Roland - Your 828 Sub Mod

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Hi Roland:

You were kind enough to send me some details of your 828 mod a short while ago, and I've been pondering it since.

You discussed that seperating the enclosure left about 200L of reflex volume below to install an 18" JBL woofer (that's a little rich for me, but several Eminence drivers model well, too).

But you didn't have too much to say about altering the Vb of the midbass horn, so I was wondering if you have any observations.

Using Keele's method, I calculate that the 828 horn mouth Fc is about 80 Hz. NOTE: I'm not saying that the horn actual Fc is 80 Hz in practice, merely that its area is that required for an Fc of 80 Hz using Keele's calculations. With a 515-G, the Keele Vb is about 50L, which my 828 upper chamber would be (approximately) considering bracing, driver displacement, etc...

Keele's St is also in agreement: at 80 Hz one calculates 0.75 ft^2 which is quite close to the 828's 8" X 13" St.

In short, compartmenting the upper chamber as you have models well according to Keele with respect to Vb, St and Am. I'm wondering if your experience confirms the numbers.

Thanks for any info.

[Roland N R]

Hello Wunhuanglo:

Basically I agree with your numbers. The horn rear chamber if anything should be too large for the actual measured horn cut off freq. But because of a torn cartilage in my left hip I been unable to proceed with free field testing (bring it down from the second floor) so I will save further comments till then.....hopefully about two weeks more. Will then post pictures etc

Best

Roland

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Roland:

Hope your hip comes around quickly. Sounds too painful to contemplate. Don't do anything too crazy too soon (especially trying to get an 828 cabinet down a flight of stairs!)

Regards,
Charlie