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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.
 

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Discussion Starter #42
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? :huh:

brucek
 

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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.
 

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Discussion Starter #44
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)............


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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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.
 

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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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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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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?
 

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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.

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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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:
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:
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…

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.:dizzy:

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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Nick,

brucek’s graphs were presented as a visual aid for his narrative. No, the Behringer does not add ringing to an electrical signal, nor does any other equalizer. It’s the room that does that.

Don’t let all the theory confuse you. It’s easy enough to take your own REW readings and compare equalized to baseline and see what you get.

Finally, should I not be using a waterfalls plot in determining how bad the room accoustics are for my living room?
REW can certainly show you the effects of equalizing and treatments, if you apply them.

Regards,
Wayne
 

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Just check my small experiment supported with graphs at the beginning of the thread ...This is not a simulation but true measuements
 

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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.
Thanks Wayne. I guess my point was that if cuts help with an area of higher SPL and resonance, shouldn't boost help with the opposite situation? It could be the room or a lack of low frequency capability of the subwoofers, but sometimes a little boost is useful and I do not think it would introduce resonance.

Now, there are of course other reasons to use boost judiciously or not at all, including maximizing the signal to noise ratio through the BFD (although I believe one has to look at the entire system's S/N ratio, also), and the fact that boosting a null eats up your sub amp's headroom (needlessly, if you really have a null that does not respond). However, a little boost can be used to treat a low line level output from the BFD, which was my problem with using just cuts.

Unfortunately, I think there needs to be a case-specific evaluation by each person as they tweak their system. Luckily, there forum is available to guide people along the way.

Pete
 

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However, a little boost can be used to treat a low line level output from the BFD, which was my problem with using just cuts.

Unfortunately, I think there needs to be a case-specific evaluation by each person as they tweak their system. Luckily, there forum is available to guide people along the way.
Pete
I agree with you about boosting to get back the signal that was reduced by cuts. This will have its problems but the benefits are greater IMO.
 

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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
Perhaps polarity is the right way. Here's what I'm thinking. Run the second sub through a bandpass filter with center frequency the same as the room resonance. The second sub would have the polarity inverted. Sorry I don't have a second sub to try.
 

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I should mention that there is absolutely no doubt that the BFD is NOT introducing any kind of ringing when the filter is boosted. What we're seeing is precisely the effects of the window used to generate the waterfall...not what's happening in real life.

I brought it up to point out that changes in amplitude can pretend to show differences in ringing (as so clearly demonstrated for us). Since we know that no ringing is added when a frequency range is boosted, we can use the graphs to normalize what constitutes no change in decay when amplitude is changed.

In other words, any EQ added to your subwoofer is going to show the exact same difference on the waterfall that the EQ made....and it's not because the EQ magically made the room ring less. Likewise, any natural amplitude variations with the subwoofer will also show up as ringing (and dips will show up as decaying faster).
 

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I should mention that there is absolutely no doubt that the BFD is NOT introducing any kind of ringing when the filter is boosted. What we're seeing is precisely the effects of the window used to generate the waterfall...not what's happening in real life.
That's not correct. The windowing effects are the cause of the response across the freq band not dropping sharply to the noise floor when no filters are active, but every EQ filter (gain or cut) rings at its centre frequency, the higher the Q (the narrower the bandwidth) the longer it rings. The ringing is easily observed in the impulse response by setting up the BFD in loopback and making a measurement with a sharp filter, it is more obvious with boost than cut but both ring. It is because of this time domain behaviour that EQ filters are able to counteract the ringing of modes, the decay of the EQ filter's attenuation over time matches the decay of the mode's gain, when the filter is properly set to match the mode's bandwidth and gain.
 

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That's not correct. The windowing effects are the cause of the response across the freq band not dropping sharply to the noise floor when no filters are active, but every EQ filter (gain or cut) rings at its centre frequency, the higher the Q (the narrower the bandwidth) the longer it rings. The ringing is easily observed in the impulse response by setting up the BFD in loopback and making a measurement with a sharp filter, it is more obvious with boost than cut but both ring. It is because of this time domain behaviour that EQ filters are able to counteract the ringing of modes, the decay of the EQ filter's attenuation over time matches the decay of the mode's gain, when the filter is properly set to match the mode's bandwidth and gain.
So, can I assume that would part of the key to using the waterfall to tweak your response...adjusting the bandwidth of the filters to modify the waterfall plot?

