EQ Does Improve Bass Decay

EQ Does Improve Bass Decay​

I wanted to share some examples of how EQ does reduce ringing in the bass, or rather, improves bass decay. Why? Because it's minimum phase and it must. I'm including both simulations of the EQ effect, as generated in REW, as well as an actual measurement using DIRAC (which is still EQ, even if it's very special EQ). It shows that EQ does improve bass decay. This comes up a lot and I wanted to provide some solid results to make the point.

First, a few animations to show a comparison of the waterfall (what most are used to seeing). This is an actual speaker measurement but showing the simulated effect on the waterfall when EQ is applied to flatten the response.

Here is a similar animation showing what DIRAC did as compared to the No EQ result. This is not a simulation but a real-world measurement.

However this may only be somewhat interesting, as many might suggest that cutting the peak lowers the level of the peak, but decay is a slope not a level, so what effect does it have on the slope? Well, look for yourself. These are comparison of the impulse response filtered at 63hz (1/3 octave) which was around where a large peak in the untreated response showed up.

Look at the slope of that black line.

Here is the same information as above, but using just PEQ and not DIRAC.

And finally an overlay to show just how seriously it impacted decay. Look at the tail.

And again, same information as above, but using PEQ

EQ clearly improves bass decay and does so far better than reasonable bass traps could achieve at such low frequencies. That isn't to say that LF damping is not important in a room, but my view is you need both. Now keep in mind that in general EQ will not provide identical results (and thus identical improvement in bass decay) at different positions in the room. That is why multiple low frequency sources (i.e. subwoofers) and some bass trapping is still needed.

 

[jtalden]

Interesting.
Does this apply to PEQ as well? I have no experience with Dirac Live and understand it is more sophisticated than common minimum phase PEQ. In my previous well measured room using PEQ, I did not notice that the decay slope changed significantly as the amplitude was addressed. It may have changed, but, if so, it was not nearly as significant as in this example. So I am wondering if this is related Dirac DSP approach and possibly others of its type rather than more ordinary minimum phase PEQ.

I am just starting to calibrate my new location so I will watch more closely for this effect.

 

[tesseract]

I have had this argument several times. Many think this is an acoustics-only problem, solved physically.

EQ and treatments work, use both if you can. The data supports both.

 

[Matthew J Poes]

Interesting.
Does this apply to PEQ as well? I have no experience with Dirac Live and understand it is more sophisticated than common minimum phase PEQ. In my previous well measured room using PEQ, I did not notice that the decay slope changed significantly as the amplitude was addressed. It may have changed, but, if so, it was not nearly as significant as in this example. So I am wondering if this is related Dirac DSP approach and possibly others of its type rather than more ordinary minimum phase PEQ.

I am just starting to calibrate my new location so I will watch more closely for this effect.

Absolutely, this applies to PEQ as well. This is a property of a minimum phase system. I first learned about this from Earl Geddes and later from Floyd Toole. Toole has written about it extensively, but I wanted to give it a shorter and more approachable treatment.

I've noticed a fair bit of misinterpretation of waterfall plots, but I really can't speculate on your results. If you could take measurements and share I'm sure we could figure it out, but if you had peaks and knocked them down with PEQ, they should show up in the waterfall as well.

If you look back at my first animation, that is showing EQ. The main difference in this case between the PEQ and DIRAC is that the PEQ filter was designed to flatten the natural roll-off of the speaker. DIRAC actually changed the tonal balance of the speaker and thus raised the level of the bass. I'll try to apply regular PEQ to the same speaker in a manner more akin to what DIRAC did and see if I can show the same results. While DIRAC is a mixed phase EQ system that operates differently from PEQ, at low frequencies the differences aren't so great. Both would have the same effect on the impulse response (that is, reduce ringing).

 

[Matthew J Poes]

I have had this argument several times. Many think this is an acoustics-only problem, solved physically.

EQ and treatments work, use both if you can. The data supports both.

I know Dennis, its an argument that won't die. One of the problems with waterfall plots is making sure you aren't interpreting noise. It's very common to use noise that looks like ringing. It's also fairly difficult, in my opinion, to see a change in decay of the slope of the waterfall plot given its perspective. Far easier to use other tools like the filtered impulse and Schroeder integral.

