Wanted: Proof of multiple subs and sub EQ

[Purité Audio]

Thanks you, I should have been more explicit , 'large dipolar diaphragms' rather than dipoles .
Above 40-50 Hz makes perfect sense for gradient bass.
When you mention 20dB less bass directed behind the speaker you are referring to cardiod design?
Thanks again for walking me through all this, I do appreciate it.
BW Keith

 

[DonH56]

See we have problems starting at 700hz, where a 6-8" cone/narrow baffle is near omni? Sources become localizable above 80-90hz, so you cannot use multi-sub to fix these peaks/nulls above 80-90Hz.
Ok, so what to do, EQ? Well, peaks can be pulled down but EQ cannot fix nulls, which are energy storage issues at that point where the mic is, at the listening position. If you attempted to add 8db of boost at 480hz to fill that hole on axis, off axis, you would have an 8db peak coloration of the reverberate. Not good!
Hmmm, now what to do? Call Ghostbusters? Call Ethan or Don for some mega pillows behind the speaker and elsehere? Well...you could.
Or you could put 4.8-6db less power into the room, up to 20db less directed at the wall behind the speaker, all with the exact same on axis "onset" FR as the box. Gradient bass. As Toole said, EQ cannot fix speaker directionality. I'm sure Amir will agree once he sobers up.

cheers,

AJ

This is nuts. I have never said EQ can fix nulls; I have posted many times it cannot. Nor have I ever said absorption will fix on-axis nulls, that is ridiculous. Absorbers can reduce nulls caused by boundary reflections when appropriately placed to control (reduce) the reflected waves that cause the cancellations. Absorbers are far less effective below 50 to 100 Hz (depending upon their design, absorption coefficients are available on-line or from the manufacturer) and so won't do much below that no matter the cause (they will do a little, but as I posted elsewhere even with a bunch of panels I only saw a slight improvement). Fixing a null due to the room usually involves moving the listening position out of the null or placing an active source (subwoofer) to mitigate it. And I never said anything about using a sub to fix nulls above the transition point. You should be a politician; I have rarely seen an engineer as talented in twisting intent, manipulating words, and denigrating others.

 

[AJ Soundfield]

This is nuts. I have never said EQ can fix nulls; I have posted many times it cannot. Nor have I ever said absorption will fix on-axis nulls, that is ridiculous. Absorbers can reduce nulls caused by boundary reflections when appropriately placed to control (reduce) the reflected waves that cause the cancellations. Absorbers are far less effective below 50 to 100 Hz (depending upon their design, absorption coefficients are available on-line or from the manufacturer) and so won't do much below that no matter the cause (they will do a little, but as I posted elsewhere even with a bunch of panels I only saw a slight improvement). Fixing a null due to the room usually involves moving the listening position out of the null or placing an active source (subwoofer) to mitigate it. And I never said anything about using a sub to fix nulls above the transition point. You should be a politician; I have rarely seen an engineer as talented in twisting intent, manipulating words, and denigrating others.

I never said you said any of that, just that you used absorbers behind the speakers. 'Tis all. Ok, I called them mega pillows, but that was only in jest.
I'm saying if you don't want energy directed that way, then don't. Real puzzler why you got dipoles if the rear energy is bad?
Never considered cardioids?
Politician?? Now that's low Don. And I told I'm not an engineer, but I play one on TV.

cheers

 

[amirm]

Just so that I am clear,
AJ would you say that 'generally ' you don't believe that first reflections should be absorbed, that they can add to a sense of spaciousness, comb filtering isn't harmful . But you would use multiple subs to smooth response and
use EQ to tame low bass ringing?
Amir/Ethan you are arguing that first reflections should be absorbed because you will perceive a more precise image ?
Keith

Bass correction and first reflections are completely different topics in acoustics. The former is one of getting smooth response and reducing ringing, the latter about complex science of psychoacoustics as applied to room reflections.

On room reflections, AJ's casual read of the topic has confused the thread. Not all early reflections are beneficial. The only ones shown to be so are the side reflections. They cause the image of the loudspeaker to shift toward the side wall, creating a subtle but pleasing effect. Other early reflections like floor are not good at all. They cause colorations (Bech 1998, Clark 1983 -- see http://www.audiosciencereview.com/f...ptual-effects-of-room-reflections.13/#post-15). Likewise front and back wall reflections do not contribute positively.

And yes, Ethan and a number of other acousticians argue that no reflection is a good reflection. And that the early ones are especially bad.