What is the ideal decay rate (or realistic desired decay rate where sound quality is not compromised - i.e. we obviously aren't listening in anechoic rooms), or do we just want it to be relatively even across the frequency response?

I guess could also be said that boost in particular should be performed at as wide a bandwidth as possible, with subsequent cuts to included peaks, instead of boosting individual dips - which is more likely to introduce ringing?

This is a very informative thread - it motivated me to pick up a sound card yesterday.

Pete
 

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So, can I assume that would part of the key to using the waterfall to tweak your response...adjusting the bandwidth of the filters to modify the waterfall plot?
Exactly, though centre frequency and gain might also need tweaking for best results.

PeteD said:
What is the ideal decay rate (or realistic desired decay rate where sound quality is not compromised - i.e. we obviously aren't listening in anechoic rooms), or do we just want it to be relatively even across the frequency response?
Ideally decay would be uniform across the band, for domestic rooms the ideal is thought to be around 250-300ms, though there is more tolerance of extended decay times at low frequencies than high. Look for discussions about RT60, a measure of decay time (more relevant for large venues but often discussed in the context of domestic rooms also).

PeteD said:
I guess could also be said that boost in particular should be performed at as wide a bandwidth as possible, with subsequent cuts to included peaks, instead of boosting individual dips - which is more likely to introduce ringing?
Right again :)
 

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Ideally decay would be uniform across the band, for domestic rooms the ideal is thought to be around 250-300ms,
But at what amplitude would the signal start? And at what noise floor ? Those things dramatically affect the apparent rate of decay that a waterfall graph shows. Without a set guideline for signal amplitude and noise floor parameters, a 250-300 ms “ideal” is highly approximate and arbitrary. You can make a waterfall look "worse” by raising the SPL level of the measurement. You can make it look "better" by lowering the SPL level.


baseline waterfall.jpg
Baseline Measurement

raised spl level waterfall.jpg
Measurement SPL Level Increased

lowered spl waterfall.jpg
Measurement SPL Level Decreased


In the same manner, waterfalls can also be made to look worse or better by raising lowering the graph’s floor. I really don't like the arbitrariness of this whole thing..

And where do phase changes from filtering fit into the picture? I lifted this from Rane’s Exposing Equalizer Mythology note. The bolded text (emphasis mine) pretty much reads like what you’ve often stated here at HTS, except he’s talking about phase shift, not modal ringing. Comments?


Phase shift is not a bad word. 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.

The frequency response of most performing rooms looks like a heart attack victim's EKG results. 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.

An equalizer is a tool. A tool that allows you to correct for a room's anomalies. It must be capable of reproducing the exact opposite response of the one being connected. This requires precise correction at many neighboring points with the associated phase shift to correct for the room's opposing phase shift. It takes phase shift to fix phase shift. Simple as that.

One way people get into trouble when equalizing rooms is using the wrong type of equalizer. If an equalizer is not capable of adding the correct amount of phase shift, it will make equalizing much more difficult than it has to be. The popularity of the many constant-Q designs has come about because of this phenomenon. Equalizers that produce broad smooth curves for modest amounts of boost/cut make poor room equalizers, and good tone modifiers. They lack the ability to make amplitude and phase corrections close together. Lacking the ability to make many independent corrections with minimal interference to neighboring bands restricts their usage primarily to giving a shape to an overall response rather than correcting it. Serious correcting requires sharp constant-Q performance, among many other things.

Only by adding many precise, narrow phase shift and amplitude corrections do you truly start equalizing a system's blurred phase response. You do not do it with gentle smooth curves that lack the muscle to tame the peakedness of most rooms. Broad smooth curves do not allow you to correct for the existing phase shift. Its just that simple, you must pre-shape the signal in both amplitude and phase. And that requires narrow filters that preserve their bandwidths at all filter positions.


Regards,
Wayne
 
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