 

[jtalden]

Thanks for the heads-up. I'm always looking to improve my understanding.

Rather than going back to my old data I will specifically look for this effect moving forward. I'm just now up and running at my new home having just done an quick preliminary bass EQ. My new smaller AV area requires more EQ in the bass range so the effect should be more apparent. ​

 

[Matthew J Poes]

Thanks for the heads-up. I'm always looking to improve my understanding.

Rather than going back to my old data I will specifically look for this effect moving forward. I'm just now up and running at my new home having just done an quick preliminary bass EQ. My new smaller AV area requires more EQ in the bass range so the effect should be more apparent. ​

You should see a change in bass decay after EQ. If you don't just share the mdat file and we can take a look. Make sure you are using the beta copy of REW as it measures noise floor. That helps trouble shoot where the noise floor is and to be sure you are interpreting real information.

 

[Matthew J Poes]

I just updated the article to include new animations and graphics showing what just PEQ did to the ringing. As you can see, the results are identical to what DIRAC did. Any method of equalization can improve bass decay.

 

[Wayne A. Pflughaupt]

Any way you can make the waterfall graphs larger? They're so small it's hard to fully see what's going on.

BTW, that animation is pretty cool!

Regards,
Wayne

 

[Matthew J Poes]

Any way you can make the waterfall graphs larger? They're so small it's hard to fully see what's going on.

BTW, that animation is pretty cool!

Regards,
Wayne

Hi Wayne, the width of the pictures (600 pixels) is fixed per our article guidelines so I can't share a larger image. I will send you something privately. I actually only shared the waterfalls because that is what people are used to seeing. The frequency filtered impulse response really highlights the change in decay more readily and frequencies where ringing was present. As you know, waterfalls are tricky business. They can't be compared and much of the "stuff" you see in the lower portion of the graphs can be noise in the room or energy storage in the speakers themselves. I pick up 60hz and 120hz hums in my waterfalls all the time, even if hum isn't readily audible in the room. I also find it pretty much impossible to eyeball a change in slope from the waterfall. The other graphs provide a more objective measure in the change in decay, that slope change if you will. It is just less intuitive if you don't realize what you are looking at.

Thanks for the compliment on the animations. They are basic gifs made from two images. This article was an experiment in how to display information like this in more digestible ways. I will probably do more and create more interesting animations for future articles. This was pretty easy to do.

 

[Wayne A. Pflughaupt]

Don’t know anything about impulse graphs – it is showing that an EQ filter reduced the decay time at 63 Hz? I assume that was a room mode that had a parametric filter applied? Because EQ can only reduce ringing for room modes, not across the board.

Regards,
Wayne

 

[Matthew J Poes]

Don’t know anything about impulse graphs – it is showing that an EQ filter reduced the decay time at 63 Hz? I assume that was a room mode that had a parametric filter applied? Because EQ can only reduce ringing for room modes, not across the board.

Regards,
Wayne

Yes of course. That was the point. If you look at the waterfall plots In the first image (no EQ) shows a ridge centered around 60hz. Since the filters are centered at 1/3 intervals I chose the closest band to the ridge. A similar ridge existed above 100hz but I wanted to pick a modal ridge that was below the point where typically acoustic treatments are efficient.

 

[Keith_W]

I followed the link in your signature to this article when you replied to me thread Thank you for posting, it is very interesting. I have a question though - HOW does EQ actually reduce bass ringing? Is it a function of how much energy is put into the room? If so, would you expect more ringing just by turning up the volume?

 

[Matthew J Poes]

I followed the link in your signature to this article when you replied to me thread Thank you for posting, it is very interesting. I have a question though - HOW does EQ actually reduce bass ringing? Is it a function of how much energy is put into the room? If so, would you expect more ringing just by turning up the volume?

Hi Keith,

Basically yes this is right. Bass modes operate as minimum phase and so matching them with a minimum phase filter will lead to a change in both the time and amplitude domain. It means that what you see in the steady state is what you see over time.