Edit: just to be clear, I do NOT agree with Ethan. I am of the school of Dr. Toole here. See the link above on why side reflections are not the ugly things we measure. What AJ doesn't understand is that we don't have a universal truth here. Some people do develop sensitivity to reflections where they truly become problematic for them. They can actually hear reflections as separate events rather than joint ones that we all do. The class of people who tend to do that are musicians and recording engineering/mixing types. And that includes Ethan. We can say Ethan's advice is not sound to typical audiophiles. But we cannot say Ethan has the same preferences as others. AJ simply is wrong about universality of this.

 

[amirm]

He's been shown repeatedly that multiple subs + EQ = smoother frequency response in multiple locations/seats.

He's been shown the "ringing reduction" at one location. He says great, fine, but I want to see this reduced ringing for other locations, not just the one.

That's right. And 11 pages later, you keep arguing other topics. Just answer his darn question. Quit agitating.

 

[amirm]

I never said you said any of that, just that you used absorbers behind the speakers. 'Tis all. Ok, I called them mega pillows, but that was only in jest.
I'm saying if you don't want energy directed that way, then don't. Real puzzler why you got dipoles if the rear energy is bad?
Never considered cardioids?
Politician?? Now that's low Don. And I told I'm not an engineer, but I play one on TV.

cheers

AJ, first of all that line at the end is not funny -- just annoying. Develop a real sense of humor if that is what you are attempting to do.

Second, if you agreed with what Don wrote, then a nice response should have come back. And regardless, none of this related to Ethan's OP. I won't tolerate you clinging to people's pant legs and not letting go. You can do that in a number of other forums you have been in, not here.

 

[AJ Soundfield]

Edit: just to be clear, I do NOT agree with Ethan. I am of the school of Dr. Toole here. See the link above on why side reflections are not the ugly things we measure. What AJ doesn't understand is that we don't have a universal truth here. Some people do develop sensitivity to reflections where they truly become problematic for them. They can actually hear reflections as separate events rather than joint ones that we all do. The class of people who tend to do that are musicians and recording engineering/mixing types. And that includes Ethan. We can say Ethan's advice is not sound to typical audiophiles. But we cannot say Ethan has the same preferences as others. AJ simply is wrong about universality of this.

I said no such thing. I said the opposite, that if heavily treated iso-rooms make an audiophile-studiophile happier, perfect!
But, like Toole, I'm also aware that even those folks may be in for a bit of a surprise with what they believe vs what they may actually hear:

Figure 8.2 in my book shows some results from Kishinaga et al.(1979) in which acoustical engineers expressed a preference for reduced sidewall reflections for evaluating audio products, but normal sidewall reflections for "fully enjoying the music". Kuhl and Plantz (1978) reported that audio professionals had a preference for a strong direct sound field for mixing, but for more reflections when listening at home.


Some "audio professionals" prefer strong direct sound fields for mixing but more reflections when listening at home.

My own investigation of the importance of loudspeaker directivity as revealed by the energy in lateral wall reflections (Toole, JAES 1985 and section 8.2.1 in my book) examined details of stereo soundstage and sound quality. The principal conclusion was that the recording technique is likely to be the prime determinant of directional and spatial impressions in stereo listening. The loudspeakers and the lateral reflections they generate are factors, but not consistent factors. Differences in soundstage and imaging qualities were thoroughly interrogated and found to be similar in all configurations. The effects of early lateral reflections on sound quality were most revealed by monophonic (hard-panned) components in stereo recordings or simply listening to a single loudspeaker.


Michelson and Rubak (AES preprint 4472, 1997) examined the recording techniques that influenced perceptions of distance in stereo recordings. They did the subjective evaluations in an anechoic chamber and in an IEC 268-13 room (a typical domestic room) and found no differences attributable to the room. One would think that this is a dimension of auditory perception with a degree of sensitivity to extraneous reflections − apparently not.


Choisel (2005, PhD Thesis, U. of Aalborg) interrogated stereo listeners on precision of sound image location when sounds were delivered by a very-wide, 180°, dispersion loudspeaker (B&O Beolab 5) or a normal, narrower-dispersion loudspeaker (B & W 801N), when side walls were reflecting or absorbing. The conclusion was that the perceived directions of images were unchanged by any of the manipulations, even though they resulted in large measured changes in reflected energy.


