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Chrisbee,

You need to compare your graph to the same initial 20Hz SPL (without BFD) and then compare after 1000ms. Otherwise you will never know.

Indeed, ringing at 20 Hz is not necessary a bad thing...It could be amazing:devil:
 

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Wow, I envy all of you and your room response. I'm still working on mine but here is what I've go so far.... note this is going out to 933ms. At 300 virtually all frequencies were still there...



It's interesting to see on the waterfall that right before and after my lull at 56hz, there is are substantial nodes! I'm not too sure how to deal with this yet.
 

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Discussion Starter #23
This would be impossible. Its impossible to have a peak without the resonance or the ringing.
Really, the best way to look at response is with the single frequency response. The waterfall is prettier but of little practical use.
Doug, we'll just have to disagree on just about everything in your post.

I wish I could find some better examples to show you that very similar peaks can have completely different persistence in time, but I could only find the response below (I've seen far better examples).

I find the waterfall plots very useful in identifying peaks that ring-out for a long time. I use REW to match that peak and reduce its ringing as low as possible. Other peaks that look very similar in the 2 dimensional frequency response plots that don't ring-out as much have nowhere near the impact to the sound.

Here's a quick example of two peaks that are very similar - at least they look that way using a simple frequency response plot. With the single slice 2 dimensional plot of frequency and amplitude, the two peaks look about the same.

Now examine what happens in a three dimensional plot when I move the slider to 30 slices to add time derived from the impulse response. One just keeps on going. And you hear that too. Using REW to match that room mode eliminates that ringing out in time..

Waterfalls are far more than pretty pictures....... :)

ONE SLICE
one slice.jpg

30 SLICES
30 slices.jpg

brucek
 

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"Doug, we'll just have to disagree on just about everything in your post."

I'm sorry to hear that. I program waterfall plots and have studied these response functions in detail as an engineering student. We spend about 4 courses per year for two years studying the response function in various courses from math to controls to DSP and analogue filtering.

NASA or any other company does not ever use waterfall plots to evaluate the response of their systems and the response of their control systems is similar to what we have here. I have never seen a waterfall plot in any serious controls theory, electric circuit theory paper or in any other form. Its never used for a reason - its useless !! They are only in audio, they look pretty but mean little. A well known Phd acoustician has agreed with me on this point and doesn't use them for the same reason.

They can obfusgate the situation when people mis interpret noise or the mathematical distortions in gating as an actual physical affect, as you have above.

Every single peak in the waterfall plot is due to a peak in the frequency response. If there is a section in the waterfall that displays a decaying portion without a peak in the frequency response then its noise or some other affect.

In some non mimimum phase systems you could possibly see a decaying part that does not show as a resonance in the first slice but the resonance must appear in later slices. This is very unlikely and absolutely unlikely in a modal region room response.
 

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Well...at least the ears do not lie. There is not need for a very sophisticated program to check for HT audio stuff. Anyway, the waterfalls in REW is a very useful program, and if correctly understood (I do not mean you don't) and utilized will with no doubt help enhance ringing (at low freqs of course).

Did you take the time to see the improvement clarfied by the waterfalls taken in my room...They definitely confirm the HUGE improvement that was done in my room. And at least, this is a practical measurment...:T
 

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NASA or any other company does not ever use waterfall plots to evaluate the response of their systems and the response of their control systems is similar to what we have here. I have never seen a waterfall plot in any serious controls theory, electric circuit theory paper or in any other form. Its never used for a reason - its useless !! They are only in audio, they look pretty but mean little. A well known Phd acoustician has agreed with me on this point and doesn't use them for the same reason.

They can obfusgate the situation when people mis interpret noise or the mathematical distortions in gating as an actual physical affect, as you have above.

Every single peak in the waterfall plot is due to a peak in the frequency response. If there is a section in the waterfall that displays a decaying portion without a peak in the frequency response then its noise or some other affect.

In some non mimimum phase systems you could possibly see a decaying part that does not show as a resonance in the first slice but the resonance must appear in later slices. This is very unlikely and absolutely unlikely in a modal region room response.
That is overstated Doug. As an example, I'm sure you have seen a great many instances where what appeared to be a single peak in the frequency response is the result of two or more modal resonances, it is only in the waterfall or by gating later parts of the impulse response that this becomes readily apparent. Whilst a waterfall plot can be misinterpreted, so can just about any other data representation - not least the frequency response itself, whose appearance in this context is entirely dependent on the window positions and durations used to generate it. It is very easy to produce frequency responses of widely varying appearance from the same impulse response. Waterfalls and similar/related data visualisations which portray combined frequency and time domain behaviour are very widely used in some fields of analysis, e.g. seismic data in oil/gas/mineral exploration.
 

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frequency response is the result of two or more modal resonances, it is only in the waterfall or by gating later parts of the impulse response that this becomes readily apparent.
I think that in many cases this is room noise that is also reinforced at certain frequencies by the dimensions of the room. Phase interactions between "modal noise" and signal may cause the waterfall to do weird things and be misinterpreted. I don't trust low level waterfall data because it can be corrupted by noise.

If you have a resonance and can't correct it with one filter then you need more than one, this becomes apparent only after one filter is used and you cannot ideally dampen the resonance. This occured in my room at about 140 Hz. There are three sharp resonances right around that area, easily identified with curve fitting.

Oil exploration, etc look almost exclusively at non minimum phase behavior under which conditions a waterfall plot may apply. I think that generally we should encourage users of our kind of software to look only at frequency response because the distortions associated with waterfall plots are usually poorly understood.
 