One thing I didn't do well in this article is distinguish between ringing and decay. Ringing is usually defined as a ridge or peak in the response that has elevated amplitude over time. It appears to decay at a slower rate. As it turns out, if you put less energy into a room, it decays more quickly. Why? Because there is a finite amount of absorption in a room (be it actual absorption like bass traps, or things that act as absorption like curtains, carpet, the walls, floors, ceiling, other speakers, air, your wife, you name it). If you put less energy into a room, it can be dissipated more readily. This is actually a trick that is used to make sure that the room is treated propery, you intentionally draw out the measurement, such as using a slower impulse signal, or even using a steady state signal over a long period of time like with an RTA.

So EQ actually reduces ringing because its reducing the amount of energy put into the room. That is why if decay is of interest and you want the room to contribute as little additional bass reverb as possible, eq alone isn't very helpful. Once the response is flat, the only further solution is to turn the volume down. So it stops ringing, and the decay at the ringing IS increased, but...once its all even, then you have to move onto other solutions.

Having said all that, this is a bit of a fools errand too. You don't need to increase bass decay very much if the response is flat. The room doesn't typically contribute enough reverb in the bass, so to speak, to be a major problem. Our rooms are too small. The only way to impact this below 125hz would be using a lot of very thick velocity absorbers, it would get untenable quick. This is why I promote EQ first and foremost. It's fast and simple, it works. Multiple subs second because it too works really well, it helps even the response over a wider space, but I make it second because its not easy to implement. I put bass traps last on my trifecta toward better bass because while I think its needed, it's the least desirable for most people, and in fact, has the least obvious effect.

 

[Matthew J Poes]

I should also mention that this same physical property plays out mathematically as well. A digital minimum phase filter has this same property. It's a property of minimum phase systems.

 

[Eric SVL]

This makes sense to me. If you strike a bell harder, it rings louder and for longer.

I had a strong peak at about 55hz, so I used the PEQ on my Rythmik subs to reduce the amount of energy being put into that frequency. It brought the peak from +11dB down to +4dB. Audyssey XT32 + SubEQ took it down the rest of the way. Lesson is, I couldn't have achieved flat bass without the PEQ.

 

[Matthew J Poes]

This makes sense to me. If you strike a bell harder, it rings louder and for longer.

I had a strong peak at about 55hz, so I used the PEQ on my Rythmik subs to reduce the amount of energy being put into that frequency. It brought the peak from +11dB down to +4dB. Audyssey XT32 + SubEQ took it down the rest of the way. Lesson is, I couldn't have achieved flat bass without the PEQ.

It’s rare that you would see a bass response as flat as is possible with EQ using just treatment and setup. Multiple subs located near field are likely the only exception.

 

[RichardDolan]

Thanks

 

[Jean Ibarz]

Hi,

A PEQ doesn't improve bass decay. An FIR filter may improve bass decay however, but it is a bit more complicated then that if you consider that what is captured by an omnidirectional microphone is not really what we perceive as "sound".

Can you please provide the .mdat of the screenshots you provide in your initial post, and I will try to show you why your data doesn't support your analysis ?

Best regards

 

[DanDan]

Thanks Matthew. The Noise Floor, although much better in the last few months! is say 40dB. Sound will get increasing masked by that as it drops in level. That with Fletcher Munson means that louder booms will indeed seem to last longer.
However afaik in most cases the rate of decay remains the same irrespective of level. Why wouldn't it. It is not like musical instruments which behave utterly differently at different volumes. This is just air getting pushed.
I am a bit confused by the Waterfalls. The PEQ is presumably doing very specific singular interventions. Dirac Live is attempting to impose a Target or wish curve using multiple filters.
The later graphs IMO display the benefits of it's filters. Decay at the cut frequency is reduced, mathematically irrespective of level. There are two benefits working independently.
1 By robbing the resonant frequency of energy, absolute level of the peak is reduced, and the nulls are also reduced in depth.
2 I will describe this loosely as my understanding is:- Eq has an IR. A boosted Eq can indeed ring, while a cut Eq is indeed damped.
Given two IR's at the same resonant frequency, they will attempt to piggy back on each other. The longer will try to draw out the short, and vice versa. Consider a Helmholtz. The traps is specifically tuned. Without damping it will ring, it has an IR. it will increase the level and decay in a room. Damped, it does the opposite.
Now, over time I have tried many Eqs and techniques in my own CR, it's easy I live here.
Initiatlly of course the trusty KT DN360.... Then a TC Parametric. This was also used to generate filtered and delayed cancellation from rear subs. In Mastering I have a collection of extreme Eq's of an ultimate quality, if there is such a thing.
To be honest, when set, many achieve exactly the same thing and I am pretty sure none of us could tell the difference between them. I don't hear a difference in Transparency. In use however, some behave entirely differently and one gets to appreciated the help and delivery. Sorry, an offshoot.... I have tried these mega Eqs to address the Elephant, a 35Hz length mode in a concrete brick room. Sonarworks too.
Many of these Eq's can be switched between Minimum, and Linear Phase.
None kill the mode as well as Dirac Live. I would love to know how/why.

