I know there are a few of these threads floating around or posts buried in other threads but my hope for this is to become a useful reference being more detailed on certain aspects of it.
Note this thread does not deal with the act of finding the best locations for subwoofers in your room just dialing things inonce the subwoofers are in position.
Note this thread is a work in progress and will beupdated in the near future with more info/graphs.
I have used my room as an example (pictures and graphs) for the type of setup detailed here. This can be found here on post #2
Measurement microphone and software (Omnimic, miniDSP UMIK-1 + REW…)
DSP with enough channels for each subwoofer orgroup of subwoofers. This can be a MiniDSP, internal amplifier DSP (Behringer Inuke, Crown CDi…) or other external DSP.
AVR/processor with adjustable speaker distance settings (nearly all decent models from the past 10+ years have this feature).
Before we dive in I should warn you that this can be a tedious process, do not expect to start from scratch and speed though thisin 15 minuets. You may find yourself taking hundreds of measurements depending on how thorough you go with it. The results however can be well worth it, you can expect improved dynamics and definition and in some cases 6-10+dB more output capability over sections or all of the subwoofer range (that can be like doubling the number of subwoofers and amplifier power at no cost!).
Now to begin, I’ll break things down into steps for easier reading.
Current setup baseline subs + mains.
You want to measure your current setup before making any changes so that you can use that to compare and make sure you are improving things. You only need to do this step if you want to compare your old settings to you new ones. If you have recently added new subwoofers and have yet to rerun your AVR/processor’s room correction or you are just starting out with a new setup and have not yet run it for the first time it may make sense to wait until after the subwoofer integration steps as you will likely have to rerun that correction after the fact. If you don’t mind running the room correction setup twice just to compare then by all means go right ahead.
If you are only concerned with the response at the main listening position that you only need to take measurements at that location. However it is best to try and optimize the response across all of the seating the best you can, therefore I will focus on that.
For most of these measurements it is best to send a two channel stereo signal into the AVR/processor, if you have your speakers set to large and do not run the subwoofer with stereo sources you may want to turn on the + setting for the subwoofer or similar option that sends the low frequencies to both speakers and subwoofer. The range we are concerned with is <200Hz so it may make sense to sweep below that frequency and adjust your software’s graph axis to focus in on that range.
Depending on how thorough you want to go and how many seats you have you can measure at every seat. Or 2-3 positions per row (if you have multiple rows). At the very minimum take at least one measurement per row. It is useful to have one measurement centered between all the seating as well.
Once you have that baseline set of measurements saved you will want to start fresh.
Baseline measurements for subwoofer integration.
First go into your processor’s settings and change the subwoofer xover to the highest setting. This will allow a clear view over the subwoofers entire operating range without the rolloff from the subwoofer lowpass causing unneeded attenuation of the higher frequencies. Leave this at that setting until we go to combine the subwoofers with the main speakers. If using external amps turn off those for the main speakers or simply unplug them from the AVR.
Clear all settings in your DSP/s and adjust the amps so that the signal gain (power to each subwoofer) is matched. You want to measure the native response ofeach subwoofer individually at each location for which you choose to take measurements. You can usually get away with 1-2 measurements per row + the central location here but make sure you continue to use those same mic positions forthe other integration steps. Don’t worry about getting the mic back exactly in the same spot an inch or three either way will not make too much of a difference at the bass frequencies. These measurements will be used as a reference when we start to add them together in order to make sure they combine constructively.
Now if any subwoofers are roughly at an equal distance to each seat for example multiple subwoofers at the front wall, these can be treated as a group since their outputs should already constructively combine. Thus they can be treated as asingle subwoofer and placed on a single DSP channel to be EQ’ed and delayed together.
Combined response of two subwoofers.
Measure the combined responseof two of your subwoofers, if you have more then two subwoofer locations make sure one of those measured here is the subwoofer furthest from the LP and come back to this step after step #5
for each additional subwoofer location.
Ideally the response of the two subwoofers would constructively combine and you will see a net gain(roughly 6dB for two identical subwoofers) across the entire range without any dips caused by destructive interference. However this is not often the case and requires adjustment of delay to time align the individual subwoofers at the LP. Delay is likely required and should be added to any subwoofer that is closer then the furthest one from the LP.
