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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.


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 the article, showing the benefits of parametric EQ, that there is a failure to fully understand the basic relation between ringing and signal levels. In fact, the confusion and misunderstanding xxx Fair warning, anyone who’s truly up to speed on this might subject find much of the following a bit simplistic.

It seems there is 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 time."

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 canbe 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 (the graph I showed in Post #117). 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 improved 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[/b]​

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, the equalizer is a great tool for what it does. Personally I love them, I have lots of equalizers. But you have to know and respect their limitations.

When comparing “before and after” waterfall graphs to determine if an improvement in ringing has been accomplished, the use of an equalizer has a disadvantage that must be addressed and compensated for that bass traps do not, namely reducing the level of a modal peak. As noted, the mere reduction in level can give the appearance that ringing has been improved, whether or not it actually has been. You can study the slice-spacing in the two graphs above with the 41.9 Hz issue, and indeed see a wider spacing of the slices in the “after” graph. However, improvements in ringing an equalizer may bring to a peak, if there indeed is any, can’t be fully determined unless the offending frequency in the “after” graph is level-matched to its original SPL.



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 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.] 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:


ATTACH

ATTACH​


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.



NEW POST
With the above in mind, we can now turn our attention to the Sonic 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 seen 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 spaces – 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 its limitations.

Likewise with multiple subwoofers. In may 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. Like Ethan, I’d like to see those waterfalls showing otherwise.





As noted in my previous post, if you want to prove an improvement in the rate of

Antimode

There you have it. If you want to show that EQ or some other method reduces ringing, you first need to have an understanding


http://audiosciencereview.com/forum/index.php?threads/wanted-proof-of-multiple-subs-and-sub-eq.185/page-8#post-8533
http://www.acousticfrontiers.com/20101026using-subwoofers-to-improve-sound-quality-part-2-room-modes-html/
http://www.acousticfrontiers.com/2013111room-and-sub-eq-101-how-to-use-parametric-eq-to-flatten-your-html/

http://www.acousticfrontiers.com/2012111audyssey-xt32-vs-parametric-eq-html/
 
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