 

[Jean Ibarz]

A PEQ is generally a IIR filter, so it has obviously an impulse response, which is by the way a minimal phase impulse response, and you can consider that your system response is convolved with this filter impulse response.

The difference with DIRAC, as I can tell from Matthew impulse images, is that DIRAC doesn't use a minimal phase filter. So it introduces some delay (which is necessary to advance the phase of some frequencies that are being late....). I bet too that it is an FIR filter. Getting the impulse response of the correction done by DIRAC is very simple: you just need to measure a loopback with the audiocard jack out => jack in and measure the impulse reponse of your sound card. Without DIRAC correction, the impulse is probably a minimal phase impulse response,clarity very high, decay nice, group delay constant, etc. Applying the DIRAC correction, we should see appear some variation in the group delay, a non minimal phase impulse response (some energy coming before the impulse response) and a worse decay rate. Also, it would have been better to post, initially, the impulse responses in a log amplitude scale, as the decay rate in a steady state is linear in time in a logarithmic scale.

 

[Matthew J Poes]

A PEQ is generally a IIR filter, so it has obviously an impulse response, which is by the way a minimal phase impulse response, and you can consider that your system response is convolved with this filter impulse response.

The difference with DIRAC, as I can tell from Matthew impulse images, is that DIRAC doesn't use a minimal phase filter. So it introduces some delay (which is necessary to advance the phase of some frequencies that are being late....). I bet too that it is an FIR filter. Getting the impulse response of the correction done by DIRAC is very simple: you just need to measure a loopback with the audiocard jack out => jack in and measure the impulse reponse of your sound card. Without DIRAC correction, the impulse is probably a minimal phase impulse response,clarity very high, decay nice, group delay constant, etc. Applying the DIRAC correction, we should see appear some variation in the group delay, a non minimal phase impulse response (some energy coming before the impulse response) and a worse decay rate. Also, it would have been better to post, initially, the impulse responses in a log amplitude scale, as the decay rate in a steady state is linear in time in a logarithmic scale.

Dirac is mixed phase meaning it uses FIR filters. This is well stated in this white papers and journal articles they published.

https://diracdocs.com/on_room_correction.pdf

 

[Matthew J Poes]

Hi,

A PEQ doesn't improve bass decay. An FIR filter may improve bass decay however, but it is a bit more complicated then that if you consider that what is captured by an omnidirectional microphone is not really what we perceive as "sound".

Can you please provide the .mdat of the screenshots you provide in your initial post, and I will try to show you why your data doesn't support your analysis ?

Best regards

If we are describing bass decay as the ringing caused by the modes then of course eq does. It solves for both the amplitude (initial peak) and phase (ringing tail). Normal IIR filters fully resolve ringing caused by modes, which was the point of this article.

I wrote this article 2 years ago and it was peer reviewed by a number of experts at the time. As recently as a week ago Todd Welti took a look at it and had nothing negative to say.

we were discussing the math behind why this works, where he pointed out that if you take the denominator of the close formed room and multiply it by a filter transfer function of the peak, it will cancel it it the pole(s) from the Denominator. It does this For every location in space of the closed form room. This is all linear mathematics. A room is generally minim phase at low frequencies. If you insert an inverse filter to obtain a flat magnitude response, you will in turn have a flat phase response. That means no ringing.

the omni mic comment doesn’t apply here either. That only applies at higher frequencies. This article only concerned ringing in the bass. At bass frequencies the room is generally a reverberant field dominated by reflections rather than direct sound. The ear cannot and does not resolve individual reflections much below 500hz and certainly not at all below 100hz. That means that below 100hz what we measure is what we hear.