Prompted in part by my book, staff and students in the McGill U. tonmeister program did an elaborate blind-listening study evaluating the ability of professional sound mixers to perform with randomized reflective, diffusive and absorbing side walls. In two different control-room situations, they found that the mixers adapted quickly to all of the sidewall variations and just got on with the job. Any changes in the mixes were minor, and not statistically significant. Individual preferences among the mixers included all surface treatments, reflective, diffusive and absorbing, about equally (King et al., JAES, 2012).

Ethan may indeed prefer treated room sighted, but that's about all we can say.
As far as the multi-sub EQ benefits vs single sub "bass traps", there really isn't an argument there, that train has long left the station.
Anyway, aren't you supposed to be enjoying Axpona?

 

[AJ Soundfield]

That's right. And 11 pages later, you keep arguing other topics.

That was Devious Don that threw the paper ball at you while your back was to the classroom facing the chalkboard. Quit blaming the wrong kid!

For me panels do reduce bass resonances but of course their absorption falls off rapidly so even with a bunch of panels a 30 Hz resonance was knocked down "only" about 3 dB. Significant but not a huge audible change. Their main help was to reduce some higher-frequency modes and kill the back wave of my Maggies so the image is much more stable (I know AJ disputes that; all I know is the measured FR is much better and image much more stable to me and a few friends who have heard them both ways).

Then Don points the finger at and blames me when you turn around!
I didn't say half the stuff he accused me of, just pointed out that he also apparently believes in heavily "treated" room, obviously for well beyond bass issues.

 

[Thomas savage]

Surly audio/mastering engineers, sound mixers and a like have to eliminate the variables when performing their duties? That's a very different environment and set of goals than the home listening environment.

Further more I would not think music is produced with a listener in mind that's in a heavily controlled reflection free environment.

 

[Purité Audio]

So the recording/mastering is chiefly responsible for 'soundstage' , if I recollect correctly Toole states that reflections in a domestic room are too weak to recreate the original acoustic and that you would have to use multiple channels ( and sympathetic recording) to recreate the venues original acoustic?
Keith

 

[AJ Soundfield]

Surly audio/mastering engineers, sound mixers and a like have to eliminate the variables when performing their duties? That's a very different environment and set of goals than the home listening environment.

About engineers believing treatment is needed to reduce variables, studies showing surprising results. I am serious. And don't call me Surely.

 

[Thomas savage]

About engineers believing treatment is needed to reduce variables, studies showing surprising results. I am serious. And don't call me Surely.

Umm, so your saying they adapt but can just as easily adapt back... Well the study says that?

I though on Sunday's you went by Shirley?

 

[Wayne A. Pflughaupt]

Nyal stands by his analysis and results. Wayne does not object because we did not see any reply from him.

Sorry ‘bout that, I have a hard time getting motivated to post in these “dog fight” threads, but I guess I’ll go ahead and give it a go. So strap yourself in - this isn't the Reader's Digest version.

Referring to your previous post on Page 8:

Bumpit

• http://www.acousticfrontiers.com/20...ow-to-use-parametric-eq-to-flatten-your-html/

"The results of using multiple subs can be spectacular – near flat frequency response, little modal ringing and low seat-to-seat bass quality variability in multi-seat theaters."

♦ The first link above (Nyal's blog); Ethan, Wayne...care to comment? ...A subject that we we're all interested in.

I’m afraid that I’ll have to take Acoustic Frontier’s claims about ringing with a grain of salt, because I can tell from statements and graphs in their articles that there is a failure to fully understand the basic relation between ringing and signal levels.

Indeed, there seems to be no shortage of confusion and misinformation surrounding the subject of low frequency ringing, how to address it, and even more so, determining if has been improved. Even among those I would expect would/should know better.

For example, over at Home Theater Shack you’ll find an old thread on waterfall graphs that runs for 17 pages. A thread doesn’t go on for 17 pages unless there is a lot of debate over conflicting ideas and interpretations, not to mention people just trying to get a handle on the topic. And I’d have to include myself in that category as well, at that time.

The person opening the thread, otherwise a really knowledgeable and sharp fellow, presents a loopback waterfall graph of an equalizer with a boosted filter, claiming that it is proof that boosted filters result in ringing. While there is certainly truth to that, he overlooks the obvious fact that one would only introduce a boosted filter to a depressed area that displays a reduced decay time to start with (assuming good equalization technique, but that’s a subject for another thread).

I’ve also seen threads with people who said they were happy with the audible results they achieved after equalizing their subwoofer, but were disappointed that it didn’t also result in reduced ringing. I’ve seen others who claimed that equalization introduced ringing after boosting at 20 Hz to compensate for subs that had reduced output that low, completely ignorant of the fact that increasing gain will make time-domain graphs like waterfalls appear worse than before.

