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Old 21st January 2009, 03:08 AM   #11
frugal-phile(tm)
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Quote:
Originally posted by chrisb
For reasons that Dave, Scott, or others could describe, the more resistive ports retain the enclosure tuning over a wider range of internal pressure levels (i.e. sound volumes) than a simple round port, particularly those of larger diameter.
I neglected to address this (a couple posts, including Chris' happened while i was pecking out the reply above)... one thing that many people, including professionals, forget is that the T/S parameters used to design boxes change depending on how high you turn the wick (ie in a BR the tuning is really only perfect for one volume level). More insidious is that the tuning changes as the dynamics of the music you play change (with the volume at the same level)... making the ports resistive tends to ameriolate this... the box still uses the back wave to reinforce the bass but is less sensitive to the tuning shifting around.

note that the shifting of the parameters will also be affected by the weather.

As to the laterel brace, the one parallel to the baffle, i'd loose it as Chris suggests. A brace should always be edge on to the driver. Further this points out something you are giving up by going to multiple round ports... the slot ports on the side, by their very construction, create an incredibly stiff box panel, they are inherntly self braced. As well choice of the speacer used can further enhance this. We use a dissimilar material, most often MDF, to further isolate the outer wall from the inner wall, the different impedance of each layer of material -- one of the reasons for using plywood (9 layers in 12mm, 11 lauers in 15mm, 13 layers in 18mm BB) is that at each layer inteface, you get an impedance change and therefore any vibration is dissipated/damped.

In other words with the Onken-style ports the lateral brace is unnecessary.

dave
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Old 21st January 2009, 03:11 AM   #12
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Quote:
Originally posted by Bigun
Home Depot for some Birch Ply this weekend.
Look carefully before buying... note the number of layers mentioned above... the more the better.

dave
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Old 21st January 2009, 03:13 AM   #13
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Quote:
Originally posted by Bigun
I'm a bit of an engineer at heart
I'm a math guy (Honours BSc) -- that has relevance with the comment regared my design methodology

dave
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