Subwoofers and Time Relationships

microstrip

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Gary,

As I have no experience setting subs I do it the "logical" way - I take a sinusoidal signal generator, set it at the frequency of the crossover and visualize it at channel1 of the oscilloscope, using it as trigger. Using a microphone at a fixed position I measure the time delay of my main speakers at this frequency versus channel1, switch it off, and then set the sub-woofer at exactly the same time delay, so that the two curves at channel2 overlap. Anything wrong with this technique? :confused:
 

DonH50

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Jun 22, 2010
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Interesting. Linear phase = constant group delay is the ideal in filters, at least for best pulse/step response. I used a combination of methods to align my subs to the mains, and the SW I use (R+D) by default uses (generates) a combination of impulse and noise signals so I can see time and frequency domains with one run (less effort; it still takes multiple runs to dial in, natch).

I am of the opinion that phasing is most important at the crossover point to minimize discontinuity between sub and mains, with less sensitivity to it above and below.

YMWV (your mileage will vary!) - Don
 

garylkoh

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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

And here are the Dutch papers .....
http://www.temporalcoherence.nl/index.php?pagina=artikelen&lang=en

I'm hoping that with the collected wisdom we have here in What's Best Forum will do it. Early in the Internet days, we used to say that the Network is smarter than any single individual. We have passionate, smart people here, and may be if we get close, funding will magically appear :D
 
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garylkoh

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Gary,

As I have no experience setting subs I do it the "logical" way - I take a sinusoidal signal generator, set it at the frequency of the crossover and visualize it at channel1 of the oscilloscope, using it as trigger. Using a microphone at a fixed position I measure the time delay of my main speakers at this frequency versus channel1, switch it off, and then set the sub-woofer at exactly the same time delay, so that the two curves at channel2 overlap. Anything wrong with this technique? :confused:

Nothing - it's perfect if you have the instruments and the ability at the subwoofer to control the delay fine enough.

Gary, can we get a good estimate of group delay in a sealed subwoofer in a limited frequency range, say 20 - 80 Hz?

Lee

One way if you have the tools that microstrip has is as he described. Give me till tomorrow to describe how to do it if you don't have any tools other than a Radio Shack meter. What's important is not the absolute group delay of the subwoofer, but the relative group delay between your main loudspeakers and the subwoofer.

:cool: all your questions are converging. Music, equipment, brain, time, space it's all related :D
 

garylkoh

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Now for the last step. The first step is to find the relative group delay between the loudspeaker and the subwoofer. As I mentioned in the last post, there are several factors that are out of our control. If we have the tools to measure, it becomes pretty easy, but let's say we have a Radio Shack spl meter, and hopefully a test disc with test tones or a signal generator like:

http://www.performanceaudio.com/cgi/product_view.cgi?products_id=4115

Pick a frequency with a convenient wavelength - this will be a wavelength that is short enough to move your subwoofer around in your room. I usually pick 80Hz so that the wavelength is 14ft. It doesn't have to be absolutely precise unless you really want to know the numbers.

First, position your spl meter 10.5ft or 17.5ft (3/4 wavelength or 1 1/4 wavelength) from the main loudspeaker. Play the signal to get about 70dB. You don't want to play too high a level to keep room interactions down.

Place the subwoofer right in front of the loudspeaker. Play the signal with the crossover set at the frequency of the signal and adjust the gain to again get about 70dB.

Now, play the signal through both the sub and the speaker and move the sub towards the spl meter until the sound level is a minimum. It won't completely cancel because of room interactions. Also, because the sub will be very much nearer the meter than the speaker, it should measure far louder once the speaker is turned off.

Subtract 7ft and that is where the subwoofer needs to be for perfect phase/time alignment with the loudspeaker at any frequency. Now, inscribe an arc with diameter between the listening seat and the subwoofer - you can put the subwoofer anywhere on that arc and still the "correct" alignment. Use this arc placement to find the place in the room where the subwoofer sounds best.

The difference in the group delay between the sub and the speaker at 80Hz in seconds can be calculated as distance between the speaker and sub divided by 1126. The inherent group delay caused by a 80Hz 2nd order low pass filter is 3.125ms - which translates to about 3.5ft. Any difference in the distance can be attributed to residual group delay caused by something outside the low pass filter.