To start physically measure the difference in length between the furthest subwoofer to the central measurement location and from the other subwoofer you have chosen to measure with it. Now add delay to the closer subwoofer equal to the difference indistance between the two. That will often get you close, now measure the combined response with the delay added. Adjust the delay +- a little (0.5ms or so) and re-measure to see if it can be improved upon. You want to try and get constructive summation of outputs at least up to your eventual crossover point for the mains(usually 80 Hz). Boundary interactions can cause issues here so you may have to settle for less then perfect summation, just choose something that looks best over the largest potion of the range.
One limitation of the 2x4 miniDSP here is its maximum delay, only 7.5ms. You can however squeeze a little more in if you use a low pass above the eventual crossover point. A higher order lowpass will result in more delay (or phase shift).
Delay continued. (Skip if you have only one row of seating or care only about the response at the main LP.
If you have multiple rows you want good integration in each row, not only the central measurement location. Measure the combined and delay adjusted response from step #4
at the other rows of seating. You want to adjust the delay to achieve a decent result across multiple rows, this can be hard to achieve and is often impossible to get perfect. The amount of delay for ideal integration in one row will often leave a notch in another. Your goal is to find a balance in the delay settings that minimizes these notches.
Additional Subwoofers (Skip if you have only two subwoofers)
Turn on another subwoofer and go back tostep #3
in order to integrate this subwoofer with the others. Treat the already integrated subwoofers as a group leaving them onbut continue to use the furthest sub as the physical reference point for adding delay. Once you have integrated all the subwoofers you can move on.
Manual EQ of Subs (If using room correction with auto subwoofer EQ this step is optional)
Now that your subwoofers are time aligned and working constructively you can EQ them together and as one applying the same EQ to all which gives you equal power distribution, no sub will bottom or clip before another if using identical subwoofers. You could also EQ each flat individually if using non identical subs but that sometimes requires additional EQ work after they are combined and you will often reach the limits of one subwoofer before the others. However individual EQ can sometimes be beneficial if say you are getting too much output at the higher frequencies from nearfield or rear subwoofers causing localization issues when applying global EQ to all the subwoofers.
For this step if I usually measure 2-3 locations per row and average all of them applying EQ based off the averaged response.
Integration with mains
Readjust your subwoofer’s xover back down to where you had it. Turn on the amps for your mains or plug them back in. Now is the time to rerun your processor’s room correction if applicable.
After that is done measure the response ofthe mains with subs turned off. Then the subwoofers turned on with the mains turned off/unplugged. Finally both turned on for the combined result. Like the integration of multiple subwoofers the combined response should show constructive output through the crossover region without any dips that were not in the individual responses. If that isthe case you should be good to go.
However if they do not combine constructively you will need to play with the subwoofer delay in your processor’s speaker distance settings. Unlike dialing in the subwoofers you can't often measure the physical distance offset between the subs and speakers here and expect the acoustic summation to be ideal at that distance, group delay in the low pass filter and subwoofer delays you set earlier effect the acoustic offset of the subwoofers. Your best bet is trial and error here, choose a distance, measure and adjust that distance + or - a little bit maybe 2ft and re-measure.
You will know you are moving in the right direction when the combined response has fewer peaks and dips around (below, at and above) the crossover. The same is true if you have the mains set as larger with the subwoofer in the plus (or similar) bass management setting. You may need to change it by a few feet to start to see a difference in the number of peaks and dips. When you get close the output from both should be constructive across the whole crossover range however if you see just one larger dip you may want to try flipping the phase 180 degrees on the subwoofers.
Like the subwoofer integration and if applicable you do want to check how the mains integrate with your subwoofers in your other rows (measuring subs only, speakers only and speakers+subs for each row). Adjust for the best balance across your multiple rows. Sometimes you can use the output from the main speakers to fix a dip in the subwoofer only response in one of your rows if it’s near the crossover by adjusting the subwoofer distance setting to align the speaker output with that row.
You may or may not want to make some final EQ changes to the subwoofers based on the combined speaker/subwoofer response to flatten any output bump you might have created if they were not integrated correctly by the processor. Or manually dial in a house curve.
Put in a movie/music sit back and enjoy!
Sorry for the long read, I hope this was not too difficult to understand and will help anyone who reads it optimize their system to it's full potential.