Waterfalls

[macrae11]

Wayne A. Pflughaupt wrote:

Assuming Rod Elliot was referring to equalization, we get a contrasting (conflicting?) view from Rane’s Exposing Equalizer Mythology by Dennis Bohn (bold emphasis added):

“Phase shift is not a bad word. It is the glue at the heart of what we do, holding everything together. That it has become a maligned term is most unfortunate. This belief stands in the way of people really understanding the requirements for room equalization.

Associated with each change in amplitude is a corresponding change in phase response. Describing them as unbelievably jagged is being conservative. Every time the amplitude changes so does the phase shift. In fact, it can be argued that phase shift is the stuff that causes amplitude changes. Amplitude, phase and time are all inextricably mixed by the physics of sound. One does not exist without the others.”

I think what Mr. Elliot failed to consider is that equalizers also introduce phase changes, which probably accounts for how it’s usually possible to EQ phase-related response problems around a sub’s crossover region.

Overall though, that was a very good article. Thanks for linking it. I especially liked the part about mics not "hearing" the way our ears do. That's probably why you don't get much of an audible improvement equalizing subs beyond smoothing out the worst problems. I.e., piling on lots of "minutiae" filters smoothing out every little ripple in response you probably won't be able to hear the difference with them in or out. I sure can't.

Regards,
Wayne

You are correct, but I belive Mr. Elliot was thinking on a slightly different line. Of course when dealing with EQ's amplitude and phase are related(with the exception of a linear phase EQ). In the real world(no EQ) they are not so incontrovertably linked. You can have the identical phase of two signals with two completely, even opposite amplitudes. Also the time which Mr. Elliot and yourself are referring to are on different scales. a 10 degree phase shift at 80 Hz (wavelenghth of 4.2m) takes a little over a 1000th of a second. Room nodes can cause differences in time of an 80Hz note of a second or more. So an EQ(even a very poor "phasey" EQ) can come nowhere close to adjusting that kind of time.

PS. Someone please correct me if my math is wrong, it's been quite some time since I've had to work out any formulas.

[Wayne A. Pflughaupt]

macrae11 wrote:

I'm not familiar with how these softwares could correct in the time domain to correct a room node that cause a(for example) low D to sound twice as long as it sounds in the program material.

I will research it a bit more, but if you have a simple explanation that could enlighten me, I would welcome it.

From the little I’ve seen, they don’t really have much effect on time domain. I haven’t seen any comprehensive in-room testing, other than Ethan Winer’s EQ vs. Bass Traps extravaganza. The test has some problems, though, mainly that the sheer number of measurements to sift through is mind numbing, and the guy who set up the equalizer did a really, really bad job of it.

Ethan also did an evaluation of the effectiveness of the Audyssey MultEQ DS-21 mentioned, which claims to reduce extended signal decay (aka ringing). His conclusion was that it was a mixed bag as far as dealing with ringing was concerned.

Since there seems to be a dearth of any comprehensive in-room testing, other than these two (which are the only ones I’m aware of) I’ve done some of my own, although I haven’t explored it fully or posted any results here at the Shack. After studying waterfalls ‘til I’m bleary-eyed, I’ve found pretty much the same thing as Ethan, that it’s a mixed bag using an equalizer as a fix for extended low frequency room decay.

From what I’ve seen, the improvement waterfalls show for modal (or “time-domain”) EQ filtering has only been apparent in the short-duration 300 ms window. When the window is lengthened to 600 ms, any advantage modal filters showed over other equalizing techniques pretty much vanished.

Here is a sample to show what I’m talking about. The Hz markers represent peaks that REW found, and the “Modal Filters” waterfalls have REW-recommended filters set at those frequencies (except for the red 49 Hz marker). The “Smoothing Filters” graphs show the result of filtering that ignored REW’s recommendations and used four filters that merely smoothed response. Notice that this “faster decay, but only up to a point” effect is especially noticeable for the 26.9 and 23.5 Hz markers. As you can see, once the window is lengthened to 600 ms, any apparent advantage of modal filtering over smoothing virtually disappears. (Hit F11 and you should be able to get all four graphs on your screen at the same time.)

