Gain Matching vs. Level Matching - Page 2

If i'm following you Bosso, with a 2 sub setup you would suggest one near/one far placement level matched at the main seat?. In my particular room I know that setup gave me the flattest response 13hz-80hz sans eq. I've since co-located my subs for the greater output on a trial basis.

You must consider precedence effect. If one source is 10dB or more louder than another your brain's focus goes to the louder source. Your directional information should be coming from the mains, but if the overall level of the subs is too loud compared to the mains, and you have subs in back of you as well as in front, then precedence effect can make locatable what would normally not be. If you want to run your subs hot enough that precedence effect occurs then you'd have to reduce the rear sub volume to redirect your brain's focus in the right direction.
My single sub is in back of me, and I can't locate it when the mains to sub balance is correct.

Isn't the default for many recevier's LFE to be crossed at 120Hz? Maybe that is one cause of the localization issue?

Thats the LPF for the LFE channel. While its all the way up to 120hz, I believe there isn't much content above 80hz in that channel.

I find the same, I don't run my "hot" over top of my mains and I really can't locate it. Bill, you explained it better than I and that's been my experience as well.

A number of people have measured and set to 80hz with no decernable loss of information. (it was discussed in a thread not long ago)

It seems to me that example Bosso used is exactly the type that gain matching is meant to address. To get these two subs level-matched would require one of them to run 12db hotter than the other. I suspect this much headroom is not an option with most commercial subwoofers.

Alan, would you care to test if the localization issue is real and not a "hearing with your eyes" variety? Here is one way to do it:

Seat yoursel on a swivel chair at or near your LP . Have an assistant blindfold you, then spin the chair and turn the subwoor test tones when the chair stops. You will be asked to identify direction the sound comes from. Do it a few times to get statistically significant results. Share the results here

From my personal experience, I prefer level matching the subs to the MLP. I have used multiple subs this way and never over driven one to distortion or amp/driver failure. By virtue of multiple subs being in different locations in the room, the response will not be the same. There is no practical advantage of gain matching over level matching subs. The phase and timing issues are another question.

These aren't necessarily direct questions for you Bill.

But where is the line drawn between "non locate-able" frequencies and volume?

Do frequencies that are normally not locate-able become more so when louder, percentage wise, than the mains?

Or does it become more locate-able at a certain gain level because of harmonics caused by approaching over driving the system.

If Alan's new subs are better at playing high levels without distortion and the problem subsides, that would be telling. Problem is Alan is biased by knowing where the subs are.
I'm not using biased in a negative manner Alan .

I think zhekas idea is a good one. Another thing to try is to apply a steep (48db) lpf to the subs. Problem there is the steeper filter will kill more of the
frequencies below the crossover and would have to be adjusted higher to resemble the curve up until the cross. If that's even possible.
If the localization subsided, you could infer that high freqs making it past the lpf was the cause.

I think this goes beyond how the subs are matched.

There's a ton of possibilities here. It would take some rigorous testing to be sure of whats really going on.
Interesting though.

[quote name="Bill Fitzmaurice" url="/t/1460606/ then precedence effect can make locatable what would normally not be. [/quote]

Apparently i need to slow down and read better. sry

Are you saying then that a sound at a frequency that we normally cant locate becomes so when it is louder than other sounds on the track?

Its been a while but I remember playing lf tones against the noise floor and I dont believe they were suddenly easy to locate.
In that case the precedence effect would have been on the order of ~ 40 db.

The Precedence Effect or Haas Effect is primarily about the time arrival of sounds. It's only when the level of a reflection is is at least 10 dB higher than the direct sound that the directionality is impacted.

"Haas found that humans localize sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction. A single auditory event is perceived. A reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a secondary auditory event (echo)."
http://en.wikipedia.org/wiki/Precedence_effect

However, most of that work was done at "speech" frequencies. It's not clear if these same principles carry over to the much longer wavelengths of bass frequencies, where the steady state response dominates. The steady state response is a combination of multiple reflections and it can take more than 30 ms to develop.

Craig

And that's basically my point. We call certain frequencies non locate-able. Why would they ever become locate-able for reasons of relative volume between subs and mains,
or volume between subs? It should be simply. not. locate-able. Alan is saying he hears where is sub is, not that he hears it first.