If you don't want the sub at that spot so far out into the room and it's a home theater, you can then use the relative group delay in feet to do what Lee suggested in the OP. This process should work with all subwoofers.
 

mojave

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Subtract 7ft and that is where the subwoofer needs to be for perfect phase/time alignment with the loudspeaker at any frequency.

Am I subtracting 7 ft from the distance of the sub to the microphone or the sub to the speaker? Am I using 7 ft because that is half the wavelength of 80 Hz? If so, then if using a different frequency (i.e. 100 Hz) the distances used in calculation will be different, too.

It sounds like it would be easier to use the RTA function of Room Equalization Wizard while playing an 80Hz tone and adjust the distance setting during playback to find the lowest SPL. Not everyone can adjust their distance setting, but it is easy enough when using a PC as the source.
 

microstrip

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Nothing - it's perfect if you have the instruments and the ability at the subwoofer to control the delay fine enough. (...)

If soemones has a good soundcard and microphone, the freeware oscilloscope software pscope that can be downloaded at

http://www.zeitnitz.de/Christian/scope_en

can do the task.

BTW, most computer soundboards or soundcards can not be used for sound measurements because they lack a volume control and AGC (automatic gain control) does not allow relative measurements. But even those can be used for this specific purpose.
 
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garylkoh

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Am I subtracting 7 ft from the distance of the sub to the microphone or the sub to the speaker? Am I using 7 ft because that is half the wavelength of 80 Hz? If so, then if using a different frequency (i.e. 100 Hz) the distances used in calculation will be different, too.

7ft from the sub to the speaker - and you are correct. If using a different frequency, the distances used will be different. However, using any frequency, you will get the sub back to the same place.

It sounds like it would be easier to use the RTA function of Room Equalization Wizard while playing an 80Hz tone and adjust the distance setting during playback to find the lowest SPL. Not everyone can adjust their distance setting, but it is easy enough when using a PC as the source.

Yes - it is far easier if you are using a PC as a source and room equalization.
 

Mark Seaton

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May 21, 2010
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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!!

Hi all, I just returned from a much needed vacation. I want to add a few quick comments, and can add more after later after I better catch up with things here.

Gary,

As I have no experience setting subs I do it the "logical" way - I take a sinusoidal signal generator, set it at the frequency of the crossover and visualize it at channel1 of the oscilloscope, using it as trigger. Using a microphone at a fixed position I measure the time delay of my main speakers at this frequency versus channel1, switch it off, and then set the sub-woofer at exactly the same time delay, so that the two curves at channel2 overlap. Anything wrong with this technique? :confused:

While conceptually not wrong, this will essentially get you quickly in the ballpark rather than provide a precise answer. From this point you can make fine adjustments to get the best resulting integration using other tools such as REW or other shareware programs. REW did recently add a phase measurement and display which can be very helpful here, and in the end, you are most interested in the total combination of the subwoofer and each speaker.

The reason your described approach is limited goes back to the nature of what you are measuring and the behavior of real loudspeakers. In short, any time measurement of an impulse is dominated by the highest frequency content, and we are interested in better integrating the sound in the crossover region. I specifically use the word region and not single frequency. As such, you need to examine the effects over a frequency range, not just a single frequency, especially when measuring in a real room and not a mostly anechoic environment.

The vast majority of loudspeakers by nature will have the higher frequencies arriving first with lower frequencies arriving at an increasing delay as frequency lowers. How much and how fast this delay increases depends greatly on the loudspeaker design (physical and crossover choices).

Even further examination of this will also explain where many past rules-of-thumb or audiophile generalizations come into play regarding subwoofer crossovers and setup.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
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Now for the last step. The first step is to find the relative group delay between the loudspeaker and the subwoofer. As I mentioned in the last post, there are several factors that are out of our control. If we have the tools to measure, it becomes pretty easy, but let's say we have a Radio Shack spl meter, and hopefully a test disc with test tones or a signal generator like:

http://www.performanceaudio.com/cgi/product_view.cgi?products_id=4115

Pick a frequency with a convenient wavelength - this will be a wavelength that is short enough to move your subwoofer around in your room. I usually pick 80Hz so that the wavelength is 14ft. It doesn't have to be absolutely precise unless you really want to know the numbers.