First, let's clarify a couple of definitions. In the following discourse, when I refer to "decay time" I mean the time it takes a signal to fade away, relative to its gain or loudness. For example, an 85 dB signal will quite naturally fade down to the noise floor sooner than a 100 dB signal.

When I refer to "rate of decay" I mean the speed at which the sound decays – what's commonly known as reverberation time or RT60: If we have 85 dB signals in two separate rooms and one fades to the room's noise floor in 300 ms, and the other in 250 ms, the latter has a faster "rate of decay." These probably aren't "correct" scientific definitions, but maybe they'll be helpful to keep things understandable and making sense.

Beyond this, it should also be understood that "ringing" is merely a succinct term that means "low frequency decay" – with (again) no distinction between the “decay time” and “rate of decay.” You’ll typically see even people knowledgeable on the subject saying things like, “if you do ‘xxx,’ an increase (or decrease) in ringing can be observed,” with no indication if they mean “decay time” or “rate of decay.” (I suspect that most of them don't know a difference exists.)

Confused already? Don't be - it took me years to sort this stuff out. Let's try to untangle it all.

Now: Let’s take a look at the effect that signal levels (gain) have on a waterfall. Here’s a waterfall graph from a thread at Home Theater Shack some years back:

Looks pretty scary, huh? Notice that the signal is peaking at nearly 110 dB. Now, let’s look at the same measurement with the signal reduced to peak at 85 dB:

Wow. Just like magic it looks much better, doesn’t it? We suddenly have a fabulous-looking waterfall, but the only real difference is that its signal level has been reduced.

So as you can see, merely reducing the level of the signal makes for a noticeably “better” waterfall graph, even if nothing has been factually improved. The apparent decay time of the signal has been “improved” merely because the quieter signal will obviously fade away quicker than the louder one...

... but that is not the same thing as improving the rate of decay, as you see happening in the right side of this graph:

To be clear, when Ethan says he’s looking for evidence of an improvement in ringing, he’s talking about the rate of decay.

Now let’s look at the relation between room modes and signal level. Again, I’m sure this isn’t the best or most scientific explanation, but a room mode is merely a build-up of bass energy that causes a substantial increase in level (gain) at a certain frequency. As we’ve seen, any increase in gain nets an increase in decay time: A room mode takes longer to fade away merely because it is louder than the rest of the signal. Again, this is not to be confused with the rate of decay. However, a mode’s rate of decay actually can be and often is worsened along with the increase in signal gain.

What can we do about the huge “sore thumb” signal level of the room mode? Enter the equalizer. An equalizer is merely a device that alters signal gain at specified frequencies.

Baseline (purple) vs. Equalized Response (black)​

With a parametric equalizer we can set a precise filter – bandwidth, frequency and negative gain value – that counteracts the mode and basically robs it of energy. We can see the effect with this "before and after" that features a nasty mode at 41.9 Hz. Counteracting the mode with a precisely-set parametric filter eliminates its audible and unpleasant “boomy” effect.

In the second graph, the level of the signal after equalization was raised to match the SPL reading the mode was displaying before being equalized. In other words, 41.9 Hz are at the same SPL in both graphs. Naturally, increasing the signal level makes the graph look worse overall (as discussed above). However, we can clearly see that after equalization, the frequency where the mode was located (41.9 Hz) now displays a significant improvement in rate of decay. But, we can also see that the rate of decay has not improved beyond the room average.

Why is this? The next thing to understand is that ringing is essentially the same to low frequencies as reverberation (or echo) is to the upper frequencies. Both have to do with the rate of decay: If you have a room with a lot of hard surfaces, it has a lot of reverberation because the signal bounces around all over the place and takes a long time to fade away. Add some room treatments, furniture, carpet etc. and the reverberation virtually vanishes. Why? Absorption. The furnishings and treatments absorb the signal and thereby the reverberation is truncated – i.e. the rate of decay the "live" room exhibited has been radically stunted. It should be self evident that an equalizer is no cure for a "live" room that has lots of echo and reverberation, nor is any other electronic device.

In the same manner, absorption is required to improve low-frequency decay times – a.k.a. “ringing.” Typically this means bass traps or something similar. An equalizer can only make adjustments in gain levels to problematic frequencies; it cannot absorb acoustical energy. It can make a waterfall graph "look" better to the untrained eye by reducing the signal level of peaking frequencies, but again - that's not to be confused with an improvement in the rate of decay. This is the mistake we commonly see with claims that equalization improves ringing.