I don't know the answer, It just seems important if we want to diag the symptom.

I agree. The only exception is if I have one sub that is capable of playing much louder than another. What I do in that case is to have gains set relative to the capabilities of each sub. That way, both subs are playing closer to their maximum potential.

On a side note. I don't understand why manufacturers of subwoofer amps include a phase setting and not a distance setting. Phase is of very little use when integrating multiple subs. The only use it has is if integrating the subs with a non subwoofer. It can be used to sync up the crossover frequency. Using phase to sync up multiple subs makes no sense since phase is frequency dependant. You can sync up one frequency and all the other frequencies won't sync. In almost every case, it's better to just leave it at 0 or turned off. Distance is much more useful when syncing multiple subs as it's not frequency dependant. If you have 2 subs and each one has it's own distance setting in a receiver, you wouldn't need it, but a much more common setup involves 2 subs on the same channel. It would be very useful to have in that situation.

Being able to directionally locate depends on two factors, the distance between your ears and the processing power of your brain. The first can't be changed, but the second can be aided by external factors. If the volume of the source is relatively flat throughout the audio bandwidth the brain will pay attention to the midrange frequencies as far as directional cues are concerned, ignoring the low frequencies, because it takes less processing power that way. This effect is exacerbated if the overall level is low, having to do with equal loudness. You hear better in the mids at low volumes, so your brain has to work less to directionally locate in the mids at low volumes. For these reasons you might not be able to directionally locate even at 200Hz with low level sources.
As levels go up in general, and as low frequency levels go up in particular, it takes less processing power for the brain to directionally locate in the lower frequencies, so it's better able to so do. At high levels you can directionally locate even at 40Hz, provided that the 40Hz content is not only sufficiently loud but also sufficiently louder than higher frequencies.
If you run only your subs, or run them really hot, directionally locating them with an 80Hz low pass isn't difficult, because your brain's focus will be purely on the subs output. Add in the rest of the audio bandwidth at a normal balance and it will concentrate on the mids rather than the lows and they won't be locatable. So while for the most part being able to directionally locate subs means they have too much midbass content it also can mean they're simply too loud. Looking at it another way, if you're going to run with the subs really hot you also should be running them with a lower crossover frequency.

I advise against gain matching when the distance disparity is too great, as in one sub at your side and the other sub at 4M. and yes, I advise against such a placement configuration because neither a phase control nor a distance control is likely to be able to reconcile the arrival time disparity.

Remember, a phase control is a simple all-pass filter. The filter must have a center frequency, so it's typically close to the most-used crossover point of 80 Hz and is meant to reconcile the sub with the mains, not separately placed subs. As someone already mentioned, that will help at 80 Hz but hurt at other frequencies.

The best scenario is to begin placement with all sub equidistant to the LP. As you deviate from there, you have to deal with the consequences of a) cancellation/reinforcement from same waves arriving at different times and b) level matching to avoid localization, more so as distance disparity increases.


Always level match the subs from the LP. It's just common sense.


You have it exactly backwards The 2 subs in my measurement are perfectly gain matched. To level match them would require the closer sub to be reduced in level, not the other way around.

It also needs to be mentioned that the near field sub will always exhibit a different FR than the properly placed sub. The native response of a sub (should be) tailored to work with room gain to result in a flat response at the LP in a given room. The closer you get the "mic" (your ears) to the sub, the more its response reverts back to a native response. This is clearly evident in the 2 traces on my graph.


We're not talking about locating the sub vs the mains, which is why I said earlier that locatable frequencies are irrelevant to this discussion. We're talking instead about the same frequencies from 2 different sources, so relative volume is the whole bowl of wax here. Taking equal bandwidth from 2 sources can be applied to the mains as an example. What happens (psycho-acoustically) when the L/R mains are fed a mono signal (as all subs are) if the right main is outputting at +12dB higher a level than the left main?