First, position your spl meter 10.5ft or 17.5ft (3/4 wavelength or 1 1/4 wavelength) from the main loudspeaker. Play the signal to get about 70dB. You don't want to play too high a level to keep room interactions down.

Place the subwoofer right in front of the loudspeaker. Play the signal with the crossover set at the frequency of the signal and adjust the gain to again get about 70dB.

Now, play the signal through both the sub and the speaker and move the sub towards the spl meter until the sound level is a minimum. It won't completely cancel because of room interactions. Also, because the sub will be very much nearer the meter than the speaker, it should measure far louder once the speaker is turned off.

Subtract 7ft and that is where the subwoofer needs to be for perfect phase/time alignment with the loudspeaker at any frequency. Now, inscribe an arc with diameter between the listening seat and the subwoofer - you can put the subwoofer anywhere on that arc and still the "correct" alignment. Use this arc placement to find the place in the room where the subwoofer sounds best.

The difference in the group delay between the sub and the speaker at 80Hz in seconds can be calculated as distance between the speaker and sub divided by 1126. The inherent group delay caused by a 80Hz 2nd order low pass filter is 3.125ms - which translates to about 3.5ft. Any difference in the distance can be attributed to residual group delay caused by something outside the low pass filter.

If you don't want the sub at that spot so far out into the room and it's a home theater, you can then use the relative group delay in feet to do what Lee suggested in the OP. This process should work with all subwoofers.

While I understand the intent, a real room complicates this approach where it can certainly get you closer, but simple measurement equipment looking at the total frequency response will allow for better results.

In short, room interaction with a subwoofer dominates the measurements, and group delay/phase is directly related to most changes in frequency response. As such, you want/need to measure the subwoofer in the chosen location, as the room's effect on the frequency response can change the group delay as much as the absolute distance.
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
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www.genesisloudspeakers.com
In short, room interaction with a subwoofer dominates the measurements, and group delay/phase is directly related to most changes in frequency response. As such, you want/need to measure the subwoofer in the chosen location, as the room's effect on the frequency response can change the group delay as much as the absolute distance.

Thanks, Mark. Glad you had a chance to enjoy a vacation.

I was answering the OP where JL had stated to Lee that all subwoofers had a 9ms group delay, and hence use 10 feet as the offset. I attempted to give a more generalized solution with simple instruments.

I now that there are better ways to integrate using proper software and instruments, but this is not my area of expertise, and I'll defer to the real experts :)
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
381
141
390
47
Chicago, IL
www.seatonsound.net
Thanks, Mark. Glad you had a chance to enjoy a vacation.

I was answering the OP where JL had stated to Lee that all subwoofers had a 9ms group delay, and hence use 10 feet as the offset. I attempted to give a more generalized solution with simple instruments.

I know that there are better ways to integrate using proper software and instruments, but this is not my area of expertise, and I'll defer to the real experts :)

Hi Gary,

No offense intended, and I agree that a 9ms offset is a gross generalization that doesn't relate to setup and the number is mis-applied here. I suspect this comes from a few correct factors that are mis-applied.

Speaking to all reading along: If we look at a simple 4th order L-R low pass filter at ~60Hz, the pass band group delay IS about 9ms with about 7.5ms at 60Hz. Real drivers have some group delay of their own due to driver inductance, box type/alignment, and the function of the connected electronics. The electronics will range from insignificant, to a few ms depending on amp type and associated signal manipulation and circuitry. So in isolation it is true that typical use of a subwoofer will make for some group delay. In the system context of a low and high pass speaker integration, this will not require ~10' of virtual delay compensation.

Due to the way a 4th order LR high pass works, the same high pass frequency on the main speakers would make for a match in group delay (which is why they sum in phase in ideal use). Real speakers in real rooms don't match this ideal, and have group delay of their own, making generalizations impossible. While we can't make specific delay predictions for all speakers, we do know that all speakers will have some non-zero and increasing group delay near the typical crossover frequencies, making the required offset less than the absolute group delay of the subwoofer.
 

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