I'm in agreement with (he may have inadvertently stated), the rest being incorrect.
The snippet that appears to concur with your facts. Yes Matt, I know the difference between facts/opinion lol
EQ reduces audibility of excess modal excitement. I've used it for decades. Along with less excitement to begin with.
Go green, waste less power as heat.

cheers

Hah well that’s a new spin.

I actually have recently come around to that idea. At least as it pertains to boundary interference effects.

ok well sorry for accusing you of anything. I am just frustrated by this thread.

 

[Sledge]

Dear Gentleman,
thank you for your effort, I have learn something (mostly that I have to improve my english) - mostly from the original documents linked in the linked threads (but available elsewhere), as those materials contain more or less complete information (with respect to the scope) and not just links to another links to another links or axioms.... My "opinion" of non-working correction seems to be faulty (but I never excluded this possibility), I only cannot see the way how modification of TF of the loudspeaker can overcome insufficient acoustical damping of the room (and I did wrote this)...unfortunately the most detailed explanation here was "it can", which really didn't help with better understanding.

Matthew - one wish - could you please find, where I wrote that something on the waterfall is "proof of non-minimum behaviour" as you wrote and used it as a proof that I cannot understand the waterfall plots?
DanDan - could you please describe your concept of direct physical feedback from the room back to the loudspeaker? E.g. how the speaker is affected by the presence of the room (impedance response od close-field SPL)?

Last question to Matthew - does the parametric EQ correction work for all room modes - e.g. does the minimum-phase EQing work with the comparable efficiency to the axial mode e.g. 42,5 Hz in the room 1 with the length 4 m as for the room 2 with the length 12 m? Are there any limits of usability of this correction (not the whole Dirac) with respect to room dimensions (lets keep it related to the wavelength for higher simplicity)? Because your article didn't mention any limits of applicability except the "ringing in the bass" statement....

Thank You

 

[DanDan]

Sledge, the title of the thread suggests it's purpose, i.e. to show that Eq does improve Bass Decay. It is not a question. It is thread on a forum, open to confirmation, exploration. Denial on the basis of theory or test would even be welcome. But overall this thread has been poisoned by people saying :- I don't think that is possible..... Prove it to me...... Explain how it happens.... No way.
I don't see any point at all in such intrusions. If anyone wants to learn the principles involved they are well published and widely available. If anyone doesn't believe the well and widely published tests, do your own.
The various linked articles, threads and papers describe how and why. The best of them tend to be Math and Physics heavy. This is why I introduced my simpler analogy of the shorter resonance of the Speaker/Eq combo riding on the back of the Room/Speaker combo resonance. Damping it to a halt a little sooner. Consider how a speaker acts when Soffit Mounted. Much more Bass with much shorter cone excursion. Room and Speaker are an interactive combo. Consider also that speakers can be used as microphones.....
That's my hypothesis fwiw.
Another more expert perspective from JohnPM in the linked GS thread:- "It is important to recognise that EQ filters do not modify "the room", they modify the Transfer Function from the point where the speaker is to the point where the mic is. Between those points the room acts like a filter, is it really such a leap to realise that one filter can alter the effect of another? "
I would have liked to see more posts like your last question to Matthew. I think it valid to tease out how useful or not this is and in what circumstances. Again, John Mulcahy explains how to identify if and how well it will work in the REW manual. P57 Minimum Phase.... https://www.roomeqwizard.com/REWhelp.pdf But again in my simple terms:- It works best at the lowest mode, a lucky coincidence as that one is the hardest to treat physically.