You could argue what frequencies are more or less locatable, but that's irrelevant. The mains are assumed to be exactly equidistant from the LP. Even in that perfectly equidistant placement scenario, your head would lean to the louder speaker. Move 1 of them right next to you and the other at 4M away and it becomes clear (to me) the futility of gain matching. The balance control of stereo days was to level match the 2 speakers without needing 2 volume controls and is still used in car audio because you sit off center from the 2 sources.
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I also wanted to talk a bit about distance delay settings vs phase control settings. Years back, Seaton helped me wrap my head around the similarity, but I had the problem seeing it because people were saying "the distance setting is the same as a phase control". No, it's not. A phase control, as I mentioned earlier, is an all-pass filter. That filter affects a wide range of frequencies, but more so at its center frequency and less so out from that point. An all-pass filter must have a center frequency, which value is never known on the phase control on a sub plate amp, but is usually generally aimed at 80 Hz.

Rod Elliot has an excellent article on building a subwoofer phase control: http://sound.westhost.com/project103.htm

The digital delay distance setting, OTOH, affects the entire signal (all frequencies in the signals BW) equally. The only way you could claim that the 2 methods are the same is in a single scenario; if you apply the same phase adjustment to 2 sources, there would in effect only be a delay and otherwise there would be zero change. Likewise, if you apply the same distance delay to 2 sources there will only be a delay and otherwise there is no change. Once you apply the effect of either method to only one of 2 sources, you open Pandora's box.

Because a multi-subwoofer systems response at the seats is primarily the result of the phenomenon of reflected wave interference, using a phase or distance delay control to attempt to alter that result in a constructive way is like playing pin the tail on the donkey. Placement is the primary tool. Everyone would prefer the press of a button to placement experimentation, but you'll always end up with a compromise in the end if you skip the placement step.

Finally, one of the reasons I prefer the up/down-firing sub configuration vs front firing is that it presents only reflected pressure waves. The front firing sub will always present the direct-radiated wave first and all reflected waves 2nd. When, for example, you apply a PEQ cut to tame a peak, you are dialing a dip in the signal that's sent to the sub. The front firing sub then presents that distorted direct-radiated sound first, followed by the "corrected" reflected sound.

It has become obvious to me over the past 15 years of infinite placement and tweaks variations in the same room that using the configuration and placement to achieve the flattest result is the far superior method to ignoring those factors and "running 'X' auto-EQ" or ham-fistedly distorting the input signal with a dozen PEQ filters. Of course, YMMV, etc.

My processor is ok but im going to need a memory upgrade soon.

Nice post Bill, thanks.There are a lot of these rules that we've used over the years here that aren't completely hard and fast. Over time with much repeating, they become universal.
I "get" it now, makes perfect sense.

Great thread and thanks for all the info Bosso and everyone else. I have apparently been doing things wrong in my setup it looks like. I am running 4 subs off 1 sub output on my preamp and because of this I have to use the same distance setting for all 4 subs. The front 3 subs are all at the same distance and this is where I have my distance setting set to, but the rear sub is much closer to the LP. Going off my SMS1, I have always just dialed in the phase on the rear sub until it produced the best FR on the SMS graph which does not look to be causing any issues with the front subs once it is dialed in going off the graph. If I turn the phase all the way to either 0 or 180, there is clearly cancellation going on, but a spot about mid point seems to let the rear sub and the front subs play well together.

Having said all the above, it sounds like to really do things right I need to have the ability to use two separate delay/distance settings, correct? I am looking into getting a new preamp anyway down the road which will have 2 sub outs and two separate delay settings, but this might motivate me to make the upgrade a bit sooner than expected.

Hi Alan,

Obviously there is a lot of confusion going on around such matters. As with most tools and approaches, I see too many trying to strictly apply a tool or approach beyond it's expected usefulness. The detail added later of your subwoofers not being identical models was a key bit that probably contributed to some of the spiral and confusion in the discussion.

Myself or Craig John might be guilty of the term "gain matching." This was intended to primarily apply to cases of using the same subwoofers or exactly scaled devices. When you have identical subwoofers, matching the gain results in equal workload from each subwoofer. This means the maximum limits of each will be reached at the same time. This is also the assumption used in all of the modeling from the Harman White Papers.

