2 x Jack 112 Lite

[Charles Jenkinson]

Does anyone know what the 'rigid foam' is (in terms of typical material or product trade name), referred in the plans for the phase extension plug and dome plug? The only foams I know are soft furnishing type foams (which are not what the plans refer to), florists foam (crumbly) or polystyrene foam (made of little balls stuck together) and lastly polyurethane foam (for insulation/flotation) which of the foams i know probably has the best properties for the function, i.e. can be cut or shaped, painted, won't readily disintegrate with handling.

[Rune Bivrin]

Polyurethane foam is preferable, but extruded polystyrene works well enough. Particularly if you cut it with a hot wire.

[Charles Jenkinson]

Thank you Rune.

Also, the foam phase plug extension shows a central circular rebate to prevent dome slap. Is the approach to fixing dome slap to wait until it is heard and 'shave a bit off' or calculate a specific clearance required to the cone and dome surface based on a maximum xmax for the driver being used. Granted that the speaker gaskets will compress, what is the suggested minimum clearance from the cone and dome to the foam? ...man, I could rap this thing if any more words rhymed.

[Grant Bunter]

Charles , you're spot on.

Construct/cut, fit , install driver. Check for slap and trim if required. IIRC 5 to 7 volts on the driver should show up any problems.
No slap?
Pack the driver away for a while and continue building...

[Charles Jenkinson]

I'm thinking this cone dome foam slap test is perhaps the 30Hz and 10 volts associated with the driver bedding in, which gives approx. ¼" displacement. There's a couple of other thoughts that go through my mind: If the Jack is full range and doesn't need subs for medium power applications, and there's no voltage limit on jacks (as per recent thread in jack section) then are they vulnerable at low end from over-excursion, and therefore does the 30Hz and 10 volts cover the worst case displacement...?

[Grant Bunter]

I'm thinking this cone dome foam slap test is perhaps the 30Hz and 10 volts associated with the driver bedding in, which gives approx. ¼" displacement. There's a couple of other thoughts that go through my mind: If the Jack is full range and doesn't need subs for medium power applications, and there's no voltage limit on jacks (as per recent thread in jack section) then are they vulnerable at low end from over-excursion, and therefore does the 30Hz and 10 volts cover the worst case displacement...?

Charles,
IRL do you expect the Jacks to cover 30Hz?
30Hz at 10 volts is a burn figure only, due to free air movement.
If your driver and foam pass the 30Hz test in free air, you've covered max excursion already.
With a fully built (and sealed) cab, due to throat pressure, excursion will be less, so no problem.
Well that's how it appears to me...

[Charles Jenkinson]

I don't expect the jacks to cover 30Hz in the cabs, i.e. reproducing music; the sensitivity response graph indicates the knee of the drop off to be around 55Hz. From reading what others say, my understanding is the jack is more like a direct radiator at lower frequencies, i.e. it isn't horn loaded, so there isn't any increased pressure. I'm thinking that the lower frequency response on a jack at high levels is heard as audible distortion, so the operator turns it down, because there is not sufficient 'horn characteristics' to attenuate audible distortion, as occur with the subs. From your input (Grant) and a bit of throwing it around between us, I can't think of anything else other than the 30Hz 10V with speaker in cab to test for cone slap, and the limit of good performance across the full frequency range being the onset of audible distortion....

[Gregory East]

The excursion at 55hz will be too limiting, more like 80hz before they start to get going productively.

[Charles Jenkinson]

It would be interesting to see a corresponding graph of excursion, at the 2.83v reference measurement - I guess it has to be calculated for each design, and then needs multiplication factors for different spl's. I find the physics of all this quite interesting but have little or no time to jump in.

[Charles Jenkinson]

I have sawn one line this afternoon. I sawed 3 on Thursday evening. The bloody plywood sheets are only square-ish in one corner, and the framing square i bought is not square. It's about 1.5mm over square on the 600mm side. I've calibrated it and stuck a note on.

[Bill Fitzmaurice]

The excursion at 55hz will be too limiting, more like 80hz before they start to get going productively.

Excursion maxes at 80Hz, with the tuning frequency excursion null at 40Hz, so there's no need to high pass for excursion control. But as response drops off below 60Hz there's no point in wasting amp power and voice coil capacity by feeding it with content below 60Hz either.

[Charles Jenkinson]

I can recommend the Stanley FatMax fine finish 11tpi hand saw for cutting out the thinner birch plywood sheets.

[ripNdeb]

I have sawn one line this afternoon. I sawed 3 on Thursday evening. The bloody plywood sheets are only square-ish in one corner, and the framing square i bought is not square. It's about 1.5mm over square on the 600mm side. I've calibrated it and stuck a note on.

http://www.newwoodworker.com/fxfrmsqr.html

[Charles Jenkinson]

I tried to modify my screwfix framing square but it didn't work. I had a row of about 10 punch marks which appeared to stretch the metal on the side required. But then I checked the inside of the square and it hadn't budged. I then checked the 24" leg of the square and it had a 1mm taper built in, i.e. it was wider at the corner of the square than at the end of the leg. I decided to cut my losses and order another off the internet, a Fisher - the same as my Dad has had for years, and that's bob on square. I am however going to take the rubbish one back to screwfix and give them some customer feedback. Lesson: don't buy and use a screwfix framing square.

On the other hand, maternity leave has given opportunity for progress. I have cut out most of big pieces and started on the detailed cutting required.

I decided not to route out the baffle, for sake of time. It's not turned out too bad, but I should have left the drill holes back from the corners a little (a couple of pics below). I'm intending filling the corners and reshaping nearer the time for painting/finishing, but again, I don't think anyone's going to be looking that closely in my throat, when there's plenty on offer to look at on the outside.

Using the tilting top hats requires a little bit of relief on the front side of the baffle to allow the baffle to sit where it needs. I thought about recessing the top hat, but with the wood only being 6mm thick (jack lites), it would require another plate gluing on the inside to take the tophat fixing flange, I decided it was a faux pas, in terms of mechanical integrity. Anyone inadvertently banging the cab down onto a pole top might break the inside support piece out.

[Charles Jenkinson]

I have a few queries. Any answers or pointers would be appreciated.

Plywood phase plug extensions: Assuming that the first one may be functionally helping to bond the protective steel mesh to the back of the baffle, can the second (6mm thick) disk be omitted, if the foam phase plug is made thicker to compensate?

Piezo testing: How is an individual piezo best powered for testing? Can a second piezo (for checking polarity) just be added in parallel to the master one on the same suggested power source?

Cutting piezo's: Is there any merit in using a finer tooth blade on my circular saw bench to mitigate cracking of the plastic? Does the piezo plastic benefit from being warm when being cut, to mitigate cracking?

PL bead application: (A bit anal this one) I've cut my nozzle so that the diameter of the hole in the nozzle is approx 4mm. If I extrude the bead at 4mm diameter (assuming I've cut my parts reasonably accurately) is a 4mm diameter bead acceptable for fixing and sealing all joints in a J12L?

Many thanks,
Charles