# EQ can decrease ringing



## ToBeFrank (Feb 27, 2010)

I've read on here that EQ does not decrease ringing of a room mode because it does not change the decay rate. I think I can prove that wrong. I bought a BFD because Audyssey wasn't helping with my room modes in the lower frequencies. I set up the BFD filters before Audyssey. Below are the slopes of the decay rates of a room mode at 31.8Hz. The first graph is with Audyssey but before adding the BFD. The second is with the BFD and Audyssey. You can see the BFD significantly changed the decay rate in the first 250ms. After that the slope is about the same as without the BFD.


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## Nuance (Nov 2, 2008)

Do you have any before and after waterfall plots? I too was under the impression PEQ could not reduce ringing; only bass traps and room treatments can. It would be nice if that were untrue...


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## ToBeFrank (Feb 27, 2010)

Here are 3 graphs. The first is Audyssey with no BFD. The second is BFD and Audyssey. The third is BFD and Audyssey, but I set the level at 31.8Hz the same as the no BFD graph. You can clearly see the increased decay with the BFD. To me, this says that you can affect ringing using EQ.


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## ToBeFrank (Feb 27, 2010)

The spectral decay is where you can really see it. First is with Audyssey. Second is BFD and Audyssey (level matched at 31.8).


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## glaufman (Nov 25, 2007)

Upon first inspection, it certainly looks like you've got something there! I wonder though, will there be detractors who want to see the same thing with Audessey disengaged completely in both shots? Yes, I realize the Audessey provided the same contour in both shots, but nonetheless I wonder what people will come up with :T


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## ToBeFrank (Feb 27, 2010)

It's even more obvious without Audyssey. Here are the before and after graphs with just the BFD. They are level matched at 31.8.


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## Wayne A. Pflughaupt (Apr 13, 2006)

True, an equalizer can have an effect on ringing. As John M has explained in previous posts such as this one, a room mode will exhibit a slower rate of decay compared to the rest of the frequency spectrum. If we apply an EQ filter matched for that mode, we’ve robbed the mode of energy and essentially defeated it. The signal now decays at a rate more typical of other frequencies. 

However, the best an equalizer can do is bring the mode’s rate of decay back in line with what the rest of what the room is exhibiting. Switching to a longer waterfall window of say 650 ms, you may find that the EQ’s effect on long-term ringing is minimal. I.e., the equalized vs. unequalized signal levels at that duration may be about the same. 

This “before and after” shows what an actual improvement in ringing (faster decay) looks like:


















Note the significantly faster rate of decay above 140 Hz with the lower graph, which added bass traps to a room: At about 15 slices, the signal has dropped as much as 50 dB at some frequencies in less than 200 ms, compared to the baseline which shows decay times at twice that rate or more. You simply can't get this kind of "action" with an equalizer. (Note, any decrease in signal peaks you see are merely the effect of absorption from the traps.)

It should be kept in mind that the improved rate of decay the waterfall shows for a room mode tamed by an equalizer is only valid at the location where the measurement was taken.

Also - while the measurement platform is stable enough (REW for example), the transducers involved - the speakers and elements in the microphones - are not. Their physical (and consequently electrical) properties are altered with changes in temperature, humidity etc. As a result, when you take a second REW reading six months or a year later you'll find it doesn't look exactly like your original one. A waterfall graph generated today with last year's EQ filters isn't going to look as good as it did back on the day you fine-tuned the filters for minimal ringing.



ToBeFrank said:


> The third is BFD and Audyssey, but I set the level at 31.8Hz the same as the no BFD graph.


 It’s good that you were able to figure out that "before and after" level matching makes for a more valid comparison. :T

A couple of other tips for more meaningful waterfall graphs:

1. Keep the levels up to get a better picture. Decay rates are related to RT-60 acoustical measurement used to calculate reverb decay times – i.e., the time it takes the signal level to drop by 60 dB. Obviously, a measurement taken at a low level minimizes what a waterfall can tell you. A good waterfall graph should keep the levels at least 40-50 dB above the noise floor. 

2. Along the same lines, lower the graph limit to something reasonably akin to your room's true noise floor. Most residential rooms have an ambient noise floor lower than the 45 dB limit of our usual graphs. It’s best to lower the floor for a waterfall to 30-35 dB. If you live in a rural location, or have a substantially soundproofed room, you might want a 25 dB floor. 

