Waterfalls - Page 5 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-16-07, 02:57 AM
Senior Shackster

Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Waterfalls

Quote:
Isn't the 100ms that you refer to a function of the duration of the impulse response analyzed?
An impulse, by definition, has a length of 0 (it's instantaneous). This is impossible in a bandwidth limited system, so one would expect to see something a little bit longer...but it shouldn't be longer than say 1ms - certainly not 100ms.

Are you saying that you only did the sweep up to 200Hz? If so, that might explain it. Why aren't you sweeping to the top limit of your soundcard?

Regardless, simply increasing the volume on the BFD is going to make it seem like it takes longer to decay. The fact that your EQ shows the same thing simply verifies that it is achieving an amplitude change...not so much changing the ringing at that frequency.

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
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Old 12-17-07, 01:06 PM Thread Starter
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Waterfalls

Quote:
An impulse, by definition, has a length of 0 (it's instantaneous).
Yes, but REW doesn't use impulses to measure the response. The impulse response is derived from the system's transfer function, which is determined from the system's response to the sweep. In this case I am interested in an end frequency of 200Hz for the response and waterfall plots. An end frequency in REW is set to the highest frequency desired (in this case 200Hz), and the resulting sweep will span from 0Hz to twice the frequency set (with an overall limit of half the soundcard sample rate) to provide accurate measurement for the selected range (in this case 400Hz).

Sorry, I'm not sure what you're disagreeing with, or even what point you're trying to make?

brucek
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Old 12-17-07, 02:32 PM
Senior Shackster

Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Waterfalls

Quote:
Quote:
Am I the only one that finds it odd that the decay rate of your BFD is over 100ms?
I don't believe that it's the decay rate of the BFD at all.
Exactly, so how do you reconcile the measurements to correlate with reality?

What I'm trying to point out is that the EQ does not ring or resonate or whatever you want to call it. It is not changing over time. You are misinterpreting an artifact of the waterfall calculation.

For a given point in the room where reflections arrive within a wavelength of the frequency in question, the system could be simplified as minimum phase. EQ only works in minimum phase situations.

However, you have not shown that the standing waves have been removed. In fact, they are most certainly still there. The easiest way to verify that the standing waves are still there is to locate the nulls of the standing waves...which do not move to different positions in the room or change frequency as EQ is applied to the signal.

I suppose you could just not sit in the nulls, but it's not quite so easy. The point I was making above is that you're only minimum phase for a single location in space. If you move the microphone even a few inches (like the width of your head), your EQ no longer works perfectly. Sure, some might argue that it is a small compromise, but the point is that it's not perfect. Proper acoustical treatment gets rid of the standing wave, which in turn improves the situation at far more locations in the room (it's not perfect either, but the compromises are much less).

One more comment which has to deal with psychoacoustics. When you walk into a room, there is a certain sonic characteristic to that room, even with no music playing. When you use EQ on the music to try and not trigger behaviors in the room, you end up sending conflicting signals to the listener who is partially expecting that bass guitar to sound like it would sound in their room. In other words, when you sit there listening to music, you are partially going to be identifying some attributes of the sound as being the room and not the music....if your music is precompensated for the room, and then your ears compensate what they hear from the room, then you end up with double compensation and the music sounds disconnected. Perception is definitely different for everyone, but the point is that it's much easier when the room isn't imparting on the sound because then there is never any amount of the listener trying to filter it out - it just sounds way more natural.

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
DrWho is offline
Old 12-17-07, 03:06 PM Thread Starter
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Waterfalls

Quote:
The point I was making above is that you're only minimum phase for a single location in space.
Yes, and I completely agree and have said the same thing in my post #13 that I'll reprint below:

The modal response of a room acts exactly like a 2nd order filter and matches the BFD generated filters in all aspects. At modal frequencies, a room resonates in gain and Q exactly as if you fed a sub signal through a 2nd order parametric filter. This fact allow us to fashion an identical 2nd order filter with the opposite gain and bandwidth that matches the room mode so it will completely disappear (at the point of measurement).

This doesn't apply outside the low frequency range where signals are no longer considered minimum phase, where primary reflections (second order) from the walls, ceiling and floor arrive at the listening position anywhere in the room with a phase shift of quite a bit less than a cycle. So, the effective limit here of about 80Hz-100Hz is reasonable for equalization in most rooms...(an 80HZ signal has a wavelength of about 14ft)............

Quote:
Proper acoustical treatment gets rid of the standing wave, which in turn improves the situation at far more locations in the room (it's not perfect either, but the compromises are much less).
Mmmm OK, I don't know the size of the treatment that might be required though and the WAF of such treatments at the frequencies in question. Personally, I think EQ is an acceptable and effective route to follow. You and I have different opinions on that I guess. We'll leave it at that.

brucek
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Old 12-17-07, 06:20 PM
Senior Shackster

Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Waterfalls

How often does it show up where EQ predictions are made that do not result in the measured outcome? If the system were minimum phase, wouldn't the measurements correlate exactly with the predictions? Whenever this happens, one might argue that making the frequency response as flat as possible doesn't lead to the most accurate reproduction. Of course this notion is nothing new - there are articles dating back into the 70's.

There are also articles that discuss windowing limits for low frequency waterfalls too - it's a tradeoff between time and frequency resolution (can't have both at the same time). I'll see if I can't find that article too, but basically the data is meaningless without a proper window and obviously in this case, the wrong window is being used because it's showing decay rates that cannot be true.