Modal Filters at 300 PP 14-01-34.JPGSmoothing Filters at 300 PP 14-08-30.JPG
Modal Filters (Left) vs. Smoothing Filters (Right) @ 300 ms


Modal FIlters at 600 14-01-34.JPGSmoothing Filters at 600 PP 14-08-30.JPG
Modal Filters (Left) vs. Smoothing Filters (Right) @ 600 ms

You can see the same effect in Ethan’s “with and without Audssey” comparison graph. Notice the broad peak between 20-56 Hz, where Audyssey seems to improve the rate of decay in the short term, but ultimately does not reduce or eliminate it in the long term. Also note that even significantly reduced gain of the same peaked area effected by the equalizer did not reduce the ringing.

Regards,
Wayne

[macrae11]

Wayne A. Pflughaupt wrote:

From the little I’ve seen, they don’t really have much effect on time domain. I haven’t seen any comprehensive in-room testing, other than Ethan Winer’s EQ vs. Bass Traps extravaganza. The test has some problems, though, mainly that the sheer number of measurements to sift through is mind numbing, and the guy who set up the equalizer did a really, really bad job of it.

Ethan also did an evaluation of the effectiveness of the Audyssey MultEQ DS-21 mentioned, which claims to reduce extended signal decay (aka ringing). His conclusion was that it was a mixed bag as far as dealing with ringing was concerned.

Since there seems to be a dearth of any comprehensive in-room testing, other than these two (which are the only ones I’m aware of) I’ve done some of my own, although I haven’t explored it fully or posted any results here at the Shack. After studying waterfalls ‘til I’m bleary-eyed, I’ve found pretty much the same thing as Ethan, that it’s a mixed bag using an equalizer as a fix for extended low frequency room decay.

From what I’ve seen, the improvement waterfalls show for modal (or “time-domain”) EQ filtering has only been apparent in the short-duration 300 ms window. When the window is lengthened to 600 ms, any advantage modal filters showed over other equalizing techniques pretty much vanished.


Regards,
Wayne

Thanks for that Wayne. I still can't say that I fully understand the system before I do my own in depth research, but this certainly clears things up.

I also think it somewhat proves my point. The DSP system can lower the initial output of the trouble frequencies, but still does not affect the longer term effects of the room.

[Wayne A. Pflughaupt]

macrae11 wrote:

Thanks for that Wayne. I still can't say that I fully understand the system before I do my own in depth research, but this certainly clears things up.

I also think it somewhat proves my point. The DSP system can lower the initial output of the trouble frequencies, but still does not affect the longer term effects of the room.

I agree. Some of my esteemed colleges here (and on other Forums) are believers in time-domain equalization, but they haven’t posted or otherwise offered any evidence to back it up.

Again I think it's great that perhaps these DSP systems sound great to you. I might like them as well. All I'm saying is that they're not correcting issues in your room, and they are colouring the original signal in it's attempts to do so.

I think it would be more accurate to say that they’re attempting to correct the room’s effect on the speaker’s response, coupled with the natural response of the speaker itself (i.e. equalizing above ~500 Hz).

Regards,
Wayne

[brucek]

macrae11 says:

I also think it somewhat proves my point. The DSP system can lower the initial output of the trouble frequencies, but still does not affect the longer term effects of the room.

Wayne A. Pflughaupt says:

I agree. Some of my esteemed colleges here (and on other Forums) are believers in time-domain equalization, but they haven’t posted or otherwise offered any evidence to back it up.

Guys,

Just as a parametric filter operates in the frequency domain, it also has a time response that acts like a modal resonance in a room.

It's easily demonstrated (see below) by doing a frequency response sweep of a Behringer parametric equalizer and adding a single filter.