Regards,
Wayne


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## ToBeFrank (Feb 27, 2010)

Wayne A. Pflughaupt said:


> Switching to a longer waterfall window of say 650 ms, you may find that the EQ’s effect on long-term ringing is minimal. I.e., the equalized vs. unequalized signal levels at that duration may be about the same.


I'm not sure I'm buying into this one. As my graphs above show, the initial decay slope differs significantly in the first 250ms. After that the EQ quits having effect and the natural decay rate of the room takes over, i.e. the slope of the decay is the same after 250ms between pre-EQ and post-EQ. However, since in the first 250ms I increased the decay by ~10dB and the natural decay is the same after that, the long term ringing should be reduced by ~10dB. I'll test it out.



> This “before and after” shows what an actual improvement in ringing (faster decay) looks like:


The word "actual" in there implies my graphs don't show an improvement. I don't agree. Can you not see the change in the slope of the decay in the first 250ms? Yes, it doesn't change the slope after that, but as I stated above, that change ultimately affects the long term.



> It should be kept in mind that the improved rate of decay the waterfall shows for a room mode tamed by an equalizer is only valid at the location where the measurement was taken.


Again, I'm not sure I agree. At 31.8Hz, the wavelength is 35.5 feet. A mode here will be affecting the level in the room across more than just the measurement position. A reduction in the mode will similarly affect more than just the measurement position. Indeed, when I made measurements around my listening position, the filter significantly flattened those measurements as well.



> It’s good that you were able to figure out that "before and after" level matching makes for a more valid comparison.


Actually, I don't think I did it correctly. When I said I matched the levels I meant I adjusted the graphs to match. I don't think this is correct. I need to actually adjust the output level of the sub as close as possible to the pre-EQ level to compare the decays. This is the only way to truly compare the results.



> 1. Keep the levels up to get a better picture. Decay rates are related to RT-60 acoustical measurement used to calculate reverb decay times – i.e., the time it takes the signal level to drop by 60 dB. Obviously, a measurement taken at a low level minimizes what a waterfall can tell you. A good waterfall graph should keep the levels at least 40-50 dB above the noise floor.


Agreed.



> 2. Along the same lines, lower the graph limit to something reasonably akin to your room's true noise floor. Most residential rooms have an ambient noise floor lower than the 45 dB limit of our usual graphs. It’s best to lower the floor for a waterfall to 30-35 dB. If you live in a rural location, or have a substantially soundproofed room, you might want a 25 dB floor.


The noise floor in my living room is 40-45dB. Wouldn't setting my graphs lower than that just be including the noise in the waterfall? Also, once the level has decayed to below that, isn't it inaudible and I don't care about the decay anymore?

BTW, I'm not blindly saying everything I say above is correct. I'm actually going to test it out with REW. I'm just stating what makes sense in my wee little brain. onder:

BTW 2, I'm not saying to use EQ over acoustic treatments. I've got several DIY treatments in my living room. But obviously, it's hard to treat these low frequencies.


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## JohnM (Apr 11, 2006)

A filter that accurately matches a modal resonance can have a very dramatic effect on decay at the mode's frequency (but remember that applies at the point where the measurement was taken, other places in the room will have a different response).

Here is a good example of what is possible, from a measurement that has a strong resonance at 50Hz (the 60dB decay time of the resonance is 1.23 seconds). The waterfall (300ms window, 500ms duration) looks like this:
 

With a single filter at 50Hz, -18dB the waterfall changes to this:
 

The resonance is in the noise floor of the plot at around 20dB. The decay of the filtered response is dramatically faster than the original, here is the unfiltered measurement 250ms in:

 

and here is the filtered measurement 250ms in:

 

It is already reaching the noise floor. The level at the filter centre frequency has dropped from 72dB to 22dB, a 50dB reduction from a -18dB filter.

To get these kinds of results the filter must be very precisely matched to the mode characteristics, compare the result with a filter at 49.5Hz, just 1% off the mode's centre:


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## ToBeFrank (Feb 27, 2010)

JohnM said:


> To get these kinds of results the filter must be very precisely matched to the mode characteristics, compare the result with a filter at 49.5Hz, just 1% off the mode's centre


This is something I've been wondering about... what's the best way to figure out the center frequency and bandwidth? Trial and error? I can start with the filters REW recommends, but I've found I can get improvement by modifying them. I feel kind of like I'm just fumbling around and guessing.


