MJK's Fostex FF125wk BLH project

[DrBoar]

I have plans for using them as my next computer speaker using 4" Tangband driver, I have the drivers as well as a cutting plan for a 12mm MDF board. X-mas, visitors and other things has thrown spanner in the works but now things are clearing up.

[Scottmoose]

Be interesting to hear how you get on.

[chazdrumzalot]

Originally Posted by DrBoar

I have plans for using them as my next computer speaker using 4" Tangband driver, I have the drivers as well as a cutting plan for a 12mm MDF board. X-mas, visitors and other things has thrown spanner in the works but now things are clearing up.

Are you still planning to make them full-size and with corner loading?

[Scottmoose]

I'd hope so, since if you make them in any other size it would be a completely different enclosure, would need to be treated as such, and designed from scratch to achieve whatever the goals were accordingly. You can't just change the physical scale of a box & expect it to work as the original design was supposed to. While to an extent a design can be scaled, to do it successfully you first have to back-engineer the original, not just in terms of physical dimensions but the inherent design objectives with x drive unit, relate these to whatever unit you intend to use, adjust accordingly for the requirements of the new driver, and then design the box to a new scale that will achieve these.

The TB presumably has very close specs. to the Fostex, in terms of its mass corner, useable gain BW etc. Otherwise it won't work as Martin intended.

[chazdrumzalot]

Originally Posted by Scottmoose

I'd hope so, since if you make them in any other size it would be a completely different enclosure...

my point exactly. For me personally, when i think "computer speaker" I think desktop, and that's clearly out of the picture with this design!

[chrisb]

Originally Posted by chazdrumzalot

my point exactly. For me personally, when i think "computer speaker" I think desktop, and that's clearly out of the picture with this design!

yup, desk-top/ very near-field application like this is where a small (2-3 liters is a nice range) enclosure has a lot of functional and aesthetic advantages

[Scottmoose]

I wouldn't use a back horn for such duties, that's for sure.

[DrBoar]

Regarding the Tangband

Stated cone area is to big. 57 cm2 correspond to a diameter of 86mm and then much more than half of the surround is included in the cone area
Diameter of cone and surround is 90mm, getting to half the surround 82-83 mm that is 54 cm2 but then the center is not part of the cone and there is 5.3 cm2 so in alles about 50 cm2 not 57.
They have been played a bit before and now I have had them unmounted and played them quite loud with cone movement of +/- 1-2 mm. With some few hours of play time they still had a Fr of 85-90 Hz. Trashing them with Ramstein Mutter and also the Sensucht album as loud as I dare with several mm of cone motion and me out of the house, repatedly have moved down the Fr to 73-75 Hz so far.

[Godzilla]

What's the purpose of the chamber behind the driver's chamber? I read something about an absorber/tuning?

[Scottmoose]

Right. See the thread I linked to. It's an internal Helmholtz resonator / absorber. It works in the same way as the bass-traps sometimes used to kill a problematic room-mode, which are essentially a big empty box tuned to said problem frequency.

In this case, Martin has tuned it to, or just above, the intended Fh (upper corner-frequency) of the horn in order to provide a higher order acoustic low-pass slope. Since most such resonators are narrow band devices, the slope will ultimately resemble a cauer filter, so the trick is to shunt the secondary peak > 40dB down to maximise the benefits.