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Old 24th October 2003, 03:00 PM   #1
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Default sub construction advice

I'm after some input on subwoofer box construction for a box I want to be very inert.

The finished product I would like to look something like this:

http://www.geocities.com/etude316/audio/wip.html

I'm most interesed in the middle version but with an added plinth with a down-firing flared vent which can be plugged.

1. Constrained layer damping - who has used that technique or has comments on its use. In particular I'm after some ideas on what to use as a barrier between the inner and outer boxes

2. Anti-vibration mounting - if the box is not heavy enough to stop the whole box moving (highly likely), then I think I will need anti-vibration mounting to prevent vibration being transferred to the floor. Is this a good idea? Who has attempted this?

3. Materials. To get the curve I'm planning to bend 3mm sheets of MDF and glue them together. The curved part will probably be about 25mm thick with a damping layer in the middle. The flat parts of the box will be probably about 30mm thick. I'm thinking of using materials other than MDF. plywood, masonite, etc


Any suggestions, comments, recommendations are welcome
thanks in advance

cheers,
Paul
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Old 24th October 2003, 05:58 PM   #2
advance is offline advance  Switzerland
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Default Recommendations based on my experience

Quote:
1. Constrained layer damping - who has used that technique or has comments on its use. In particular I'm after some ideas on what to use as a barrier between the inner and outer boxes
a) Idikell 4001
b) Hawaphon
c) Very High Density Cork (1mm)
d) Lead (0.5 to1mm)

other materials are:

e) Sand (layer of approx. 10mm)
f) Pellets (layer of approx. 10mm)

excellent layer combinations are (outer to inner layers)

1) Multi-ply Plywood - Hawaphon - Idikell
2) Multi-ply Plywood - High Density Cork - Multi-ply Plywood - Idikell
3) Corian - Idikell


Quote:
2. Anti-vibration mounting - if the box is not heavy enough to stop the whole box moving (highly likely), then I think I will need anti-vibration mounting to prevent vibration being transferred to the floor. Is this a good idea? Who has attempted this?
a) Carbon Prepreg
b) Very High Density Cork (10mm)
c) RDC = Resonance Deadening Components, Clearlight Audio

layer combinations are:

1) Box - Carbon - Marble - RDC - Floor
2) Box - HD Cork - Marble - RDC - Floor
3) Box - Marble - RDC - Floor


Quote:
3. Materials. To get the curve I'm planning to bend 3mm sheets of MDF and glue them together. The curved part will probably be about 25mm thick with a damping layer in the middle. The flat parts of the box will be probably about 30mm thick. I'm thinking of using materials other than MDF. plywood, masonite, etc
a) Corian or Creanit (expensive but very inert)


Datasheets and examples:
http://home.tiscalinet.ch/cooltune/P...s/Menhir01.htm


advance
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Old 25th October 2003, 09:02 AM   #3
advance is offline advance  Switzerland
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Default Two additional links....

A very interesting articel in german:

http://www.picosound.de/D_gehmat.htm#Funktion

H.D. Harwood & R. Matthews BBC - Factors in the Desgin of Loudspeaker Cabinets:

http://www.bbc.co.uk/rd/pubs/reports/1977-03.pdf

advance
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Old 3rd November 2003, 03:27 PM   #4
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Default Re: Recommendations based on my experience

Quote:
Originally posted by advance


a) Idikell 4001
b) Hawaphon
c) Very High Density Cork (1mm)
d) Lead (0.5 to1mm)

advance
Great info advance It looks like you know a lot about it.

I would like to extend my JMLab Electra 905 bookshelf speakers to semi-active 3-way using active x-over at 380Hz and building the 3rd bass section with a driver Focal 11v7511. Enclosure will have about 90 litres, bassreflex tuned to 30Hz and there will be 2 channel power amplifiers on the back wall of every bass enclosure. I plan to use some sandwich material for the enclosure - e.g. 18mmMDF, 4mm cork, 18mm MDF, or to use something like bitumen roof tiles instead of cork.

Why do you mention VHD cork? And why just 1mm? Is VHD cork same as floor cork tiles? What about softer wall cork tiles?

Is there a chance to have resonancies on higher frequencies in such bass encosure (>1kHz)? Should I apply some damping material such as Idikell to the inner walls?

Thanks

Daniel
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Old 4th November 2003, 06:28 AM   #5
advance is offline advance  Switzerland
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Quote:
"Great info advance It looks like you know a lot about it."
Thank you . All my knowledge is based on experience acquired over many years (by listening, reading and experimenting). I am not an expert or an authority on audio.

Quote:
"I would like to extend my JMLab Electra 905 bookshelf speakers to semi-active 3-way using active x-over at 380Hz and building the 3rd bass section with a driver Focal 11v7511."
Our hearing is extremly sensitive in the region of 300 Hz to 3000Hz. So, an X-over at or below 200Hz would be a better choice (IMO).

Quote:
"Enclosure will have about 90 litres, bassreflex tuned to 30Hz and there will be 2 channel power amplifiers on the back wall of every bass enclosure."
Do not forget to brace your 90 litres enclosure! Electronics are also sensitive to vibration. Therefore, a power amplifier on the back wall of a bass enclosure is a bad idea (IMO).

Quote:
"I plan to use some sandwich material for the enclosure - e.g. 18mm MDF, 4mm cork, 18mm MDF, or to use something like bitumen roof tiles instead of cork. Why do you mention VHD cork?"
May I suggest two other solutions (inner to outer layers):

4mm MDF // 5mm bitumen roof tile // 22mm MDF // 1mm Cork // 3mm Multi-ply Plywood
(the speaker should be fasten to the 22mm MDF layer and also the bracing)

or

2 x 5mm Bitumen Roof Tile // 13mm Corian

Quote:
"And why just 1mm?"
Mechanical stability! However, 1-3mm Non-VHD may be fine....VHD would be perfect...

Quote:
"Is VHD cork same as floor cork tiles?"
No.

Quote:
"What about softer wall cork tiles?"
Should be ok if limited to 3mm...

Quote:
"Is there a chance to have resonancies on higher frequencies in such bass encosure (>1kHz)? Should I apply some damping material such as Idikell to the inner walls?"
Yes there is. The higher x-over you choose and the lower order filter you use, the more you have to deal with such resonances.


advance
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Old 4th November 2003, 06:51 AM   #6
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Default Re: sub construction advice

Quote:
Originally posted by paulspencer
2. Anti-vibration mounting - if the box is not heavy enough to stop the whole box moving
The best anti-vibration mounting is a pair of drivers mounted push-push. Much more effective (and more practical) than trying to solve the problem with the mass of the box.

Then all your box needs to do is not have walls that move from the pressure changes inside the box -- and a low pressure box such as a TL or aperiodic box make this problem easier to tackle.

dave
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