Subwoofers and Time Relationships

RBFC

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I had an interesting discussion with Barry Ober (www.soundoctor.com) the other day. Barry is the chief technical consultant for JL Audio, makers of some very nice subwoofers.

We discussed setup issues that affect sound quality when integrating (one or more) subwoofers into your system. Barry, who seemed immediately both personable and knowledgeable, was adamant that we consider the time element of integration first and foremost before frequency response concerns were addressed.

He stated that JL subwoofers have a group delay of 9 milliseconds (mS). Here is how the science and math work to show you the placement and setup details for optimal time alignment with your main speakers:

Speed of Sound = 1126 feet per second OR 1 foot in 0.000888 seconds

Group Delay of 9 mS = 0.009 seconds

Therefore: 0.009 divided by 0.000888 = 10.135 feet equivalent distance offset


So, you would need to place your JL sub 10.135 feet in front of your main speakers to have perfect time alignment in wave launch. Since very few folks choose this setup configuration, it stands to reason that very few people achieve optimal results with the JL subwoofers. If you cannot achieve time coherency, then frequency response tuning/adjustments will never fix what is fundamentally wrong.

Now, since many of us can't physically locate our subs 10 feet in front of the main speakers (spousal acceptance factor and other reasons), how do we achieve time coherency?

There are a couple of methods.

  • Processor speaker distance settings: Most all of today's surround processors and receivers have "speaker distance" adjustments to calibrate the system for proper surround envelopment. This adjustment can also be used to "move" the subwoofer back the required 10 feet extra. So, instead of setting your subwoofer at 10 foot distance, you would use 20 feet. This setting would produce the required increase in main speaker delay to properly time integrate the JL sub with the mains. If you have an upper limit of distance for speakers, say 15 feet, then you could set the sub at the maximum distance of 15 feet and reduce the distance of all other speakers by 5 feet. This 10 foot offset for the sub would maintain the distance differentials of all the other speakers and also create the proper time offset for the sub.

  • JL subwoofers have a "Phase" control (which is really just a delay). Measured at 80 Hz, 180 degrees of phase "delay" is equal to 6mS of delay. So, if processor adjustments are not feasible or adequate, you can fine tune the sub's "time position" using the phase control. If, for instance you choose to place the subs in a location that (coupled with your processor time/distance delay settings) produces a POSITIVE offset for the sub, then you can use the phase control to achieve the theoretical 10 foot offset.

The end effect is that your sub is "electronically" placed 10 feet forward of your main speakers to account for its group delay.

If you tend to place your subwoofers BEHIND your main speakers, as many of us do, you must then add an extra foot of sub distance in your processor (in addition to the 10 foot offset already discussed) for each foot behind the main speakers to the sub.

I set up my system this way yesterday, and it made a large positive difference in the sound quality of my setup. Kick drums were rendered far more realistically, and bass instruments were placed in space far more precisely than before.

Once you get things oriented correctly in the time domain, then you can run the ARO (EQ) feature of the sub to improve the frequency response characteristics.

I was admittedly a bit skeptical as I listened to Barry at first, but the straightforward logic of the discussion made perfect sense to me. This short discussion is but the tip of the "bass iceberg", and I hope to invite Barry here to delve much more completely into this subject.

Lee
 
Last edited:

garylkoh

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So, you would need to place your JL sub 10.135 feet in front of your main speakers to have perfect time alignment in wave launch.

May be I don't know as much about integrating subwoofers, but that is only true if the crossovers inside your main loudspeakers exhibit no group delay.
 

RBFC

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Agreed, Gary. Barry stated that most main speakers exhibit group delays in the MICROseconds range, placing them in the inconsequential zone. All modern sealed subwoofers exhibit 9mS of group delay.
I'd suggest you request this information from the manufacturer of your main speakers, then do the math.

Lee
 
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garylkoh

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Lee - I was kidding. Most of my loudspeakers all have a built-in subwoofer.

The group delay on a loudspeaker is dominated by the crossover. If the loudspeaker has a 2nd order crossover, it exhibits a 180 degree phase shift. At 90Hz, that's a delay of 5.56ms. If the loudspeaker has a 4th order crossover, the delay is 11ms. Even if the woofer crossover does not have a high-pass, there will be temporal coherence problems with integration. I've posted elsewhere a link to a study which shows that the human threshold of perception of temporal coherence is of the scale of 6microseconds. Most power amplifiers exhibit a group delay far greater than that.
 