This is hardly the only approach to setting up subwoofers, nor will it always be preferred. If one of the subs is a small fraction of the distance to the listener, I usually prefer to have a different delay of some form and set that delay being mindful of the real distances and using measurements to best integrate the subs. If you have more than enough subwoofer headroom, are out of adjustments and have other issues to contend with, by all means experiment to find a setup which yields a better result.

If we really want to get specific with the terminology and apply it to using different subwoofers, we should further separate cases of equal power contribution where all subwoofers produce the same acoustic power as would be observed outdoors and alternately limit matching where you would insure one sub doesn't run out of gas before the other. When the subwoofers are identical these two states occur simultaneously.

Per most all discussions here on AVS, absolutely correct.

Ideally you would take some measurements to confirm how well the subwoofers are or are not integrating. You would then experiment with adding delay to the near subwoofers to offset the earlier arrival of the sound while watching the combined frequency response. If digital delay is not directly available and your subwoofer has a low pass filter, you can use this to approximate the effect, where any low pass filter creates group delay. Most are fairly constant below the XO frequency. The lower the crossover, the greater the delay. Some simple modeling software for filters can help you approximate and understand what is going on, but it's an empirical adjustment for sure. Finally you will want to take a look at the distance settings in your receiver where the relative distances between the speakers and subwoofer channel can be set to help reduce localization by insuring the bass from the speakers arrives first. This generally comes from reducing the distance entered for the subwoofer or increasing the distance setting of the LCR.

If the adjustments available don't give the desired results, start experimenting with other adjustments in level and the like. Most often taking detailed measurements of both individual subs, speakers and their combined results will help achieve what you are after.

Remember all the Harman Papers by Toole & Welti assumed the same signal fed to each subwoofer. Of course real world rooms don't always fit the models. Sometimes you get lucky and don't need any delay adjustments. The more extreme the difference in distance to the listener is, the less likely this will be the case, and the more likely you may have some general directional perception to contend with. In such cases, some means of delay adjustment and EQ can be very useful.

Correct. If this doesn't work out, it is much more likely that moving subwoofers around is what is really needed, not level adjustments.

Level matched is always done at the main seat, gain matched is not. I never said otherwise.

P.S. I re-read my post I can see how it might been read that way. For the record I do know the difference between gain vs level and how to do both.

So, outside of a receiver, which typically can only handle delay of 1 or 2 channels, what can be used to adjust delay?

Why don't subwoofer amps have a built in delay knob like they do for phase?

Almost any type of external DSP / EQ. Minidsp, behringer, QSC, etc.

I believe it is because an analog variable phase control using an all pass filter is much simpler and cheaper to implement than an analog time delay. It is easier to implement digitally, so for those amps that have a user adjustable DSP, I'm not sure why a delay would not be included.

How exactly would a MiniDSP be used for sub distance/delay? Would I hook this up between my SMS1 and the sub I want to use it on? How exactly do you make adjustments on it? Is it software based to be used on a CPU?

I would like to get a second sub distance/delay option into my system in the easiest way possible and hopefully be able to keep everything the same otherwise (still use the SMS1, same preamp, etc.....), so this MiniDSP might do the trick (?).

Exactly. It's also important to realize you often need as many channels of delay as subwoofers. If you have 2 subwoofers at the front of the room, it would be rare that you would need to delay them separately. Most setups I only need 2 separate channels. Occasionally there is need for a 3rd.

Echoing ironhead's comments, this is only is practical in amplifiers which employ DSP. Adding delay really only comes into play with multiple subwoofers, as for full range speakers you generally will find that you need to delay the speakers, not the subwoofer. In most cases it's best handled externally, but I'm sure it might become a future feature on a subwoofer which already has a menu or software based interface. The real issue comes back to the reality that in order for this to be useful, you need measurement capability where such customers often will want the flexibility of an external device to manage multiple subwoofers rather than paying for such a device in each subwoofer.

I guess lowering output of the closer sub is a better approach than pushing the distant one beyond its capabilities. But then you effectively limit the closer sub to 1/15th of its power capabilities which cannot be a good thing either.

Wow! I go to work for a day and all the REAL experts show up! Alan, between bosso, Mark Seaton and Bill Fitzmaurice, you have 3 of the finest minds in the business helping you out, and they have each contributed some EXCELLENT input. I learned a lot so thanks for starting this thread.

Craig