May graphics are fine and they show what I claim they show. I have no idea where the mdat file for this is, it’s a 2 year old article and the file is long lost I’m sure. If you feel you can repeat this experiment and somehow show something different, feel free. As I noted, this was peer reviewed so if you come up with something different, be prepared to have it scrutinized. Your claims seem to assume the room is mixed phase, which at low frequencies is generally not true. I have measurements of 100’s of rooms and I’ve never seen any significant mixed phase behavior below 100hz.

 

[Matthew J Poes]

Thanks Matthew. The Noise Floor, although much better in the last few months! is say 40dB. Sound will get increasing masked by that as it drops in level. That with Fletcher Munson means that louder booms will indeed seem to last longer.
However afaik in most cases the rate of decay remains the same irrespective of level. Why wouldn't it. It is not like musical instruments which behave utterly differently at different volumes. This is just air getting pushed.
I am a bit confused by the Waterfalls. The PEQ is presumably doing very specific singular interventions. Dirac Live is attempting to impose a Target or wish curve using multiple filters.
The later graphs IMO display the benefits of it's filters. Decay at the cut frequency is reduced, mathematically irrespective of level. There are two benefits working independently.
1 By robbing the resonant frequency of energy, absolute level of the peak is reduced, and the nulls are also reduced in depth.
2 I will describe this loosely as my understanding is:- Eq has an IR. A boosted Eq can indeed ring, while a cut Eq is indeed damped.
Given two IR's at the same resonant frequency, they will attempt to piggy back on each other. The longer will try to draw out the short, and vice versa. Consider a Helmholtz. The traps is specifically tuned. Without damping it will ring, it has an IR. it will increase the level and decay in a room. Damped, it does the opposite.
Now, over time I have tried many Eqs and techniques in my own CR, it's easy I live here.
Initiatlly of course the trusty KT DN360.... Then a TC Parametric. This was also used to generate filtered and delayed cancellation from rear subs. In Mastering I have a collection of extreme Eq's of an ultimate quality, if there is such a thing.
To be honest, when set, many achieve exactly the same thing and I am pretty sure none of us could tell the difference between them. I don't hear a difference in Transparency. In use however, some behave entirely differently and one gets to appreciated the help and delivery. Sorry, an offshoot.... I have tried these mega Eqs to address the Elephant, a 35Hz length mode in a concrete brick room. Sonarworks too.
Many of these Eq's can be switched between Minimum, and Linear Phase.
None kill the mode as well as Dirac Live. I would love to know how/why.

Dirac isn’t really minimum phase or linear. It’s a mixed phase implementation that can mimic the phase behavior of the room (Usually called either minimum phase or non-minimum phase based on the pole(s). Rooms are mostly minimum phase across the entire bandwidth but reflections in the room do cause mixed phase behavior at certain frequencies. Dirac basically solves this by inverting the actual transfer function of the room with certain constraints to avoid over-fitting. No basic eq program could replicate that. Mathcad could. I’m sure there are other options to create an inverted room transfer functions based on the same multiple measurement points and constraints.

I can’t speak to that pesky 35hz. Not sure why DIRAC would be better. It does have some impulse response shaping that might be doing something like feeding back a delayed anti phase signal to counter the ringing. I doubt it does anything that sophisticated and it shouldn’t be necessary. The likelihood that the 35hz length mode is mixed phase is near 0. A normal properly matched IIR filter that inverts it should be just as good.