RBFC

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I must apologize to Barry Ober regarding a few mistakes in my previous posts. While the gist of this is correct, I got a few details wrong and have corrected them. My fault for regurgitating a 2+ hour conversation from memory.

The math as presented is correct, though.

To summarize the corrections:

1. By setting the subwoofer farther away in "speaker distance settings", you are effectively increasing the delay of the main speakers, NOT REDUCING THE DELAY OF THE SUBWOOFER.

2. I originally stated that JL subwoofers exhibited 9mS of group delay. ALL MODERN SEALED SUBWOOFERS EXHIBIT 9mS OF GROUP DELAY.

I hope that this clarifies the original posts.

Lee
 

RBFC

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Lee - I was kidding. Most of my loudspeakers all have a built-in subwoofer.

The group delay on a loudspeaker is dominated by the crossover. If the loudspeaker has a 2nd order crossover, it exhibits a 180 degree phase shift. At 90Hz, that's a delay of 5.56ms. If the loudspeaker has a 4th order crossover, the delay is 11ms. Even if the woofer crossover does not have a high-pass, there will be temporal coherence problems with integration. I've posted elsewhere a link to a study which shows that the human threshold of perception of temporal coherence is of the scale of 6microseconds. Most power amplifiers exhibit a group delay far greater than that.

Gary,

As I'm far from the authority on this subject, I sincerely hope that Barry takes me up on my offer to post here. He could certainly do a better job dealing with these issues than I can.

Lee
 

garylkoh

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Gary,

As I'm far from the authority on this subject, I sincerely hope that Barry takes me up on my offer to post here. He could certainly do a better job dealing with these issues than I can.

Lee

Lee, it doesn't matter. What is important that your instructions for setting up the subwoofer 10ft further than it actually is on the HT processor is correct (for most loudspeakers anyway). I also set the center channel distance NEARER than actual. It compensates by delaying the center channel a little to make voices and the movie soundstage deeper.
 

garylkoh

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2. I originally stated that JL subwoofers exhibited 9mS of group delay. ALL MODERN SEALED SUBWOOFERS EXHIBIT 9mS OF GROUP DELAY.

Not true - depends on the design of the subwoofer amplifier and its crossover. There will always be a phase shift at the crossover frequency that is equal to 90 x the order of the filter.
 

RBFC

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Well, even in my primitive ape-brain, the math made perfect sense.

I may guess that since all speakers utilize power amplifiers (OK, passive subs need not apply), that the group delay would effectively cancel out across the board there. Perhaps because the crossovers (in the mains) are handling frequencies not covered by the subwoofer, the analysis of group delay effects may be undertaken a bit differently???

This is a truly fascinating subject, and I am once again on a steep learning curve from zero to 100 mph....

Thanks,

Lee
 

RBFC

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Not true - depends on the design of the subwoofer amplifier and its crossover. There will always be a phase shift at the crossover frequency that is equal to 90 x the order of the filter.

I quoted the 9mS figure directly from Mr. Ober's comments to me that stimulated my edits above. Not being a speaker manufacturer, I am in no position to question this either way. As soon as I can glean more details at my end, I will definitely continue this discussion.

Lee
 

garylkoh

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Sorry, Lee. It just rattled my cage for JL Audio to generalize about ALL subwoofers.

Here's a good primer on phase, time and distortion: http://sound.westhost.com/ptd.htm

Nevertheless, this is an interesting and useful subject to integrate subwoofers into your system.
 

RBFC

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Gary, no offense taken. As an admittedly naive understudy, I wanted to present this information in the interest of exposing the lack of scientific understanding when setting up subwoofers in a system. Perhaps you have additional (or less) considerations when producing a speaker with an integral subwoofer... certainly there must be some similar obstacles and some different. I find Mr. Ober's rationale, and the immediate results in my system, quite compelling. I am eager to learn much more about this topic.

Thanks for participating in this discussion and in my education.

Lee
 

Steve Williams

Site Founder, Site Owner, Administrator
I had an interesting discussion with Barry Ober (www.soundoctor.com) the other day. Barry is the chief technical consultant for JL Audio, makers of some very nice subwoofers.

We discussed setup issues that affect sound quality when integrating (one or more) subwoofers into your system. Barry, who seemed immediately both personable and knowledgeable, was adamant that we consider the time element of integration first and foremost before frequency response concerns were addressed.

