Pensil 7 (Alpair 7)

[markaudio]

Originally Posted by togura

The removable back panel looks good.
I want to know what kind of screws was used to secure the back panel to the cleats. I guess wood screws would get loose after several removes of the panel. Should I use tee nuts and machine screws? Any suggestions how to use tee nuts would be great.
Thanks
Tatsuya

Hi Tatsuya,
I use M4 self tapping wood screws on the Pencil 12 because from experimentation, 220 gram of the white fibre fill works well. The fibre fill I use is the commercial stuff found on many speaker boxes. I get the fibre-fill in 1.25 metre X 3 metre rolls.

Having said this, I'd more recommend using the M4 machined fittings (see pics) if more experimentation is needed.

I'd suggest working on this basis:

1 - Back removal up to 5 times - M4 self-tapping wood screws should be Ok, but don't murder the threads by over-tightening.

2 - Back removal 5 times up - use the machined fittings.

Cheers

Mark.

[Henkjan]

+1 on the machined fittings. I prefer them usally over screws. just make sure you fix the T-nut with a drop of glue, to reduce the risk of pushing them out when you insert the bolt (btdt).

btw: in case you use MDF as construction material, screws are usually only good for 1 time fixing only, if you plan to remove just once you'll want to use the fittings.

[planet10]

Originally Posted by Henkjan

+1 on the machined fittings. I prefer them usally over screws. just make sure you fix the T-nut with a drop of glue, to reduce the risk of pushing them out when you insert the bolt (btdt).

We've given up on t-nuts -- one failure and you are feedup -- and use threaded insert fittings instead.



dave

[gazzagazza]

Whats the situation with sealing the rear when using the threaded bushes? I guess you need to make sure they are inserted flush or just sub flush to be sure the back sits flat against the battens. What are people using to ensure the panel seals when screwed home?

[CSS/XBL]

Originally Posted by planet10

We've given up on t-nuts -- one failure and you are feedup -- and use threaded insert fittings instead.



dave

Yes, I've switched to threaded inserts as well and stock 10-32 as well as 1.5" and 1.75" bolts, mostly for woofers.

Bob

[Henkjan]

there it shows that also in types of screws we have our preferences, not just in audio after some disapointments with the threaded inserts (came out with the bolt) I used T-nuts since

[markaudio]

Originally Posted by gazzagazza

Whats the situation with sealing the rear when using the threaded bushes? I guess you need to make sure they are inserted flush or just sub flush to be sure the back sits flat against the battens. What are people using to ensure the panel seals when screwed home?

I'd suggest either liquid or taped sealers (see pic) taking care to keep the threaded holes clear prior to fitting the back panel. Gasket paper (or most thick paper) also works well and can be easily cut into strips and glued in place.

I recommend using the taped and/or paper seals while experimenting with damping material volumes, once satisfied, apply a fine bead of liquid sealer to finish. This should assure a leak proof rear panel.

Nylon washers under the screw heads would assure a complete seal if you want to be 110% sure. I usually find there's enough give in the wood to use the screw without washers.

Cheers

Mark.

[gazzagazza]

Thanks for the suggestions on sealing the rear panel Mark.

[gazzagazza]

As I've progressed the build I've made a series of measurements to see, for my own interest, what the nature of the panel resonances might be.

I set up one of the enclosures, on its side on a mat, on a stone floor.

I set up a measurement mic to be about 250mm above the enclosure surface at just more than halfway along the panel.

To get repeatable results I struck the panel midway by dropping a hard rubber ball onto it from my shoulder height. Each measurement was peak held over 5 drops. This proved highly consistent.

I'll post a series of spectrums to follow.

[gazzagazza]

First measurement. Undamped side panel without enclosure back.

The enclosure is built from 18mm pine ply, glued on all edges, with an additional batten glued along the inside of all joins.

The spectrum shows the energy distribution. The levels recorded are not calibrated, so treat them as relative to the other spectrums posted only.

You can see three sequential sets of 5 drops to show the consistency.

Peak energy here is in the 200Hz to 500Hz band.