Are 2" driver exits/horn entrances really 1 15/16"?

[originaltubino]

I am going to buy a bit or hole saw to make a hole the size of a 2" driver exit. What size for a 2"? Who's buried in Grant's Tomb, right?

But they don't measure 2", they measure about 1/16" shy of that, AFAICT. Is this one of those things everyone (but me) knows, like 2x4 lumber isn't 2" x 4"? I want to buy the right tool. Thanks for any help for what is probably a very basic question.

[John]

The drivers are a bit shy of 2" at the throat exit. What exactly are you trying to do???

[originaltubino]

The drivers are a bit shy of 2" at the throat exit. What exactly are you trying to do???

Right, and the entrance of horns matches that, just shy of 2"... Okay, here's what I'm doing: I just purchased a pair of handmade Yuichi A290 wood horns (beautiful ones made by Woody Banks). A separate adaptor piece bolts first to the driver, and then the adaptor bolts to the horn. The adaptor pieces do not yet have the holes drilled for the driver mounting bolts (but they have the ~2" holes that adaptor circle to square). In order to get the mounting holes placed just right on the two pieces (and any future ones I make), I decided to drill a wood template that I could clamp to the blank, then drill through for perfect hole placement. Kinda odd maybe, but I know it would allow me to easily drill these two pieces correctly, and in the future I would be happy to have it. (I see more horn woodworking in my future.)

So I used the throat plate part of a spare horn to mark the mounting holes in a piece of hardwood, and got them just right -- so far so good. But to line up the template with the adaptor, I need to line it up with the 2" hole, which is easiest to do by making that hole in the template. I've got the pilot hole exactly centered between the mounting bolt holes, so almost done, just need to make the right size hole now to line up the hardwood template on the adaptor to use the drill press to make 4 holes.

To reiterate, I want something to drill THROUGH to line up the holes perfectly, so couldn't use the spare horn... I could squeeze a very stubby 1" pencil in there to mark holes through the mounting plate, but I need lots of vertical access for the drill press.

I'll make it 1 15/16" or 1 31/32", and it will work fine. Thanks!

Still curious about the exact hole size, and why it isn't really 2"...

[withTarragon]

There may be another snag. At the exit of the driver, the wall may actually be flaring out a "few" degrees. I am not sure if all of the newer JBL drivers do this or not.

Depending on the material you are working with, sandpaper or a file may allow you to approximate this (matching the flare) close enough. It is probably worthwhile to explore this since the transition and any flaws at the throat can be problematic. Geddes makes a big deal out of this and the enginers at Klipsch also mention the issue of "getting the throat right".
Good luck.

[Earl K]

Hi ,

- I use the 2441 mechanical drawing ( attached below as a pdf ) for my measurements .

- You'll see that the official word is that the drivers' exit is 1 15/16" .

- I'll only guess that it is this size because Bart Locanthi ( the original designer ) inherited this dimension from Western Electric ( & their 594 driver ).
- If W.E. used this size it may have been because the flare rate of the throat section ( combined with the distance measured back to the diaphragm ) conspired to arrive at an odd ball measurement.

BTW; ( when making metal mounting plates )
- I usually map out all holes to be drilled first by measuring out from a center point ( obviously before the 2" center hole is punched / with GreenLee Hole Punches ) .
- These points are placed on pencil drawn circles of appropriate diameters
- ( I use a good compass from a geometry set to draw the circles ) .
- I will typically "oversize" these mounting holes ( somewhat ) to aid in alignment .


>< cheers

[spkrman57]

Not that we want it to be that way, most folks don't even notice the mismatch that occurs.

I have several aluminum horns that are made for 2" comp drivers and they are off by a large size difference. I'm planning on just using them for cone driver use since the horns are no-name brands anyways.

Ron

[originaltubino]

Specs, technical tips, and history too! All great answers to a mundane question. Thanks, everyone!

[Tim Rinkerman]

Just for good measure, I believe the JBL 375 compression driver got its name(number?) because of its 3.75 inch voice coil, and the 175,well...you figure it out...

[Horn Savant]

Hi ,
- I'll only guess that it is this size because Bart Locanthi ( the original designer ) inherited this dimension from Western Electric ( & their 594 driver ).

Also note 594 VC seems to be exactly 4"!