# Rew eq filters for ringing



## marty1

I thought I would have another go at REW eq filters for my bfd to see if I can improve my response and it looks pretty good, a lot better than I have been able to achieve manually.

The only thing is it doesn't seem to reduce ringing, in fact although my own attempt to manually eq the bfd did not smooth out the response as good as rew, it seems that with rew's own filters applied there seems to be an increase in ringing compared to my attempt. It may have something to do with the fact that rew applied some boosts where as I only applied cuts?

Is there somewhere in the filter task settings that can account for ringing and put filters in place to improve this side of the response as well as smooth out the response.

I've attached the response of my filters and then rew filters so you can compare.

Marty


----------



## Wayne A. Pflughaupt

Hey marty,



> The only thing is it doesn't seem to reduce ringing...


As you have discovered, an equalizer doesn’t reduce ringing in the strictest sense of the word. What it does is deprive a room mode of energy. That does have the effect of reducing the mode’s ringing, but it only brings it back in line with what the rest of the room is exhibiting. It won’t make any improvement beyond that.

If your intent is to reduce ringing, you need bass traps. These graphs show the effect of bass traps on ringing; you simply can’t get this with an equalizer.










*Empty Room









With Four Bass Traps*​

For a more dramatic demonstration of the above graphs, open up their direct links into two browser tabs and switch between them.

http://media.soundonsound.com/sos/sep04/images/realtrapsbefore.l.jpg
http://media.soundonsound.com/sos/sep04/images/realtrapsafter.l.jpg




> it seems that with rew's own filters applied there seems to be an increase in ringing compared to my attempt. It may have something to do with the fact that rew applied some boosts where as I only applied cuts?


Any increase in signal level will show up on a waterfall graph as an apparent increase in ringing. It isn’t necessarily so; it’s a common mistake to confuse a higher signal with increased ringing, but one has nothing to do with the other: Ringing – improving it or making it worse - is about the _rate _ of signal decay. Signal levels have no effect on that. For a better understanding of signal levels vs. rate of decay, see this post.

Regards, 
Wayne


----------



## marty1

Thanks Wayne, helpful as ever 

I have gik acoustic bass traps pretty much everywhere, I'm getting a few more from Bryan soon so it will help some more.

The trouble is the ringing or the decay times are bigger low down where bass traps are not that effective so eq is really the only way to bring it under control. I did notice at the bottom of the filter tasks it has the modal resonances which I thought may indicate that rew will account for them and try and bring the decay time to below 300ms.

Marty


----------



## JohnM

To target filters at specific resonances you will need to do some manual work. Use 'Find Resonances' in the Modal Analysis part of the EQ window to generate the list of resonances in the measurement. Find one of the problem frequencies you see in your measurement in the list of resonances and note the exact frequency and T60 value. In the EQ filters add a filter of type 'Modal' and set the frequency and Target T60 values to those in the resonances list. Adjust the filter gain and check the 'Predicted' waterfall to see how the ringing has been affected. Try small changes in the Modal filter's centre frequency to make sure it has the correct value, you can also try changing the Target T60 value to see whether the resonance analysis found the best setting. It is a fairly interactive process, but it is straightforward by watching the effect in the predicted waterfall plot.


----------



## marty1

Thanks John I will give that a try.

Marty


----------



## Wayne A. Pflughaupt

marty1 said:


> The trouble is the ringing or the decay times are bigger low down where bass traps are not that effective so eq is really the only way to bring it under control.


Unless you have some modal peaks down there, EQ is not going to help.



> I did notice at the bottom of the filter tasks it has the modal resonances which I thought may indicate that rew will account for them and try and bring the decay time to below 300ms.


You might want to fully define your objectives, because “300 ms” is a pretty vague target. You can “achieve” 300 ms by reducing the signal level, or by raising the graph’s lower vertical value from 45 dB - which is already too high - to something even higher.

Also keep in mind that ringing accomplishments achieved via equalization are only “good” at the measured position today. Move the mic to another location and it's “gone.” Take some sweeps six months from now your measurements won’t look quite the same; neither will the waterfall graphs they generate.