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But isn't this why the room acoustics is so important? One should not have to apply more than one filter to correct a resonance at one frequency if the room is doing its job! My belief is if you have to apply multiple filters for a narrow frequency spike or bump, it's the room or a very poorly designed sub driver or enclosure "ringing". Granted it still has to be corrected for the system to sound "right" but not multiple filters.

My engineering degree is not in acoustics, so this is just my opinion. :)
 

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You cannot have a reasonable room treatment if say you have a problem at 30 Hz...
 

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One should not have to apply more than one filter to correct a resonance at one frequency if the room is doing its job! My belief is if you have to apply multiple filters for a narrow frequency spike or bump, it's the room or a very poorly designed sub driver or enclosure "ringing". Granted it still has to be corrected for the system to sound "right" but not multiple filters.
My degree is in EE not acoustics as well - but even if you design a room you cannot guarentee not getting multiple modes at one frequency or a narrow band because rooms are never perfectly square nor built to precise dimensions. Higher order modes are dense & sensitive to small differences in planned and actual construction.

You don't need multiple filters to correct - you can just put a big notch in for low frequencies if you want.
 

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My degree is in EE not acoustics as well - but even if you design a room you cannot guarentee not getting multiple modes at one frequency or a narrow band because rooms are never perfectly square nor built to precise dimensions. Higher order modes are dense & sensitive to small differences in planned and actual construction.

You don't need multiple filters to correct - you can just put a big notch in for low frequencies if you want.
Absolutely,...if the room is not rectangular (non divisible sides) and very well constructed then its influence on the sound becomes unpredictable and more drastic (read treatments) measures have to be applied to "dampen" resonances, or at least tame them to the point where something like a BFD can control them without degradation in sound quality.

I still don't believe that using a big notch filter would be a final answer,...temporarily, yes. But long term you're going to get tired of it and will want to address what is causing that big bump.

So this is where I agree with brucek and the use of waterfall plots because we are dealing with an analog device that behaves like a car's suspension or sub in this case (spring & shock absorber). Remember the big old "American" sedans and how their suspensions behaved,...excellent frequency response most of the time, but throw in a curve or mid corner bump and they became unsettled, and that's why I believe in this instance waterfall plots are useful.
 
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Hi,

I would like to understand whether to intervene:
I don't succeed in flattening the 153 Hz blade (DEQ2496)
(2 of my Ht satellites about 6 ft convergent, microphone to the center)



Red waterfall toslink input;
Green waterfall DEQ equalization + FBD - 6 [email protected] 1/60 oct
Yellow waterfall DEQ equalization + FBD - 30 [email protected] 1/60 oct
Orange waterfall DEQ equalization + FBD - 30 [email protected] X 3 1/60 oct.

where am I wrong?

thanks

EDIT amrvf:

excuse me,
soundcard not calibrated!!! (not loopback but microphone signal!!!)
 

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Discussion Starter #33
I would say the peak is far too narrow to even hear.

Filters of 1/60th of an octave are far too narrow to be useful. Small microphone movements at that frequency would change the result.

Looks like the signal may be room noise.

brucek
 

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I'm back. Thanks for the earlier responses.

Here are some fresh waterfall plots of my 4 x 15" IB in a very non-parallel 27' attic with multilevel floors, an open stairway in the floor. Ceilings, floor and 45 degree walls are all boarded.

Listening position no BFD:


Listening position with BFD including +16dB boost @ 20Hz 120/60.


Nearfield with BFD.


Any useful thoughts? :)
 

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Brucek wrote earlier in this thread:

"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"

In Chrisbee`s second waterfall in the post above he has gained 20 hz with 16 db. I cant see that this has emulated a room mode at the gain frequency. The room mode seems to be the same as the higher frequencys.
 

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You may find this an interesting comparison: Measured at the listening position:



Here I have replaced my original 32Hz drivers with four brand new 16Hz drivers from AE.

BFD settings have been reset from the massive +16dB @ 20Hz boost required before.

I wanted to continue the new slope into the infrasonic to match my normal listening levels. Fletcher-Munson, Equal loudness etc.

A wide trough existed between the 12Hz room mode and ~25hz. I filled this with another quick and dirty boost filter at 20Hz..

Trial BFD Filters are:

20Hz + 5dB BW35.
25+2Hz -6dB BW20.
40Hz -4dB BW30
50+5Hz -3dB BW20
 

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Brucek wrote earlier in this thread:

"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"

In Chrisbee`s second waterfall in the post above he has gained 20 hz with 16 db. I cant see that this has emulated a room mode at the gain frequency. The room mode seems to be the same as the higher frequencys.
That is because the filter Chris used was very wide, spanning 2 octaves (BW 120/60). Modes have narrow bandwidths, typically 1/10th of an octave or less. Using narrow filters to boost the response creates the same extended ringing problems as room modes.
 

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In the first post...

Am I the only one that finds it odd that the decay rate of your BFD is over 100ms? I understand that some shape is expected for a bandwidth limited spectrum, but that long?
 

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Discussion Starter #40
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.

A loopback cable (with the BFD removed from the circuit) will show exactly the same result.

Isn't the 100ms that you refer to a function of the duration of the impulse response analyzed?

For example, if connect a loopback from line-out to line-in of the soundcard and sweep a measurement to 200Hz, and increase the duration of the gate out to 1000ms (1Hz frequency resolution), I would get the following result.

waterfall 1000ms duration.jpg

But if I decrease the gate to 100msec (10Hz frequency resolution), I would get the following result.

waterfall 100ms duration.jpg

brucek
 
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