The time tail is evident, is it not?

From my sticky post on waterfalls I don't think anyone can argue that this filter is also operating in the time domain?

Waterfall plot plot of a BFD using a single filter of (40Hz, Gain +15dB, BW 10)


From that thread I noted the following:

But now I look at the resulting waterfall plot of that single filter below.

Look familiar? Sure it does.

It looks like a room mode resonance of any REW measurement at subwoofer frequencies. And it should. It's because the EQ filter, just like the modal resonances of a room, has a time response that acts like a 2nd order biquad. If I apply an EQ filter with the same Q and opposite gain of a room mode, I would completely counteract the effect of the mode. See the time component of the filter (just like a room mode). It rings out, and still isn't in the noise after 300msec. You see, EQ filters don't just affect level. This is why they're so effective at equalizing at modal frequencies below 100Hz. Yes, it is listening position dependant, and only valid at the point where the response was measured, but because of the long wavelengths of low frequencies, the region around that area is fairly large. This is in opposition to higher frequencies where equalization is a bit of a waste of time, since the effective region is so small that eq is impractical.


I remember a post (from this thread) by JohnM (author of REW) some time ago that was the trigger that really started me thinking about this subject and after lots of playing around with my equipment and investigating, I'm a believer (one of the few, it would seem).

John said:
This is a fundamental misunderstanding of minimum phase systems. Room modes (in particular those below a few hundred Hz) are minimum phase phenomena (substantially behaving as 2nd order biquads) and are effectively countered by 2nd order biquad correction filters such as those implemented in the BFD pro and many similar parametric equalisers. It is, however, unnecessary to engage in lengthy academic discourse on the topic since measurements clearly show the effectiveness of the approach.

brucek

[macrae11]

Thanks for your input Bruce. This thread has certainly gotten off topic from the OP, and it's mostly my fault I'm afraid. I apologize and I hope no one minds, as I think there is some good discussion going on here.

To get back to you Bruce, I'm assuming by the looks of your waterfall graph that you used a broadband white noise burst to create this waterfall measurement.
Here's my thought:
You're not extending the time that the 40Hz signal is present for. All you are doing is making the starting point louder, so more of the signal is heard before it falls into the noise floor. The decay curve appears to be identical, just a higher output, which shows that you haven't actually changed any time issues.

Now certainly it is easy to eliminate a filter boost by a subsequent identical but reversed filter cut. It might even sound identical to not using a filter at all, if it is a high quality digital low phase EQ. But this isn't how things work in the acoustic world.

If you were to introduce that same cut filter into program material going into a room, you would lower the initial attack of that note in the room, and the decay would then take the same amount of time for the sound level to decrease. But since the level of that frequency started at a lower point, it will fall into the noise floor sooner, having it be not audible at the correct time as if the room wasn't playing a factor. So yes the note will appear to be shorter but only because it started at a lower volume.

One thing that I think people forget is that the first instant of sound you hear coming from the speakers is not affected by the room. It is affected by how far away you are from the speakers, and the room temperature, moisture etc, but it has nothing to do with the walls in your room. Hence if you're EQ'ing for the room, you are also EQ'ing the direct signal. So that first attack of sound you hear is going to have a 15 dB cut at 40Hz.

Now one thing I will concede is that in a non severe instance of this, very few filters doing very moderate attenuation, with a very narrow Q, it is possible that this will be inaudible to the listener, and may improve the perception of ringing in the room. This is however, still a band-aid solution to a surgery problem.



Having said all, this I'll give a personal disclaimer: It's been several years, since I've really done much acoustic design, and I may be a bit out to lunch. However the laws of physics haven't changed, at least not to my knowledge, and I don't think we're at a point where we can throw the acoustic treatment out the window yet. At least until there is a way to EQ the room reflections separately from the direct signal.