Regards, 
Wayne


----------



## marty1

Wayne A. Pflughaupt said:


> Unless you have some modal peaks down there, EQ is not going to help.
> 
> You might want to fully define your objectives, because “300 ms” is a pretty vague target. You can “achieve” 300 ms by reducing the signal level, or by raising the graph’s lower vertical value from 45 dB - which is already too high - to something even higher.
> 
> Also keep in mind that ringing accomplishments achieved via equalization are only “good” at the measured position today. Move the mic to another location and it's “gone.” Take some sweeps six months from now your measurements won’t look quite the same; neither will the waterfall graphs they generate.
> 
> Regards,
> Wayne


I was under the impression that ringing exceeding 300-500ms, audibly can be mistaken for a peak and muddy the sound, so my aim was to flatten out the response and reduce ringing as much as possible.

In regards to the fact of it only sounding good at one position but isn't that the same with all eqing, I have never been able to get a good sound in more than one position, unfortunately my wife has to sit in the non sweet spot :devil: but with eq adjustments I usually find if you attend to a peak or dip at one position another position gets affected.

Regards
Marty


----------



## Wayne A. Pflughaupt

Let’s discuss some basic physics about waterfalls, signal levels and decay.

First, let's clarify a couple of definitions. 

In the following discourse, when I refer to "decay time" I mean the time it takes a signal to fade away, relative to its gain or loudness. E.g., an 85 dB signal will fade down to the noise floor sooner than a 100 dB signal. 

When I refer to "rate of decay" I mean the speed at which the sound decays. E.g., if we have two 85 dB signals and one fades to the room's noise floor in 300 ms, and the other in 200 ms, the latter has a faster "rate of decay." 

Technical papers on reverberation time and RT60 make no distinction in terminology between the two (“decay time” and “rate of decay”), so these probably aren't "correct" scientific definitions. However, I feel they'll be helpful to keep things understandable and making sense. 

Beyond this, it should also be understood that "ringing" is merely a succinct term that means "low frequency decay" – with (again) no distinction between the “decay time” and “rate of decay.” You’ll typically see even people knowledgeable on the subject saying things like, “if you do ‘xxx,’ an increase (or decrease) in ringing can be observed,” with no indication if they mean “decay time” or “rate of decay.” (I suspect that most of them don't know a difference exists.)

Confused already? Don't be - it took me years to sort this stuff out. Let's try to untangle it all.

Now: Let’s take a look at the effect that signal levels (gain) have on a waterfall. Here’s a waterfall graph from another thread:










Looks pretty scary, huh? Notice that the signal is peaking at nearly 110 dB. Now, let’s look at the same measurement with the signal reduced to peak at 85 dB:










Just like magic it looks much better, doesn’t it? 

Here's another example. Doesn't the second graph look great? The only difference is that its signal level has been reduce by 10 dB and bingo, we suddenly have a fabulous-looking waterfall.















​

So as you can see, merely reducing the level of the signal makes for a great-looking waterfall, _even if nothing has been factually improved._ The apparent decay time of the signal has been “improved” merely because the quieter signal will obviously fade away sooner than the louder one...










... but that is not the same thing as improving the _rate_ of decay, as you see happening in the right side of this graph:










So getting back to your query...


marty1 said:


> I was under the impression that ringing exceeding 300-500ms, audibly can be mistaken for a peak and muddy the sound, so my aim was to flatten out the response and reduce ringing as much as possible.


...hopefully now you can understand why I said, "You might want to fully define your objectives, because '300 ms' is a pretty vague target. You can 'achieve' 300 ms by reducing the signal level, or by raising the graph’s lower vertical value from 45 dB - which is already too high - to something even higher." 

Now let’s look at the relation between room modes and signal level. 

A room mode is merely a build-up of bass energy that causes a substantial increase in level (gain) at a certain frequency. As we’ve seen, any increase in gain nets an increase in decay time: A room mode takes longer to fade away merely because it is louder than the rest of the signal. 

Again, this is not to be confused with the _rate_ of decay. _However_ – what distinguishes a room mode from a “regular” peak in response is that it will also display a _longer rate of decay,_ in addition to the increase in signal gain. 

What can we do about the huge “sore thumb” signal level of the room mode? Enter the equalizer. An equalizer is merely a device that alters signal gain at specified frequencies.