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## JohnM (Apr 11, 2006)

Trial and error is about all you can do with REW V4, and it can be a tedious process - aside from the re-measuring, the filter needs to match the mode's T60 decay time, but to match it not only must the centre frequency and bandwidth be correct, but the bandwidth depends on the filter's gain, so if the filter gain is changed the bandwidth needs to be adjusted to get back to the correct T60 time.

REW V5 has analysis tools to make a good initial determination of the centre frequencies and T60 decay times of the resonances in a measurement, and a "Modal" filter setting for which REW will automatically adjust the filter bandwidth as the gain is adjusted while maintaining the target T60 figure.


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## ToBeFrank (Feb 27, 2010)

Thanks John. Really looking forward to V5!


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## Doug Plumb (Mar 16, 2007)

The EQ can reduce room ringing at the measurement mic location but can increase it everywhere else. This is why passive treatment is better.

All IIR Equalizers (Eq's that use BiQuads) behave the same in this manner. It doesn't matter what product you use. It does matter how well it is adjusted in terms of precision.


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## glaufman (Nov 25, 2007)

If you're using an EQ to reduce room ringing at one point, presumably you're introducing a cut. that being the case, how can that increase ringing at another location?

Or are you assuming that the overall level needs to be re-set once the cut is in place?


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## Doug Plumb (Mar 16, 2007)

It could increase ringing in other parts of the room. It will not necessarily, but may. There isn't anything to say that it will not.

Room resonance shifts a bit as you measure different locations - pls don't ask me to explain this because I cannot. It makes absolutely no sense to me, but its true.


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## Doug Plumb (Mar 16, 2007)

The resonance change is not due to the change in other poles near the one you are measuring - its a definite slight change.


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## glaufman (Nov 25, 2007)

Doug Plumb said:


> It could increase ringing in other parts of the room. It will not necessarily, but may. There isn't anything to say that it will not.
> 
> Room resonance shifts a bit as you measure different locations - pls don't ask me to explain this because I cannot. It makes absolutely no sense to me, but its true.


I will admit I may be a little lacking on both the experience and thoeretical side of this question, but as I understand it, it shouldn't, so I agree with you there. Frequencies at which modes are present is entirely 100% due to the room, not where in the room you measure. Ignoring for the moment the acoustic effect of having your mic stand in a different place (or standing in a different place for that matter), what CAN change is if you reduce a mode you may suddenly see a very closeby mode that was previously masked. And one may have not known to predict that mode because we sometimes focus an axial and forget about tangential and oblique modes. Now, in another location, you may see the same cut doesn't reduce the ringing as much, that makes sense, if you measure in a null, you may see increased decay time, (though I wouldn't go so far as to expect it) but I can't imagine getting additional ringing if you're putting less energy into the mode.

Since it seems you agree in theory, unless I misunderstood you, perhaps you have some scans to illustrate what you've experienced?


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## Doug Plumb (Mar 16, 2007)

Nearby room effects are irrelevant. I don't have scans to show this, but lots of experimenting verified this for me. I also got expert confirmation that this does occur.

Some aspects of room acoustics are a little over my head. I only studied PDE as applied to near ideal situations.


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## ToBeFrank (Feb 27, 2010)

I'm definitely now a believer when JohnM says you have to get the filter just right to nail the room mode. I made some improvement with my first set of filters (the graphs I posted at the beginning of the thread), but after some trial and error I finally hit the right combination to mostly kill my 31.8Hz room mode. I went from 10dB decay at 300ms before EQ to 30dB decay at 300ms after EQ. It's still not great if we're talking RT60, but it's a massive improvement. I watched Iron Man on bluray tonight and my bass sounds fantastic!


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## Doug Plumb (Mar 16, 2007)

I don't think that low frequency room acoustics are particularly important, although RPlusD nails room modes at least as well as anything available anywhere, I don't think its particularly important unless you have a severe mode. In which case you can null it out if it is in a region of sufficient modal density.

In my apartment I had three co-incident modes around 130 Hz within a Hz of each other. I could identify all of them independently with RPlusD and fixing the problem made a serious improvement in sound, but so did notching them out.


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## ToBeFrank (Feb 27, 2010)

Doug Plumb said:


> I don't think that low frequency room acoustics are particularly important, although RPlusD nails room modes at least as well as anything available anywhere, I don't think its particularly important unless you have a severe mode. In which case you can null it out if it is in a region of sufficient modal density.