So, how do you determine from a "before" and "after" waterfall graph if you have actually realized an improvement in ringing? This probably sounds overly simplistic, but just study the spacing between the horizontal lines. Each horizontal line indicates a "slice" (fraction) of time as the signal decays from its "starting point" until it falls below the graph's floor. So, if there is an improvement in the rate of decay – i.e., if the signal in an "after" waterfall graph is actually decaying faster than in the "before" graph – there will be wider spaces between the horizontal lines.

This is clearly evident in the graphs below that show ringing in a room with and without bass traps. Note the dramatic difference above 140 Hz that absorption makes.

Courtesy of Real Traps​

You simply can't get this effect with an equalizer – again, it can't absorb acoustic energy. And indeed, Ethan will argue, and rightly so, that the improvement in ringing the equalizer can accomplish with a modal peak is only effective at the location of measurement, not across the entire room. Don't get me wrong, equalizers are great tools for what they do. Personally I love them, I have lots of equalizers. But you have to know and respect their limitations.

The use of an equalizer has a disadvantage compared to bass traps when comparing “before and after” time domain graphs, namely that equalizers address ringing by reducing the level of a modal peak. As noted, gain-reduction of a modal peak can give the appearance that ringing has been improved, whether or not it actually has. This disadvantage must be addressed and compensated for before any “before and after” comparison can be deemed relevant. Improvements in ringing an equalizer may bring to a peak, if indeed there is any, can’t be fully determined unless the offending frequency in the “after” graph is level-matched to the baseline measurement.

 

[Wayne A. Pflughaupt]

With the previous post as background, let’s take a look at the graphs in the Acoustics Frontiers articles that NorthSky linked in Post #145.

In the article ROOM AND SUB EQ 101: HOW TO USE PARAMETRIC EQ TO FLATTEN YOUR BASS the claim is made that parametric EQ can reduce ringing of a room mode, and presents these two spectrograms as proof.

Personally I don’t like spectrograms as much as waterfalls as a tool for analyzing low frequency time-domain behavior because they’re not as readily intuitive, but here are two spectrograms presented in the article to show that parametric EQ reduced ringing of a mode at 72 Hz:

72 Hz Room Mode Before EQ

72 Hz Room Mode After EQ​

However, if you look at the graph below that shows the frequency response “before and after” equalization you can see that the 72 Hz peak after EQ has been reduced by 6 dB.

That in itself can explain why the “after” spectrogram appears to look better. However, since a spectrogram does not show the “slices” of time like a waterfall does, it’s impossible to make the determination if EQ has accomplished an actual improvement in the rate of decay for the 72 Hz peak (RT60), or merely the decay time (gain).

This is relevant because not all peaks in response are true room modes. EQ would/could only reduce the rate of decay of a true mode. If the “after” spectrogram had been level-matched at 72 Hz, we could possibly make a determination if the rate of decay had been improved, but that key element was overlooked.

In like fashion, in the article AUDYSSEY XT32 VS. PARAMETRIC EQ Acoustic Frontiers claims that EQ can reduce ringing with spectrogram graphs that most likely – once again – confuse a decrease in signal level with an improvement in the actual rate of decay.

The first spectrogram shows decay after equalization with Audyssey XT32, the second after manual parametric equalization:

Spectrogram After Audyssey XT32

Spectrogram After Manual Parametric EQ​

And once again, the frequency response graphs show why the “after” spectrogram appears better. You can clearly see that the “after” graph is on average 5 dB lower than the “before.”

Frequency Response After Audyssey XT32

Frequency Response After Manual Parametric EQ​

Adding insult to injury, at the end of the article we find this quote (emphasis mine):

“One has to try and not be misled by how differences in measurement levels can affect the charts – the scale is sized to fit the highest peak. [NOTE: Huh? Did I miss something? I see no evidence of any peak-matching in any of the graphs presented, including the baseline graph that I didn’t show here.] If that peak is significantly higher than the average bass level then the room modes will stand alone, and be easy to spot. Such a graph can be seen in the ‘before’ example. In the after examples you can see that Audyssey does not do as good a job at reducing the long time decay of the modes around 20Hz and 43Hz as hand dialed parametric EQ. You can also see that in general the right hand side of the [Audyssey] graph is significantly higher in level than for the hand dialed parametric EQ graph.”