*Baseline (purple) vs. Equalized Response (black)*​

With a parametric equalizer we can set a precise filter – bandwidth, frequency and negative gain value - that counteracts the mode and basically robs it of energy. We can see the effect with this "before and after" that features a nasty mode at 55.7 Hz. Counteracting the mode with a precisely-set parametric filter eliminates its audible and unpleasant “boomy” effect.



















So by reducing the level (gain) of the mode, the equalizer brought its decay time, _as well as its rate of decay,_ back in line with the room's average. But notice that the mode’s rate of decay at the frequency where it was located - 55.7 Hz - _has not improved beyond what the room is already exhibiting._ 

Here's another example, an even better one. In this comparison, a parametric filter was precisely set for the bandwidth and gain of a room mode at 41.9 Hz. In the second graph, the level of the signal after equalization was raised to match the SPL reading the mode was displaying before being equalized. In other words, 41.9 Hz are at the same SPL in both graphs. 

Naturally, increasing the signal level makes the graph look worse overall (as discussed above), but _*before and after level-matching is the only way to tell if an EQ filter has resulted in an improvement in the mode’s rate of decay.*_ In this case we can clearly see that after equalization, the frequency where the mode was located (41.9 Hz) now displays a significant improvement in rate of decay. And once again, we can also see that the rate of decay has not improved beyond the room average.















​

Why is this? The next thing to understand is that ringing is the same to low frequencies as reverberation (or echo) is to the upper frequencies. Both have to do with the rate of decay: If you have a room with a lot of hard surfaces, it has a lot of reverberation because the signal bounces around all over the place and takes a long time to fade away. Add some room treatments, furniture, carpet etc. and the reverberation virtually vanishes. Why? _Absorption._ The furnishings and treatments absorb the signal and thereby the reverberation is truncated – i.e. the rate of decay the "live" room exhibited has been radically stunted. Anyone will tell you that an equalizer is no cure for a "live" room that has lots of echo and reverberation, nor is any other electronic device.










In the same manner, absorption is required to improve low-frequency decay times – a.k.a. “ringing.” Typically this means bass traps or something similar. An equalizer can only make adjustments in gain levels to problematic frequencies; it cannot absorb acoustical energy. It can make a waterfall "look" better to the untrained eye by reducing the signal level of peaking frequencies, but again - that's not to be confused with an improvement in the _rate_ of decay.

So, how do you determine from a "before" and "after" waterfall graph if you have actually realized an improvement in ringing? This probably sounds overly simplistic, but just study the spacing between the horizontal lines. Each horizontal line indicates a "slice" (fraction) of time as the signal decays from its "starting point" until it falls below the graph's floor. So, if there is an improvement in the rate of decay - i.e., if the signal in an "after" graph is actually decaying faster than in the "before" graph - there will be wider spaces between the horizontal lines. 

This is clearly evident in the graphs below (and the ones presented in Post #2) that show ringing in a room with and without bass traps. Note the dramatic difference above 140 Hz that absorption makes. You simply can't get this effect with an equalizer - again, it can't absorb acoustic energy. Don't get me wrong, the equalizer is a great tool for what it does. But you have to know and respect its limitations.

















*Courtesy of RealTraps*​

So again getting back to your query...


marty1 said:


> The trouble is the ringing or the decay times are bigger low down where bass traps are not that effective so eq is really the only way to bring it under control.


...hopefully you can now understand why I said that equalization is not going to help your ringing problem. The graphs in your first post show no evidence of a room mode in the lowest frequencies. The only way to "improve" any ringing you have down there with an equalizer is to reduce the gain, but as we've seen that’s only a superficial improvement, not an actual one.

To further demonstrate, let's revisit this graph shown earlier in this post, where an EQ filter eliminated a 55.7 Hz room mode: 










This time take a closer look at the spacing of the slices seen both outside the circle, and inside where the mode was equalized. Note that the signal level (SPL) of the area below 45 Hz is something on the order of 10-12 dB higher than the signal level above 45 Hz. Yet, _the spacing of the slices is virtually the same across the board._ This should sufficiently demonstrate that once you've eliminated the room modes, you can't equalize to further "improve" ringing. All you're doing at that point is making gain (signal level) changes, not improvements in the rate of decay. 