Can you elaborate on why you think low frequency acoustics isn't important? I was under the impression this area is important to get clean, crisp bass rather than boomy bass. In my room I have bad modes around 32Hz and 45Hz. Since my only placement option for my main seating position is close to the back wall, I have these modes no matter where I place my sub (which also has limited placement options). Due to the low frequencies I'm unable to use treatments to work on them. This leaves me with EQ.



> In my apartment I had three co-incident modes around 130 Hz within a Hz of each other. I could identify all of them independently with RPlusD


This is interesting. Will I be able to try this out with the demo version of RPlusD? If I can test it out it could be worth the investment. I still have some ringing around 25Hz that I suspect is from more than one mode close together, and I've had little success getting the correct filters in REW.


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## Doug Plumb (Mar 16, 2007)

Obviously having a clean kitchen is more important than having a clean driveway, but if you see a bunch of phoney environmentalists walking up your driveway then you will want to go out and hose them down and not even consider your kitchen floor. If you can't hose them down then asking them to leave is your only choice.

The same logic applies with rooms. Some rooms have a terrible response and it needs to be corrected by placing absorbers in the corners, failing this EQ may be your only choice. 

But the region between 100 Hz and 5 KHz is far more impostant because this is where our hearing is most acute, just like you would have a greater sensitivity to dog poop on your kitchen floor where it wouldn't be an urgent matter if it was on your driveway.

"This is interesting. Will I be able to try this out with the demo version of RPlusD? If I can test it out it could be worth the investment. I still have some ringing around 25Hz that I suspect is from more than one mode close together, and I've had little success getting the correct filters in REW."

I doubt you have more than one mode at 25 Hz unless your room is very big. RPlusD can identify the separate modes, but it is sometimes not easy to make that happen and you may have to use quite a few mic location measurements to get the software to ID both at one time.

The best way to do it is to adjust the filter and see the changes, since RPlusD emulates the EQ it eliminates the measure->adjust->measure-adjust cycle. You need the add on package to do this and you could do it just as well be hand with the basic package with one mode.


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## JohnM (Apr 11, 2006)

Doug Plumb said:


> It could increase ringing in other parts of the room. It will not necessarily, but may. There isn't anything to say that it will not.


Filters have their own decay time which depends on their gain (the more negative the gain, the lower the decay time), bandwidth (lower bandwidth/higher Q means longer decay) and centre frequency (lower centre frequency means longer decay for a given gain and Q). REW V5 shows this figure on the filter adjustment panel as "filter T60". If the amplitude of a modal resonance at a position in the room is lower than at the location the filter was configured for, particularly if the amplitude is zero, the filter's own decay may be greater than the room's acoustic decay at its centre frequency. The decay tail of the filter may then be observed emerging from the waterfall in the later slices. However, this decay tail is very unlikely to have any audibly detrimental effect, as the few studies that exist suggest that we have great difficulty perceiving long decays in the absence of a corresponding peak in the frequency response. More audibly significant will be the dip in the frequency response where the filter is cutting without a modal peak present.


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## ToBeFrank (Feb 27, 2010)

Doug Plumb said:


> But the region between 100 Hz and 5 KHz is far more impostant because this is where our hearing is most acute, just like you would have a greater sensitivity to dog poop on your kitchen floor where it wouldn't be an urgent matter if it was on your driveway.


Right, but in that range I can work on it with treatments, which I've already done. However, I can't do much in the lower ranges with treatments and have to use EQ. That's why for the purposes of this thread I'm ignoring anything but the low frequency range.


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## glaufman (Nov 25, 2007)

JohnM said:


> If the amplitude of a modal resonance at a position in the room is lower than at the location the filter was configured for, particularly if the amplitude is zero, the filter's own decay may be greater than the room's acoustic decay at its centre frequency. The decay tail of the filter may then be observed emerging from the waterfall in the later slices.


Having trouble wrapping my head around that one, but suffice to say I've been corrected.:blink:


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## Doug Plumb (Mar 16, 2007)

A big cardboard box such as a TV box does a lot, but like this, anything effective is going to be big. But it can be done.


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## corock (Sep 7, 2009)

Doug Plumb, you should write analogies for a living. Great stuff.


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## Doug Plumb (Mar 16, 2007)

Thanks, I've been working on writing an analogy for the last five years or so, but that would be off topic. That comment just fell out of me.


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