He stated that JL subwoofers have a group delay of 9 milliseconds (mS). Here is how the science and math work to show you the placement and setup details for optimal time alignment with your main speakers:

Speed of Sound = 1126 feet per second OR 1 foot in 0.000888 seconds

Group Delay of 9 mS = 0.009 seconds

Therefore: 0.009 divided by 0.000888 = 10.135 feet equivalent distance offset


So, you would need to place your JL sub 10.135 feet in front of your main speakers to have perfect time alignment in wave launch. Since very few folks choose this setup configuration, it stands to reason that very few people achieve optimal results with the JL subwoofers. If you cannot achieve time coherency, then frequency response tuning/adjustments will never fix what is fundamentally wrong.

Now, since many of us can't physically locate our subs 10 feet in front of the main speakers (spousal acceptance factor and other reasons), how do we achieve time coherency?

There are a couple of methods.

  • Processor speaker distance settings: Most all of today's surround processors and receivers have "speaker distance" adjustments to calibrate the system for proper surround envelopment. This adjustment can also be used to "move" the subwoofer back the required 10 feet extra. So, instead of setting your subwoofer at 10 foot distance, you would use 20 feet. This setting would produce the required increase in main speaker delay to properly time integrate the JL sub with the mains. If you have an upper limit of distance for speakers, say 15 feet, then you could set the sub at the maximum distance of 15 feet and reduce the distance of all other speakers by 5 feet. This 10 foot offset for the sub would maintain the distance differentials of all the other speakers and also create the proper time offset for the sub.

  • JL subwoofers have a "Phase" control (which is really just a delay). Measured at 80 Hz, 180 degrees of phase "delay" is equal to 6mS of delay. So, if processor adjustments are not feasible or adequate, you can fine tune the sub's "time position" using the phase control. If, for instance you choose to place the subs in a location that (coupled with your processor time/distance delay settings) produces a POSITIVE offset for the sub, then you can use the phase control to achieve the theoretical 10 foot offset.

The end effect is that your sub is "electronically" placed 10 feet forward of your main speakers to account for its group delay.

If you tend to place your subwoofers BEHIND your main speakers, as many of us do, you must then add an extra foot of sub distance in your processor (in addition to the 10 foot offset already discussed) for each foot behind the main speakers to the sub.

I set up my system this way yesterday, and it made a large positive difference in the sound quality of my setup. Kick drums were rendered far more realistically, and bass instruments were placed in space far more precisely than before.

Once you get things oriented correctly in the time domain, then you can run the ARO (EQ) feature of the sub to improve the frequency response characteristics.

I was admittedly a bit skeptical as I listened to Barry at first, but the straightforward logic of the discussion made perfect sense to me. This short discussion is but the tip of the "bass iceberg", and I hope to invite Barry here to delve much more completely into this subject.

Lee

Interesting thread Lee

When I bought my pair of Gotham subs several years ago I virtually had the same conversation with Barry.

My bigger dilemma was integrating the subs into my 2 channel setup without the use of my processor as I use a different preamp for 2 channel vs my processor for HT. This is where the use of something like TacT or DEQX etc are most helpful
 

FrantzM

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Most speakers are not even time and/or phase coherent and that include the best speakers or best-loved speakers around ... THere is no consensus on the subjec of time alignment and phse coherence... Some designers do not think these parameters are essential some do.. The results are of course differenct and one find great speakers in oth camps.

I did implement the multi sub approach and used the time delay function of an active bass Crossover to time-align the subwoofers. Recently Mark Seaton showed that the FR did change with time alignment of the subs as well, resulting in better response ... It remains however a difficult endeavor to time align ( I know not the same as Phase coherence) speakers..
moreover there are other factors that play in the mix many of them Psychoacoustics .. It takes us for example a while (about 50 ms , don't quote me on this number) to process a 40 Hz signal .. This may explain why putting the sub in the back at a good distance of the mainspeakers at times seem to have little effect on overall subjective coherence ..
More later .. Fascinating subject ...
 

garylkoh

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Subwoofer integration into a 2-channel system, or home theater system is hugely complex. IMHO, it cannot be simply distilled into "My subwoofer has a group delay of 9ms" - sorry, Mr Ober. Hopefully you'll jump in here - but he may just have tried to over-simplify things for Lee. Mark Seaton might also want to leap in too!!

Since this is the What's Best Forum, let's try to understand this a little better.

First, some concepts and definitions. The details and all the math is here: http://www.analog.com/library/analogdialogue/archives/43-09/active_filters.pdf
but you don't need all that to understand what is necessary for the problem at hand.