There you have it: By their own statements, Acoustic Frontiers claims that the manual EQ graph shows an improvement in ringing. We should ignore the fact that the graph has the advantage of a signal level roughly 5 dB lower overall – it means nothing. And did parametric EQ accomplish wonders at 20 Hz compared to Audyssey? Well sure, it dropped 20 Hz nearly 10 dB – no wonder it “looks” better!

Thus Acoustic Frontiers shows they do not truly understand the relation between signal levels and decay. Nor how to analyze time-domain graphs.

But as I said before, confusion and misunderstanding levels in time domain graphs about the relation between ringing and signal seems to be nearly universal, even among those I’d assume should know better. Indeed, even the DSPeaker company, that specializes in bass room correction devices claimed to reduce ringing, submits these “before and after” graphs to “prove” the effectiveness of their products:

​

To be fair, it appears that there actually is reduced ringing in the second graph – note the wider spacing between the slices – but it would be easier to tell for sure if they had level-matched the peak in the “after” graph.

 

[Wayne A. Pflughaupt]

With the above in mind, we can now turn our attention to the Acoustic Frontiers article USING SUBWOOFERS TO IMPROVE SOUND QUALITY, that makes the claim that multiple subwoofers can reduce ringing.

At this point it should be obvious that adding additional subs has the potential to increase signal levels and as such make ringing look worse on a time domain graph, However, if the claim is true that adding subs can prevent the formation of modal peaks, then the claim could legitimately be made that multiple subs reduce ringing. I haven’t seen anyone present “before and after” time domain graphs showing the effects of additional subwoofers. The closest thing I’ve come across is these two graphs from Gene DellaSala at Audioholics, but they also include the effects of equalization in addition to the additional (total of five) subs. The graphs aren’t precisely level-matched, but at least are closer than most. The results are mixed: In the “after” graph you can see wider spacing between the slices in the range where the 40 Hz peak was, but not at the 60 Hz peak.

Primary Seat, One Sub No EQ

Primary Seat, Five Subs, With EQ​

Of course, every room is different. However, I doubt anyone will dispute the notion that there are definite benefits to adding subwoofers, including the potential to eliminate the formation of modal peaks. But even if the extra subs prevent the formation (and ringing) of a room mode, they aren’t going to reduce overall ringing beyond what the room would naturally exhibit. That requires absorption. Anyone who believes that additional subwoofers will take your room from this...

Empty Room​

... to this...

With Four Bass Traps​

...also has to believe that you can reduce flutter and echo in a live room by adding additional speakers.


Ethan, being a fan (and vendor) of bass traps, has also raised the question of whether or not equalization can reduce ringing beyond the location where the measurement was taken. Believe it or not, I think I may have a room where that might be the case.

Our system is in our living room, which is open to the dining room, entryway, kitchen and breakfast room, and has a ceiling that soars from 10 ft. at the front of the room to 19-1/2 ft. at the back. In short a very irregular room. I love rooms like this. Personally I would never do a dedicated “shoebox” room, even if I had the space, because of all the inherent acoustical issues that have to be overcome. It’s a bit “live” with the wood floors, but that’s fine with me. Some people argue that well-dampened rooms are better at delivering what the artist or producer intended. And that’s fine. To me a room that’s somewhat “live” gets a better representation of actual performance venues– you know, places that you’d actually visit to see that artist, or movie.

But I digress. Another benefit of an irregular room is that it doesn’t require numerous subs to get smooth response at most listening positions. I only have one sub, and after eliminating the peak at 42 Hz (yes, that was my room in the graph two posts back), bass sounds even and smooth at anyplace in the room I care to sit.

So to answer Ethan’s challenge about the effects of EQ at multiple locations with regards to ringing, I considered taking some measurements at additional locations to see if the ringing of that mode was indeed improved beyond the main listening position as well as things sounded.

But there’s really no point in taking the time or trouble. I know what I’ll find. Best case, if indeed ringing for the 42 Hz modal peak was improved at additional locations, I know it isn’t going to show improvement beyond what the room is already exhibiting naturally. No electronic device can absorb acoustical energy or improve ringing beyond that point. Again, as nifty and cool as equalizers are, you have to know and respect their limitations.

Likewise with multiple subwoofers. In many rooms they can certainly offer many advantages, but I can’t see how they are going to improve ringing beyond what the room would naturally exhibit. Like Ethan, I’d like to see those waterfalls showing otherwise.