[brucek]

I'm assuming by the looks of your waterfall graph that you used a broadband white noise burst to create this waterfall measurement.

Room EQ Wizard - REW

brucek

[macrae11]

Hi Bruce

Sorry I knew what software you were using, I was just wondering what type of signal was used to create the initial flat waterfall. After reading some more about REW it seems it uses a logarithmically swept sine wave.

Thanks

[Wayne A. Pflughaupt]

brucek,

I see blaser sent you over here - hee hee!

I don't think anyone can argue that this filter is also operating in the time domain?

Sure we can. It’s impossible to make a case from an electronically generated, faux "room" mode that displays amplitude only but has no time domain element from reflections, boundary interference, etc. For instance, is it going to look different if you install some bass traps in the room? It's merely a computer-generated loop that has no real-world significance.

Since I unfortunately did not save the REW files I used when I ran this experiment a while back, I decided to do it over and this time use a longer window for the waterfall. This time I let REW do the filtering and two modal filters (i.e. optimized for time domain) for my 42 Hz room mode were crafted, the primary one being centered at 42 Hz with a 10/60 bandwidth and cut 12 dB.

As before, a 300 ms window showed the same short-term reduction in ringing with modal filtering. But at 600 ms – different story. Here are the results with the 42 Hz peak level matched after equalization (as described in the linked thread). (Hitting F11 should get them both on-screen for easier comparision).

base waterfall at 600 reduced.jpg
filtered waterfall at 600 w-level adjustment reduced.jpg
Baseline (Top) vs. Modal Filters, Level-Matched (Bottom) @ 600 ms

In fact, even with no level matching, we can see that modal filters made only a minor difference in reducing the 42 Hz mode's long-term ringing:

base waterfall at 600 reduced.jpg
filtered waterfall at 600 no level adjustment reduced.jpg
Baseline (Top) vs. Modal Filters , No Level Matching (Bottom) @ 600 ms

If you want to convince us otherwise, show us your in-room waterfalls (long window, please). IMO Ethan’s Audyssey report puts the matter to bed, unless someone can show a fundamental problem with his methods or procedures. As his chart above shows, Audyssey did not reduce ringing. I'm sure my experiments aren't as sophisticated as his, but I'm getting the same results every time.

Regards,
Wayne

[atledreier]

Even though the 'evidence' here suggest this and that, the reality is that good EQ in the time domain, like Audyssey and RoomPerfect (Both of which I've extensively used) makes for better sounding systems, especially in a well treated room. I can't explain it, I won't even try to, but the fact still remain. It sounds better.

[brucek]

If you want to convince us otherwise, show us your in-room waterfalls (long window, please)

I suspect I could find a suitable example of REW removing a modal resonances, but I don't think that would convince you.

I do take exception to the level adjusting in your example though. If I had a signal that was at +20dB and I reduced it by some means by -20dB to eliminate its effect, you can't raise the result by +20dB and say, "see, it's still there".

REW is fairly strict in what it targets to eliminate. The peaks must be minimum phase modal resonances. Sometimes the peaks can be a combination of two or more closely spaced resonances and the filter is only seeing one of them and so the tail carries on as it decays. Sometimes REW may not exactly identify the center frequency or the exact bandwidth of the response. I think you have to play around to get it exact. The waterfall plot is certainly the display to use for that purpose.

You've obviously proved the point that the ringing out was reduced in your example and so the point is proven that the filters operate not only in the frequency, but also the time domain. I'm sure with a bit of playing around you could completely remove the room mode, if indeed it was one. Again, this is all at the point of measurement only.

My "faux" tests were designed to show that if the filter is perfectly placed, then the result is perfection.

brucek

[Wayne A. Pflughaupt]

atledreier wrote:

Even though the 'evidence' here suggest this and that, the reality is that good EQ in the time domain, like Audyssey and RoomPerfect (Both of which I've extensively used) makes for better sounding systems, especially in a well treated room. I can't explain it, I won't even try to, but the fact still remain. It sounds better.

