Multiple Small Subs - Geddes Approach

1) You've said the source type does not matter, then why didnt you like ported Summa? You could have ported it very low at 25Hz and got a shallow sloped response to blend with other bass sources.
From my experience asking questions, the port just creates one more complexity in phase. If LF extension is challenging excursion then add more woofers. Don't forget also, fixing Group Delay with DSP, adds more delay to the whole signal.
 
Hi Earl (@gedlee),

Please help me understand below questions:-

1) You've said the source type does not matter, then why didnt you like ported Summa? You could have ported it very low at 25Hz and got a shallow sloped response to blend with other bass sources.
2) Why did you like dipole? if we ignore the SPL loss for a moment.
3) How important is it to raise one of the subwoofers amongst the three subs?
4) Can we sum the subs below the first room modes in phase with other subs?
5) In a bass reflex sub, at the tuning frequency there is 180 degrees phase shift. With modern DSPs the phase at the tuning frequency can be reversed back to make more like the phase of a sealed sub. Do you think, it is worth a try to mimic a sealed sub using a bass reflex one?

Thanks and Regards,
WonderfulAudio
I would appreciate if somebody could answer point by point for better understanding.
Thanks and Regards,
WonderfulAudio
 
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Hi Earl (@gedlee),

Please help me understand below questions:-

1) You've said the source type does not matter, then why didnt you like ported Summa? You could have ported it very low at 25Hz and got a shallow sloped response to blend with other bass sources.
The original Summa had ports. But after I went with multiple subs I realized that at best porting was not necessary and it could also be detrimental because of the total lack of cone control below tuning. So I just deleted them. All my subs are closed box.
2) Why did you like dipole? if we ignore the SPL loss for a moment.
The bass was clean, cleaner than most monopoles, but still not as good as multiple subs.
3) How important is it to raise one of the subwoofers amongst the three subs?
I think that it should be advantageous, but not critical. Only one of my subs is a few feet off the floor.
4) Can we sum the subs below the first room modes in phase with other subs?
We can, but I am not sure why one would want to do that. In my room, the first mode is at about 22 Hz. Not much point in trying to go lower than that.
5) In a bass reflex sub, at the tuning frequency there is 180 degrees phase shift. With modern DSPs the phase at the tuning frequency can be reversed back to make more like the phase of a sealed sub. Do you think, it is worth a try to mimic a sealed sub using a bass reflex one?
You could never do that. One cannot control the phase difference (180 degrees) between the cone and the port with DSP so below tuning the two are just cancelling each other out - no sound to EQ - and trying to do so would just add a lot of excursion with no gain in output.
 
I think that it should be advantageous, but not critical. Only one of my subs is a few feet off the floor.
Hi Earl (@gedlee)
1)Can the corner sub be the one that is lifted? It could be aesthetically more acceptable, the other two subs not so.
2)The multi sub article at https://mehlau.net/audio/multisub_geddes/ recommends adding delay, but you have told delay is not a good idea, am I understanding right?
Thank you,
Warm Regards,
WonderfulAudio
 
Hi Earl,

Few more relevant questions came to my mind, hope you find time to answer them.

3) It seems like each additional sub we are adding contribute less and less spl, then can we optimize by having 1st sub 12 inch driver, 2nd sub 10 inch and third sub 8 inch driver, each tuned to 25hz (with PRs) just to avoid loss of loading at low frequencies. I am just thinking of saving some real estate if it works. In the city where I live rooms are typically small 🙁
4) If after multi sub approach we move the listening position, the sofa a foot ahead or back(for cleaning the room), do we need to do multi sub tuning again or should the sofa be at exact the same place that we better mark its position on the floor and always keep it there?
5) The multi sub article at https://mehlau.net/audio/multisub_geddes/ uses mp3 for whitenoise, doesn't MP3 format remove quite a lot of frequencies that are required for this multi sub technique?
6) Can we use frequency stepped sweep (from lets say freeware Arta-Steps) instead of whitenoise for this technique? If yes, then is 1/6th octave between 25Hz and 200Hz good enough? We could take just one measurement with linear mic array instead of rotating the mic by hand. There are 2 feet and 4 feet versions available at https://www.shure.com/en-IN/products/microphones/mxa710?variant=MXA710AL-2FT%20
7) Is first room mode always a peak or a null or could be anything?
8) If this technique was done with door closed (almost sealed room) and we then open the door then should this technique be done all over again?

Thank you,
Warm Regards,
WonderfulAudio
 
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For those that dont know, REW has a nice Room Simulation tab. I guess 8 subwoofer + Main Left and Main Right is sufficient to most.
I dont know how accurate it is, but it is really easy to try ideas and theory from the couch 🙂

For example my room. I like to keep my mains rather close to backwall:

Only mains:
1746278592465.png


Adding one sub at backwall at midpoint (huge dip at 50 hz is gone)
1746278811538.png


Adding 3 ms delay (delay added is the same as if plateamp had phase turning knob - Took me some time to realize that):


1746279588084.png


Adding third sub:
1746279465749.png
 

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