Regards,
Wayne A. Pflughaupt

 

[amirm]

Now: Let’s take a look at the effect that signal levels (gain) have on a waterfall. Here’s a waterfall graph from a thread at Home Theater Shack some years back:

Looks pretty scary, huh? Notice that the signal is peaking at nearly 110 dB. Now, let’s look at the same measurement with the signal reduced to peak at 85 dB:

Wow. Just like magic it looks much better, doesn’t it? We suddenly have a fabulous-looking waterfall, but the only real difference is that its signal level has been reduced.

Hi Wayne. I explain this in my Time vs Frequency tutorial. Briefly here, you need to first establish the noise floor and then use that as the baseline to set the vertical scale. In addition, the depth of the graph needs to be correct and it is not in the red. I also play with the skew of the graph.

There are many ways to get prettier waterfall graphs without doing anything. It is for this reason that I don't recommend most people dabble in waterfalls. Everyone uses the defaults in REW which I find don't work well.

 

[NorthSky]

Wayne, a huge thank you for taking the time with a very very educative read.
I need more time to fully understand, but that's very exciting time ahead.

And please, anytime you have something to share that is good for us in our audio education we are attentive and peaceful cats.

Cheers,
Bob

 

[dallasjustice]

Thanks Wayne. You've articled the dilemma of trying to demonstrate how EQ can improve sound quality. And I believe the terminology is now the battle ground. As you point out, an improvement is marked in the waterfalls with less sharply defined peaks. The most offensive room modes are the ones which are sharply defined from one another. So Ethan is right, ringing isn't reduced. Yay, Ethan wins! But in the end, what matters is sound quality. So if multiple subs and EQ can smooth out ringing across the low frequency spectrum, then there will be fewer offensive well defined modes. This is desirable to listening quality. And of course it is no replacement for good bass traps.

I've tried to compare one sub with four subs in my system. I made some Stockwell transforms I planned to post here. But I didn't. I discovered that I couldn't reliable level match one sub with the four. How can it be done? Not all subs are the same distance to MLP. Therefore they aren't all the same electrical level. I then tried the REW low frequency pink noise. But that may not be accurate because the mic also measures the excessive ringing without attenuation and the four subs measured together include an EQ which attenuated the signal by roughly 6db. I decided to use the pink noise and compensate the levels according to mic's SPL. But I'm still not sure that's the best way to compare so I gave up.

But I learned more not from looking at absolute decay times. I learned the most from looking at the shape of the loudest peaks (in REW spectrogram they are orange or red). I discovered that multiple subs with EQ reduce the decay time of those peaks, but also spread them out across a wider frequency range. The four sub setup uses FIR filters which are fairly long. So there's some latency. This isn't a problem for Acourate to measure. But REW can't reliably measure where there's too much latency. And I couldn't important the impulse into REW. So I was limited to the STransfom plots. But I'm certain that if I could import the impulse into REW, I would see waterfalls with smoother transitions from mode to mode in the four sub with EQ. This is what the listener will ultimately prefer as has been demonstrated in listening tests on target curves by Sean Olive.

So I think we are now in the area of definitions and semantics. IMO, the focus should be on listener experience.

With the previous post as background, let’s take a look at the graphs in the Acoustics Frontiers articles that NorthSky linked in Post #145.

In the article ROOM AND SUB EQ 101: HOW TO USE PARAMETRIC EQ TO FLATTEN YOUR BASS the claim is made that parametric EQ can reduce ringing if a room mode, and presents these two spectrograms as proof.

Personally I don’t like spectrograms as much as waterfalls as a tool for analyzing low frequency time-domain behavior because they’re not as readily intuitive, but here are two spectrograms presented in the article to show that parametric EQ reduced ringing of a mode at 72 Hz:

72 Hz Room Mode Before EQ

72 Hz Room Mode After EQ​

However, if you look at the graph below that shows the frequency response “before and after” equalization you can see that the 72 Hz peak after EQ has been reduced by 6 dB.

That in itself can explain why the “after” spectrogram appears to look better. However, since a spectrogram does not show the “slices” of time like a waterfall does, it’s impossible to make the determination if EQ has accomplished an actual improvement in the rate of decay for the 72 Hz peak (RT60), or merely the decay time (gain).

This is relevant because not all peaks in response are true room modes. EQ would/could only reduce the rate of decay of a true mode. If the “after” spectrogram had been level-matched at 72 Hz, we could possibly make a determination if the rate of decay had been improved, but that key element was overlooked.

In like fashion, in the article AUDYSSEY XT32 VS. PARAMETRIC EQ Acoustic Frontiers claims that EQ can reduce ringing with spectrogram graphs that most likely – once again – confuse a decrease in decay time with an improvement in the actual rate of decay.