I’m actually going to be giving this another treatment and Todd Welti has been helping me With that. So far he has only confirmed what I already knew. But at least I don’t feel crazy as I work through the project. I can also bring in Mathias and maybe he could lend some insight into what Dirac does to address these problems that’s different.

in my own wok I have found that you often see a buildup of bass energy in small tightly constructed rooms. If the walls are cinderblock or brick, and there is little resilience at low frequencies in the inner shell, you see not only excessively strong modes, but also a general increase in LF energy as you get lower. It just keeps getting worse. Group delay rises with it as well. It’s highly visible in REW if you use a wavelet.

what I have found to be effective is moving to resilient and damped wall construction. It doesn’t offer a lot of benefit above 50hz, but seems to offer some useful benefit below 50hz in a way that normal in room bass traps cannot. This same concept was studied at Purdue, where I attended graduate school, as part of a reactive wall construction for anechoic chambers. The chamberS LF limit was effectively lowered by about half an octave by making the inner shell resilient, which we called passively reactive. It was lowers a full octave Or more by making it actively reactive through the use of wall transducers which physically moved the wall anti phase to the low frequency transfer function of the space. It was too slow to be useful but was a good proof of concept. The transducers were basically bass shakers and they had a lot of problems. We didn’t have the ability to engineer better transducers. We tried a different type of driver but it wasn’t strong enough.

point to all this being that I have found it impossible to fully fix these LF problems without designing it into the room. But you are right, I’ve had really good luck with Dirac. It often equals the quality of results I can achieve using much more heroic efforts. It’s enough to make you want to cry. Out all this effort into fixing a problem only to find that a cheap piece of software can basically equal it.

I haven’t tested the new bass management suite from Dirac yet, but This would be the icing on the cake. The one remaining problem that even basic Dirac can’t address is that modes create that interference pattern in a room. I phase ringing addresses in one place can be made worse where the ringing is anti phase. Some rooms have modes such that a flat response is achieved in one spot and within 1 foot things go nuts. Dirac can’t fix that. Multiple LF sources can but setting them up has been so hard for folks that many insist it doesn’t work. It’s a simple fact that it works and it’s easy to work out mathematically why. Those who found it doesn’t work simply didn’t do it right. So if Dirac can make that easy for people then it’s a serious game changer.

the reduction of LF Spatial variance Is found to be a simple linear relationship with the number of LF sources such that the improvement can be calculated as 1/n where n is the number of discreteLF sources sufficiently spaced to constitute an acoustically unique spatial location.

on that note, have you ever tried placing subs at the front and back of the room and then eqing? That might give better results. Alternatively you could operate a subwoofer below 50hz in the back of the room with a delay applied that is equal to the time it takes for the wave to transverse the length of the room. That would cancel the length mode entirely. 35hz is well into the unimodal region where this would hold true at all locations.

 

[DanDan]

Interesting post Tx. I am not really conversant in Math. Mathias is pretty much a pal at this stage. He is very proud of the DL algorithm which makes the choices. Also 'spatially robust' is a favourite term.
I forget what exactly caught my attention some time ago, but I made some connection between something Mathias wrote and the very early Meyer SIM system. In particular the Complementary Phase CP10 Equaliser.
Btw, I have been offered one of these and am very tempted. Thoughts?
My CR is active, mostly for Mixing and Mastering, so I tend to leave it alone and live with the learned foibles. When I finish a current Album Mix, my next move is to try DL V3, which is compatible with my older Mac OS. Thanks Flavio!
I will let you know my throughts when I get to that. I think the version I got has the LF option.

Our houses here are brick, so my CR is a bunker, but perhaps oddly falls into what I call 'The Opposites Syndrome'
The strong Modal and SBIR action results in a Bass deficient room. One acoustician way back called it 'degenerative' if I remember correctly. Sounds about right for my workplace!
I did my Thesis in there, and over time have tried many things. Two Celestion Ditton 66 at the back wall have been used for cancellation. Including an 'active' version including a sensing mic. It was not really stable.
But who knows, worth trying again, and I am cooking up a plan involving really shallow LF drivers to virtually flush mount.
The Limp Bag (Newell) is also called humorously 'Invisible Alpha ™ ' by Andre Vare. I would love to try the BBC whole wal ideal sometime. https://www.bbc.co.uk/rd/publications/rdreport_1994_12
Of course the modes do their pattern, we just have to work around that, often to advantage when recording acoustic instruments. But robbing energy and a bit of damping in a mode drops the peaks a lot, and the decay a little. But I like to think it also softens the nulls and widens their Q.