Sure, I don’t think there are many who will claim that Audyssey (or any equalizer) won’t make an audible difference (hey, I’m a big equalizer buff myself). And certainly there is a time domain element with Audyssey, just like there is with any equalizer – to re-quote Rane’s Dennis Bohn, “Amplitude, phase and time are all inextricably mixed by the physics of sound. One does not exist without the others.” The debate seems to be over exactly how or why. Which I’m starting to think is kind of silly. It’s like, “Okay, it works. Who cares why?”

At the end of the day, an equalizer merely tames a room mode by depriving it of energy – i.e. reducing its amplitude. If it also happens to truncate the near-term signal decay time, that’s certainly a nice side effect. But if you could somehow totally eliminate the extended decay without reducing the amplitude, it would still sound bad (just try applying one of those “electronic room modes” to some headphones, where there is no ringing issue). So in my mind it’s the amplitude aspect that makes the audible difference, not the time domain.

Regards,
Wayne

[Wayne A. Pflughaupt]

brucek wrote:

I suspect I could find a suitable example of REW removing a modal resonances, but I don't think that would convince you.

Try me. After all, I became a skeptic by examining the graphs. I fully recognize that my own room is all I have to work with, and that things could be different in another room – or even every other room! (Probably should have mentioned that sooner - sorry... )

I do take exception to the level adjusting in your example though. If I had a signal that was at +20dB and I reduced it by some means by -20dB to eliminate its effect, you can't raise the result by +20dB and say, "see, it's still there".

It’s valid comparison because we don’t keep our systems at a static level. We turn them up and down. It’s well known that signal decay times are directly related to signal level. Level matching simply takes that factor out of the equation for examining the signal decay effects of modal filters. Otherwise a cutting filter has an unfair advantage merely by the effect of reducing amplitude: “See, it reduced the ringing!” Um no, it didn’t...

Regards,
Wayne

[macrae11]

Ok whether room EQ works or not, whether it reduces ringing or not, whether you like the sound of it or not, right now I don't care. Can anyone answer this question.

How can an EQ cut that is significant enough to reduce a modal resonance in a room, not affect the direct signal?

[brucek]

How can an EQ cut that is significant enough to reduce a modal resonance in a room, not affect the direct signal?

Well, it does affect the direct signal. The EQ adjusts the direct signal to obtain the desired result at the point of measurement (which can be a result of direct and reflected signal). The EQ results are only effective at the point where the response was measured and a small area around that point depending on the frequency.

brucek

[JohnM]

To elaborate on Bruce's comment a little, unless you live in an anechoic chamber or sit a foot from your one speaker, you never get to hear just the direct signal from the speaker. You cannot escape the contribution the room makes to what arrives at your ears.

[macrae11]

Absolutely you never just hear the direct signal. However you do hear the direct signal first by itself before any room reflections get to the listening position. So if you make a 15dB cut at 40Hz, don't yo think that would be audible in the direct signal? And if you do that the room node is still there correct? So you're not "fixing" the room node you're just sending less signal in that frequency range to excite the room node.

Am I wrong in any of my logic? I'm not trying to be facetious, just trying to get a better understanding of your logic in this scenario.

Thanks

[Blaser]

Wayne A. Pflughaupt wrote:

brucek,
I see blaser sent you over here - hee hee!

Wayne,

I usually agree with Brucek on many points and specially the waterfall thread where I have made practical measurments in my room to demonstrate and confirm Brucek's thread opening theoretical analysis.

It was close but missed....He came alone

[Blaser]

Wayne A. Pflughaupt wrote:

Level matching simply takes that factor out of the equation for examining the signal decay effects of modal filters. Otherwise a cutting filter has an unfair advantage merely by the effect of reducing amplitude: “See, it reduced the ringing!” Um no, it didn’t...

Wayne,

Look at my measurements in the first page again. It did.