So this is the situation you're facing by trying to use equalization to improve your ringing at the lowest frequencies, where there are no apparent room modes: The only "improvement" you'll see will merely be the effect of the signal level being reduced by the equalizer, so you're just chasing your tail. All you're going to get is less bass.

Conversely, it’s not uncommon to hear some say that using positive-gain (boosted) EQ filters causes ringing. Well sure, if you increase the signal level at a specific frequency, it’s going to take longer to fade away. This isn’t necessarily a bad thing: If your frequency response has a depressed area, its decay time is less than what you have at adjacent frequencies. Increasing its signal level will naturally increase its decay time – the two can’t be separated. There’s no downside, unless you happen to think your sub sounds better with certain frequencies attenuated below the rest (hat tip: it doesn’t).

But does a boosted filter increase the depressed area’s _rate of decay_ along with its decay time? In other words, will that area now take longer to fade away than adjacent areas?

We had a member here once offer the following graph as emphatic proof that positive-gain filters would indeed increase ringing. It certainly seems plausible, as you can plainly see that spacing between the slices where the boosted filter is employed is much tighter than what is seen at frequencies above and below the filter. This indicates a dramatic and detrimental increase in the rate of decay.










The problem is that the filter, and waterfall graph, was merely generated in the REW program via a loopback. In other words, the graph is an electronic representation, not an acoustic representation. What really matters is what happens with an in-room measurement.

Here’s a graph of my subwoofers after equalization, where I applied a combination of boost and cut filters, at the locations and in the amounts indicated. The 22 Hz filter was a fairly wide one employed to enhance extension. (NOTE: I prefer a linear graph for this example as it makes the upper frequencies easier to study.)







​

You can see by studying the spacing between the slices that the boosted areas _do not show an appreciable increase in rate of decay_ compared to other areas of the graph. Indeed, the boosted filter at 85 Hz shows the _same spacing_ as the negative-gain filter at 42 Hz. 

How can this be, if positive-gain filters increase ringing, or more precisely, the rate of decay? Obviously, a waterfall graph generated in a sterile electronic environment bears no resemblance to one generated in a messy acoustical world.

If boosted filters increase the rate of decay at the frequencies employed, it’s obvious that the effect is swamped by the signal decay time the room exhibits naturally. It would require a room heavily dampened with bass traps to prove that boosted EQ filters increase the low frequency rate of decay. For the rest of us, it’s a non-issue. There may be good reasons not to use boosted EQ filters (most of which are arguable as well). But an increase in ringing isn’t one of them.

Regards, 
Wayne


----------



## Barleywater

midbass with no eq:









Above is waterfall of 10" driver with potential use as midbass. Not a pretty sight.

Simultaneous solution for equalizing amplitude and phase of all frequency components in FFT of impulse response results in the following:









Ringing at 90Hz is cutoff of high pass filter. Complementary ringing is set up by low pass filter of woofer, and it disappears when woofer is added to system. Much of decay is actually redistribution of driver harmonics energies into intermodulation components.

A bass trap is effectively an EQ with center frequency, a width, and a damping factor. This may be applied directly to a driver based on its performance in the room.

When too narrow a notch is applied to a peak, the result is two smaller peaks. When too broad a notch is applied to a peak, the peak is brought down, but so are neighboring frequencies. 

Rooms are complex resonators, and so are speaker drivers. The room is passive and is pumped (illuminated) by the driver. The resonances common to room and driver from driver location are energized(unmasked). Balancing the frequency dependent energy at the driver required to get uniform response from the room is much easier than blasting the room with energy and shaping the room response with a large array of damped resonators.

Here is same process applied to 12" sealed woofer measured at 9':

Raw response spectrum:










Equalized response spectrum:









Woofer is driven by Hafler Pro 2400 bridged to mono 400watts. Without equalization the driver is prone to bottoming. With equalization voltage swings across driver terminals are greater, but excursion is tightly controlled. Articulation is super tight from upright bass to diesel locomotive. No room treatments, no bass traps and no smoothing.

