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.
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.