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
DrWho is offline
Old 12-18-07, 02:39 PM
REW Author

John

Join Date: Apr 2006
Location: UK
Posts: 6,308
Re: Waterfalls

The main factors determining the shape and behaviour of the waterfall plot are the types and durations of the window functions used. The main window function is a right half window whose type is set by the Low Freq Decay window selection in the Analysis settings (default is Tukey 0.25) and whose duration is governed by the window control below the waterfall graph. The key to the observed behaviour of the waterfall is the window function applied to the left edge, which is not user-selectable. It is a left half Hann window whose duration is half the selected window duration (making for a total duration of 1.5 times the value in the control). This value was arrived at empirically by trying various settings to find a value that makes it easy to distinguish modal effects in measured data. I may make this a user-configurable parameter in a future build, but the current settings work well for the intended purpose of this plot.
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Old 12-24-07, 06:29 PM
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Pete

Join Date: Sep 2006
Posts: 96
Re: Waterfalls

Quote:
brucek wrote: View Post
As a side note, you can see what a completely terrible idea it is to add a gain filter to boost the level of a sub at low frequencies. You do nothing more than emulate a room mode at the gain frequency
Brucek, I do not agree with this. I believe boosting versus cutting should have no impact on creating resonance. The resonance is a function of the room. If you need "boost" at a given frequency, the room is absorbing that frequency, which is why you need the boost. Now, maybe there is some effect with really narrow bandwidth (I am an engineer, but not an acoustical or EE so I won't claim to know).

What about boosting within an area of a large cut at a level less than the cut? Surely that would not create a resonance, right?

To prove your point (I would do it, but I am still doing manual reading imported in REW), I think you would need to create a frequency response curve for a real room using two methods: 1) no boosting, and 2) with boosting at selected low frequencies. Then compare the waterfall plots between the two. I think any area of resonances are likely to be the same and induced by the room, not dependent on whether you used boost or cut in a given area.

Regards,
Pete
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Old 12-24-07, 08:38 PM
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Nick

Join Date: Mar 2007
Location: Seattle, WA
Posts: 95
Re: Waterfalls

I read that Floyd E. Toole (of Infinity) stated that leveling a peak will mitigate a room mode because doing so changes the phase as well. I am not expert enough to prove or disprove it but it seems to make sense. The proof, as he said, is in listening.

Now on a slightly different twist: suppose there are two identical subs except that the phase on one sub is changed to oppose the other at a resonance frequency, keeping the phase the same at all other frequencies. Has anyone tried it?

Last edited by MakeFlat; 12-25-07 at 03:14 PM. Reason: Added Floyd's last name. Added complementary phrase to clarify the last paragraph.
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Old 12-25-07, 09:37 PM
HTS Senior Moderator

Join Date: Apr 2006
Location: Katy, Texas
Posts: 8,814
Re: Waterfalls

Quote:
PeteD wrote: View Post
Brucek, I do not agree with this. I believe boosting versus cutting should have no impact on creating resonance. The resonance is a function of the room. If you need "boost" at a given frequency, the room is absorbing that frequency, which is why you need the boost.

What about boosting within an area of a large cut at a level less than the cut? Surely that would not create a resonance, right?
I believe that John clarified here that you are essentially correct. Boosting a null shouldn’t result in ringing that’s any (or at least much) worse than it would be if the null wasn’t there.

Quote:
MakeFlat wrote: View Post
Now on a slightly different twist: suppose there are two identical subs except that the phase on one sub is changed to oppose the other at a resonance frequency, keeping the phase the same at all other frequencies. Has anyone tried it?
Are you talking about phase or polarity? I believe phase is more-or-less a time alignment issue – not sure you one could make it “oppose.”

Regards,
Wayne

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Old 12-25-07, 10:46 PM
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Silver Supporter
Nick

Join Date: Feb 2007
Location: Montreal
Posts: 14
Re: Waterfalls

Happy Holidays everyone!

This thread has been very interesting and very timely as I have significant room issues that I would like to correct and I’m considering room treatment. I haven't been able to find much info on understanding waterfalls plots.

So statements such as the following….

Blaser:
Quote:
So, is it assumable that equalization can be more practical, cheaper, easier for room mode decay (ringing) as well as FR below say 80 Hz than room treatment?
BruceK:
Quote:
Well for sure.
…really got me interested. My primary purpose on understanding this stuff is to determine if I need room treatment for my room to handle the basss ringing, since I have a BFD.

As a novice in the room acoustics realm though, this thread has left me (and perhaps others with regard to using waterfalls plots in relation to room ringing), a little confused.

So, if its ok with you guys, I was wondering if I could pose a few more simpler questions in this thread, since I’m sure I’m missing something here.

From BruceK’s original message…

Quote:
Point # 1 is regarding the effectiveness of equalization at low frequencies (15Hz-100Hz). The question posed is how can a parametric filter possibly correct room resonances. The assertion being that an EQ filter only lowers the relative SPL level in the room at that frequency, and as a result the ringing may be reduced since it drops into the noise, but it can't really correct the problem. Sorry, I don't agree..............................
I could be wrong, but it appears to me, that Bruce took the output directly from the BFD to apply filters to a flat frequency response. When the gain was increased by 15db at 40 Hz, a whopping 300+ ms of ringing was observed.

My initial though was: How is this possible from a direct output of the BFD to REW? No room reverberations had a chance to occur.
Am I confusing two different acoustic effects?
But later in the thread, Dr. Who and BruceK, and others tried to clarify this. The BFD is likely not really introducing that much of a decay.
But if the BFD isn’t really introducing 130 ms of ringing to the flat signal, then is it really introducing 300+ ms of ringing when the 15db gain is applied at 40 Hz?
If not, where does this leave us in relation to Point #1, if we can’t really trust the waterfalls plot?
Am I just reading this all wrong?
Am I missing something basic here.

Finally, should I not be using a waterfalls plot in determining how bad the room accoustics are for my living room?

Needless to say, any insight would be greatly appreciated..
Thanks!
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