Andrew


----------



## lbrown105

really curious about this eq for increasing decay slopes (thereby reducing decay times for a given SPL). The thread/post attached seems to contradict this thread about effectiveness of PEQ to increase decay slopes. In the thread someone tries to give evidence by using eq to bring down a specific peak, increase the overall volume and and keep the starting SPL at that freq equal and showing a reduced decay time which indicates a steeper slope since starting at the same SPl value.

Does this evidence contradict this thread? Not sure I am rreading this correct but if a PEQ can increase the rate of decay of <50HZ freqs it sure would be great vs trying to install 4 ft thick bass traps!!


http://www.hometheatershack.com/forums/rew-forum/7135-waterfalls.html


----------



## Barleywater

Thank you for digging up Waterfalls thread. This is in complete agreement with my post #9, and both my post and Waterfalls thread contradict Wayne's view.

At low frequencies you are inside the speaker.

Andrew


----------



## Wayne A. Pflughaupt

lbrown105 said:


> really curious about this eq for increasing decay slopes (thereby reducing decay times for a given SPL). The thread/post attached seems to contradict this thread about effectiveness of PEQ to increase decay slopes. In the thread someone tries to give evidence by using eq to bring down a specific peak, increase the overall volume and and keep the starting SPL at that freq equal and showing a reduced decay time which indicates a steeper slope since starting at the same SPl value.
> 
> Does this evidence contradict this thread? Not sure I am rreading this correct but if a PEQ can increase the rate of decay of <50HZ freqs it sure would be great vs trying to install 4 ft thick bass traps!!
> 
> 
> http://www.hometheatershack.com/forums/rew-forum/7135-waterfalls.html


Not sure exactly what the contradiction would be, since there is no consensus in that thread. I mean, a thread doesn’t go on for 17 pages unless there is a lot of debate over conflicting ideas and interpretations, not to mention people just trying to get a handle on the topic. When someone says something like...


> As I have frequently mentioned I boost at maximum at 20Hz to compensate for my high Fs 32Hz non-spec drivers. Here's a waterfall I generated from an older file. *It seems that boosting with the BFD does indeed increase ringing.*





> I don't think anyone can argue that this filter is also operating in the time domain?
> *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.*
> 
> 
> *Waterfall plot of a BFD using a single filter of (40Hz, Gain +15dB, BW 10)*


...it's clear they don’t understand the relation between signal levels and decay times, and as such have a fundamental misunderstanding of the subject matter.

When the Waterfalls thread was opened, waterfall graphs and signal decay analysis was a new thing to most of the regulars here (JohnM being the obvious exception), and many of the posts in the thread are quite frankly in error or at least ill-informed. And I’d include many of my own to be in that category. I personally have learned a lot more about signal decay since then, and especially how to analyze the waterfall graphs. 


That said, a parametric EQ will do a great job of taming the 50 Hz problem in your room and bring its decay time back in line with what the rest of the room is exhibiting, assuming it’s being caused by a mode (which typically a problem like that is).

Regards, 
Wayne


----------



## Barleywater

Wayne,

Here you contradict yourself:

Post #8: 



> Note the dramatic difference above 140 Hz that absorption makes. You simply can't get this effect with an equalizer - again, it can't absorb acoustic energy.


Post #12:



> That said, a parametric EQ will do a great job of taming the 50 Hz problem in your room and bring its decay time back in line with what the rest of the room is exhibiting, assuming it’s being caused by a mode (which typically a problem like that is).



Regards,

Andrew


----------



## Wayne A. Pflughaupt

Hey Andrew,

I’ll sum up Post #8 in a nutshell:

* An equalizer will reducing ringing in a room mode, but only to the point of (and not beyond) what the room is naturally exhibiting.
* If you desire a reduction in ringing beyond that point, absorption is required.
* An equalizer will not reduce ringing in areas of the frequency spectrum where there is no room mode. For that, absorption is required.

Regards, 
Wayne


----------



## lbrown105

thanks guys for summing this up for me. Makes mrore sens and I totally understand that earlier people were going through a learning curve. Much like I am trying to do now with PEQ and FR and time domain equalization. I'll try to address the modes and bring it back to the room respnse. From there looks like absorption. Still have a lot more questions but I'll try to read more first before asking in case I can find the answers already posted.

cheers


----------