Group Delay is simply a derivative of phase with respect to frequency. It is sometimes simplified to mean the average delay over some frequency band. If the group delay is the same across the entire frequency band, it's sometimes called "envelop delay" and then you can use also "time delay".

Otherwise, group delay is frequency dependent. Take the usual filter needed in any subwoofer - a low-pass (or high-cut) filter. The phase of a 2nd order crossover is as follows:



To translate phase shift to group delay = phase shift / (360 * frequency)

Hence, a low-pass crossover set at 80Hz has a delay of 3.125ms at 80Hz. At 160Hz, the phase shift is -135deg, and the delay is 2.34ms.

To complicate things, many subwoofers have a very high-powered amplifier. If there was no protection built-in, when presented with a big low-frequency transient like dropping your stylus on the record, or Lyle Lovett (love him!!) the amplifier would rip the cone off the woofer.

So, many subwoofer designers implement a high-pass crossover into the amplifier at 20Hz or 16Hz or whatever frequency makes sense to him (when battling with the marketing folks). If this high-pass crossover is implemented, then it will have a group delay of 12.5ms at 20Hz, and 3.125ms at 40Hz.

Other subwoofer designers implement a compression circuit to limit the power of the amplifier, which might also have a group delay. To get even more complicated, if the subwoofer has servo control (Velodyne, Martin Logan, Genesis) there is usually an all-pass filter to phase align the servo-circuit and the amplifier circuit (although for it to work properly this should be an envelop delay).

So, you have to look at group delay and frequency together as at the frequencies that matter with subwoofer integration, they are closely and tightly correlated. As Frantz pointed out, it changes the frequency response of the system. If the phase of the sub and the phase of the speaker are aligned, you'll get a bump, and if they are not, you'll get a dip.

In my next post, I'll try to wade through how to integrate a subwoofer with all this knowledge we have. Things will get very interesting...... stay tuned.
 

Nyal Mellor

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As Frantz mentioned, the sensitivity of the ear to group delay varies by frequency. A 9ms group delay error at 50Hz is much different to 9ms at 1kHz. To my knowledge there has been very little research done into this topic and so really we don't know too much about it. Some of my own experiments have shown that time delaying subs so the peaks of the impulse responses occur at the same point in time as the main speakers creates a very small improvement in SQ. Note that correcting for time delay is not the same as group delay correction. Time delay correction generally refers to playing a full bandwidth pink noise measurement, looking at an impulse response and calculating the time difference between the peaks. Group delay correction in my mind refers to having a finer grain frequency resolution, perhaps one third octave or even higher resolution such as that on a DEQX which has a 60000 point FIR filter. On a scale of importance I would put it way down the list maybe even below cables!
 

Steve Williams

Site Founder, Site Owner, Administrator
Hi

Most speakers are not even time and/or phase coherent and that include the best speakers or best-loved speakers around ... THere is no consensus on the subjec of time alignment and phse coherence... Some designers do not think these parameters are essential some do.. The results are of course differenct and one find great speakers in oth camps.

I did implement the multi sub approach and used the time delay function of an active bass Crossover to time-align the subwoofers. Recently Mark Seaton showed that the FR did change with time alignment of the subs as well, resulting in better response ... It remains however a difficult endeavor to time align ( I know not the same as Phase coherence) speakers..
moreover there are other factors that play in the mix many of them Psychoacoustics .. It takes us for example a while (about 50 ms , don't quote me on this number) to process a 40 Hz signal .. This may explain why putting the sub in the back at a good distance of the mainspeakers at times seem to have little effect on overall subjective coherence ..
More later .. Fascinating subject ...

great response Frantz. I completely agree. Setting up my twin Gotham subs to integrate with my main speakers was a true labor of love and if you ask me now if I think it is perfect I can only speculate and say it is for me the best I have heard in my room. I don't use TacT or DEQX so all of my measurements were "initially" done using the JL Audio microphone set up, then with the use of the Velodyne microphone setup, then with multiple meters and then finally by ear (which BTW changed overall settings minimally). This took 3 months of daily meticulous listening with some of the best albums I have that demonstrate bass (darn, if I hear The Boll Weavil Song again...:) ) and then another 4-6 months of intermittent tweaking but always having the recommended meter settings in memory for easy recall.



Long and the short of it Frantz is right on, certainly by my observations as outlined above.
 

garylkoh

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The big problem arise because group delay is frequency dependent. Then, the next problem is that wavelength is frequency dependent. Then, sound takes a finite time for the wave to reach any spot in the room. These three factors play havoc with frequency response.

