Resizing horn dimensions

[uvellani]

I would like to use this horn design - http://madisound.com/pdf/fostexcabs/85kencrev.pdf - but with one driver per box instead of the two in this design. I am not sure how to go about resizing the horn? Do I just halve the volume of the compression chamber and the cross section of the horn? And leave the length as is? Or is it more complicated than that?
I love the sound of the FF85k driver and would like to build something that I can use for listening in a nearfield setup.

Thanks
UV

[GM]

Greets!

Rear chamber Vb halves, as does the throat area, ergo the horn has to be considerably longer since it has to have the same expansion rate from the new throat to the original one.

GM

[mrlots2do]

Originally posted by uvellani
http://madisound.com/pdf/fostexcabs/85kencrev.pdf

While thinking about what GM said, I was trying to determine the throat dimension to be changed. I found mistakes in enclosure dimensions. I've circled key indicating measurements and get different throat dimension heights. The cut list dimensions say part 4 (500mm) and 10 (845mm).

Note to self: yeah, I get 45mm for throat height, but those center three dimensions are wrong by 25mm.

[AndrewT]

Hi,
in a conventional horn the cross-sectional area increases along the length of the horn. The doubling in area usually occurs at a constant distance along the horn (flare rate) but this length increment can vary.

It is worth trying to check the design to see if they have incorporated a constant flare rate or varied it.

Once you know what they have done it becomes easier to visualise the changes you need to make to the length.

Fostex appear to use little bits of offcut to avoid putting any of the horn elements at an angle. I would simplify it by sloping many of the internal elements.

The three vertical elements at the bottom are deliberate attempts to trap particular frequencies (or exagerate them) which? Would a sloping floor be better here?

[GM]

Greets!

Technically, neither, they are supposed to be filled with damping material such as sand, pea gravel, or similar to both mass load the cab and damp unwanted HF output, so using a sloped board isn't a good plan IMO. FYI, some folks are satisfied with just draping some felt over them, so as always, YMMV.

GM

[uvellani]

Thanks all for your input.
What I have decided to do is try the ff85k in this design - Buschhorm mk2 - which is a design for a 3'' FR(fe83e). That seems an easier cabinet to build rather than experimenting with modifying the recommended horn for the ff85k.
The fe83e has a higher qts(0.72 vs 0.47) and higher fs(140vs122) than the ff85k. Any thoughts on how the ff85k might respond in this cabinet? Like I said I am not looking for thumping bass. But what I would like to achieve is a balanced and full sound from about 3-4 feet away.

Thanks

[GM]

Greets!

Dunno, technically the rear chamber and throat are much too large, ergo the horn is much too short, so in theory there's going to be a big broadband notch in the mids/lower mids where the horn rolls off too early to blend to the driver's falling response. This can be offset though by adding enough series R to raise the FF85's effective Qts to ~match the FE83's. This will probably make the highs too rolled off though, so a bypass cap to suit may be required.

GM

[AndrewT]

Hi,
chop the back chamber in volume and use the spare space to lengthen the horn at the throat end.

Think about how you could cut the width behind the driver to 70mm or tapering to 70mm to make the smallest part of the throat nearer square rather than 150 by 15 tapering to 25, at the existing throat.

I'm thinking an inverted V forming the back chamber and an extra vee shaped horn 25mm by 150 at the top reducing to 25mm by 70 at the bottom making the new throat. There may be enough room to fit in two folds and two lengths of horn each 25mm deep front to back and spread the taper over the two lengths.