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How to lower the resonant frequency of a structure?

I recently built myself some 'Zero Baffle' spkrs, using:
* 4x SB Acoustics 'Satori' 6 1/2" mid/bass drivers
* and 1x Satori tweeter.

See pic:




They are 2-way active - XOs and extensive EQing is provided by my miniDSP nanoDIGI - and they hand over to subs at a choice of frequencies stored in the nanoDIGI (48dB L-R slopes): 90Hz, 110Hz, 130Hz and 150Hz.

They sound terrific but, as a result of a friend bringing over some CDs to have a listen ... I've been made aware of a problem that I would like to get fixed.

I did an experiment - using my sig-gen to feed various frequencies into the mid/bass drivers. Level was set quite high - and, starting with the sub XO set to 90Hz, I slowly wound the frequency up from 80Hz to 400Hz.

The steel frame resonated at 130Hz; changing the sub XO to 110Hz reduced the buzz a bit - upping the XO frequ to 130Hz decreased it a bit more ... and increasing the XO point to 150Hz removed the buzz (as 130Hz was now going into the sub).

My question to the brains trust is ... what do I need to do, to lower the resonant frequency of the frame which supports the drivers? (Ideally to below 90Hz!)

Now, the frame which the drivers are bolted to is as follows - made from 5mm x 13mm steel flat, bolted to a 20mm thick base, which stops them falling over. See here:




(There are 2 of these 'Us' for each spkr; sorry the pic has come out so small. :-(( )

It seems to me I have 2 options:
1. use a denser material than steel - say, bronze or brass 'flats'.
2. or increase the mass of the frame - so have solid sides instead of a frame made from flats.

Can anyone suggest the pros & cons of each?

Thanks,
Andy


Edits: 05/13/22
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Topic - How to lower the resonant frequency of a structure? - andyr 21:39:29 05/12/22 (47)
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