At any position, if the particular frequency is in phase, you will get a 6dB reinforcement. If the frequency is 180 deg out of phase, you can get perfect cancellation. The problem is that neither the loudspeaker nor the subwoofer puts out single frequencies. They both put out a continuous range of frequencies with a continuous difference of phase anywhere from 0 deg to 180 deg. Using just equalization, you can reduce peaks, but do nothing to the dips.

Let's use 60Hz as a convenient frequency in the following example. At 60Hz, wavelength is 18ft and propagation time is 16.7ms. Half wavelength (180 deg phase shift) is 9ft and propagation time is 8.3ms. In other words, if you have two loudspeakers (or one loudspeaker and one subwoofer) putting out a pure 60Hz tone and one is 18ft from you and the other is 9ft or 27ft from you, you should hear absolutely nothing. However, if one is 18ft away from you, and the other is 18ft or 32ft away, you should also hear nothing. (It's more complicated than that because of wall/ceiling/floor reflections in small rooms).

Where does that get us with subwoofer integration (especially with full-range loudspeakers)?

There are lots of ways to find where to place subwoofer(s) - Art Noxon has one on the Tube Traps website, Geddes has placement for multiple subwoofers, you can craw around the floor with the subwoofer on your listening seat. However, these are all specific to the room. The OP talked about optimal time alignment with your loudspeakers.

I know some ppl think that time coherence doesn't matter, but I think that it's critical for PRAT - which again to some people does not exist. For those who do, bear with me.

Plowing on..... phase shift is continuous 0deg to 180deg (see graph in my previous post) centered on the crossover frequency, if the subwoofer is on an arc 1/4 wavelength of the crossover frequency closer to the listener than the loudspeaker it supports, the loudspeaker and the subwoofer will be time and phase aligned. This is because at the crossover frequency, it will be 90 deg out of phase. So, for a crossover frequency of 60Hz, the subwoofer will be 4.5ft closer to the listener than the main loudspeaker it supports.

This assumes that:

  1. The loudspeaker exhibits no group delay at the crossover frequency of the subwoofer - a ported woofer on a loudspeaker has its own group delay, and if the loudspeaker has a high-pass crossover to protect the woofer from low frequencies, this will have the same group delay characteristics as the crossover on the subwoofer
  2. The subwoofer exhibits no group delay other than the low-pass filter - the amplifier in the subwoofer may exhibit its own group delay
  3. The input to the subwoofer and the input of the loudspeaker exhibits no group delay - if you are driving the subwoofer with the output of the preamp, and the loudspeaker with the output of the power amp, the group delay of the power amp will come into play (if you use network cables like MIT and Transparent, all bets are off

I still have not answered the OP..... that will have to be the next post.
 

garylkoh

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As Frantz mentioned, the sensitivity of the ear to group delay varies by frequency. A 9ms group delay error at 50Hz is much different to 9ms at 1kHz. To my knowledge there has been very little research done into this topic and so really we don't know too much about it. Some of my own experiments have shown that time delaying subs so the peaks of the impulse responses occur at the same point in time as the main speakers creates a very small improvement in SQ. Note that correcting for time delay is not the same as group delay correction. Time delay correction generally refers to playing a full bandwidth pink noise measurement, looking at an impulse response and calculating the time difference between the peaks. Group delay correction in my mind refers to having a finer grain frequency resolution, perhaps one third octave or even higher resolution such as that on a DEQX which has a 60000 point FIR filter. On a scale of importance I would put it way down the list maybe even below cables!

The most comprehensive studies on time alignment has been by Milind Kunchur of University of Southern Carolina - and I think that I saw one of your posts that reference to him. However, there have also been others critical of his findings. There is another Dutch study that escapes the mind at this time.
 

Nyal Mellor

Industry Expert
Jul 14, 2010
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The most comprehensive studies on time alignment has been by Milind Kunchur of University of Southern Carolina - and I think that I saw one of your posts that reference to him. However, there have also been others critical of his findings. There is another Dutch study that escapes the mind at this time.

Hey Gary, in any event, no where near as much research as been done as is needed to understand reliably the pscychoacoustic side! Here is hoping for a Floyd Toole 2 to magically appear and get funding to do lots of in depth listening experiments to understand better what is happening.

Link to pages for anyone interested http://www.physics.sc.edu/~kunchur/Acoustics-papers.htm
 

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