By Matthew J Poes on Mar 21, 2018 at 5:50 PM
  1. Matthew J Poes

    Matthew J Poes Staff Writer
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    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.

    [​IMG]

    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.

    [​IMG]

    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.

    [​IMG]
    Look at the slope of that black line.

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

    [​IMG]

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

    [​IMG]

    And again, same information as above, but using PEQ

    [​IMG]

    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.
     
    #1 Matthew J Poes, Mar 21, 2018
    Last edited by a moderator: Oct 11, 2018
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Discussion in 'Tech Talk' started by Matthew J Poes, Mar 21, 2018.

    1. jtalden

      jtalden Member

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      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.
       
    2. tesseract

      tesseract Senior Admin
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      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.
       
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    3. Matthew J Poes

      Matthew J Poes Staff Writer
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      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).
       
    4. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
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    5. jtalden

      jtalden Member

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      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.
       
    6. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
    7. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
    8. Wayne A. Pflughaupt

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      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
       
    9. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
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    10. Wayne A. Pflughaupt

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      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
       
    11. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
    12. Keith_W

      Keith_W Member

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      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?
       
    13. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
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    14. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       
    15. Eric SVL

      Eric SVL Active Member

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      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.
       
    16. Matthew J Poes

      Matthew J Poes Staff Writer
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      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.
       

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