The first spectrogram shows decay after equalization with Audyssey XT32, the second after manual parametric equalization:

Spectrogram After Audyssey XT32

Spectrogram After Manual Parametric EQ​

And once again, the frequency response graphs show why the “after” spectrogram appears better. You can clearly see that the “after” graph is on average 5 dB lower than the “before.”

Frequency Response After Audyssey XT32

Frequency Response After Manual Parametric EQ​

Adding insult to injury, at the end of the article we find this quote (emphasis mine):

“One has to try and not be misled by how differences in measurement levels can affect the charts – the scale is sized to fit the highest peak. [NOTE: Huh? Did I miss something? I see no evidence of any peak-matching in any of the graphs presented, including the baseline graph that I didn’t show here.] If that peak is significantly higher than the average bass level then the room modes will stand alone, and be easy to spot. Such a graph can be seen in the ‘before’ example. In the after examples you can see that Audyssey does not do as good a job at reducing the long time decay of the modes around 20Hz and 43Hz as hand dialed parametric EQ. You can also see that in general the right hand side of the [Audyssey] graph is significantly higher in level than for the hand dialed parametric EQ graph.”

There you have it: By their own statements, Sonic Frontiers claims that the manual EQ graph shows an improvement in ringing. We should ignore the fact that the graph has the advantage of a signal level roughly 5 dB lower overall – it means nothing. And parametric EQ did accomplished wonders at 20 Hz compared to Audyssey? Well sure, it dropped 20 Hz nearly 10 dB – no wonder it “looks” better!

Thus Sonic Frontiers shows they do not truly understand the relation between signal levels and decay. Nor how to analyze time-domain graphs.

But as I said before, confusion and misunderstanding levels in time domain graphs about the relation between ringing and signal seems to be nearly universal, even among those I’d assume should know better. Indeed, even the DSPeaker company, that specializes in bass room correction devices claimed to reduce ringing, submits these “before and after” graphs to “prove” the effectiveness of their products:

​

To be fair, it appears that there actually is reduced ringing in the second graph – note the wider spacing between the slices – but it would be easier to tell for sure if they had level-matched the peak in the “after” graph.

 

[dallasjustice]

Fazenda also researched the so called "source and sink" subwoofer method. This is using subs on opposite walls in opposite electrical polarity or phase. This method most certainly reduces rate of decay without any EQ. This fazenda experiment shows that listeners preferred it over other sub setups.

http://www.aes.org/e-lib/browse.cfm?elib=16324

 

[AJ Soundfield]

So to answer Ethan’s challenge about the effects of EQ at multiple locations with regards to ringing, I considered taking some measurements at additional locations to see if the ringing of that mode was indeed improved beyond the main listening position as well as things sounded.
But there’s really no point in taking the time or trouble. I know what I’ll find. Best case, if indeed ringing for the 42 Hz modal peak was improved at additional locations, I know it isn’t going to show improvement beyond what the room is already exhibiting naturally.

Hi Wayne,
I'll say it again to be clear, if the frequency response at the different spatial locations are smooth/free of peaks, as they would be with optimized multi-subs/EQ (both, not one or other), then the corresponding "ringing" will be reduced...and the eyes of those fascinated with pretty waterfalls will be happy. I think.
We'll just keep avoiding that dreaded audibility word.

I know it isn’t going to show improvement beyond what the room is already exhibiting naturally. No electronic device can absorb acoustical energy or improve ringing beyond that point. Again, as nifty and cool as equalizers are, you have to know and respect their limitations.
Likewise with multiple subwoofers. In many rooms they can certainly offer many of advantages, but I can’t see how they are going to improve ringing beyond what the room would naturally exhibit.

What do you mean by "natural" room ringing?
I walk into a room, where is the natural ringing? Must an acoustic source be involved?
Let's say I place a pressure source 3' out from both walls in one corner, place a pressure mic at the LP, measure the pressure/decay, will the "natural" ringing show?
Ok, now I place a velocity source (gradient) with the same free field response as the pressure source, in the exact same spot in place of the pressure source. The axial, tangential and oblique modes are now differently excited, the pressure/decay response at the LP mic is quite different.
I rotate the gradient by 45 degrees in the exact same spot. The modes are once again excited quite differently, the pressure mic once again show a different response at LP.
Of the 3 different responses, which will properly show the "natural" ringing of the room?

cheers,

AJ

btw, your measurement examples do show the effectiveness of "midbass" traps.