Modified Model 19 speakers

[yup]

Hey guys,
Here is the box I am planning to build for my Model 19 clones...

It was optimized to make the best use of 4' x 8' sheets of plywood (requires 3 sheets for 2 speakers - including bracing)

web page

Please take a look and give me your comments before I begin construction.

Drivers:
woofer 416-8C
horn 811b
tweeter 802-G

This enclosure is not as big as the model 19. I believe I can get away with this since I am using the C and not the B. (also, this is as big as I can feasibly fit in my living room and stay married)

My plan is to start with the port cross sectional area slightly larger than I need and then add spacer shims to the inside of the box to make the effective port length longer so I can fine tune it to my room. I can use thread inserts on the inside to line everything up accurately.

The inside of this box is 8 cubic feet, not counting the area lost for the horn, woofer, and bracing - or counting the effective volume gained from fiberglass insulation.

My main question is what port dimensions should I start with? Remember the resonate frequency needs to be tuned slightly high to start with.

Thanks,
Jeff

[intotubes]

Is this essentially the same as a Valencia?

Looks cool. Not too big IMO.

[GM]

Greets!

A pity you can't go taller. Anyway, (2) 4" dia. vents should suit your needs, so call it ~25.13"^2 for a single rectangular one.

GM

[yup]

Thanks GM...

Are you suggesting to go taller purely for box volume or HF horn placement, or both?

What would you consider an "optimum" box volume for the 416-8C? From what I have read here it seems that it wants to be in a slightly smaller box than the 416-8B (Model 19). Is this correct?

Unfortunately, I would need to go over a foot taller with this design to get the same volume as the model 19...


intotubes,
From what I can find, the Valencia's only came with 416-a/z & 416-B's. Do you know the internal volume of the Valencia?

Thanks,
Jeff

[yup]

It looks like the Valencia has an internal volume of approximately 7.5 cu-ft (without subtracting horn and woofer displacments, etc...)

So this design is definately similar to the Valencia except for the different woofer.

Jeff

[GM]

Greets!

You're welcome!

Both.

'Optimum' is a floating point depending on your desired performance criteria. I have two good corners, so a large, low tuned EBS alignment works best for me, with usable output to 16 Hz. The stock M19 tuning is for near wall placement, with controls to allow some adjustment for well out into the room placement at the expense of eff. and LF tonal balance.

Historically, Altec cabs were tuned for midbass 'punch' (underdamped) since there was little content on vinyl below 50-60 Hz, aka the 'West Coast Sound' that reviewers trashed in the '70s when more BW became available for the masses and SS replaced tubes. The M19 was their attempt to conform somewhat to the new status quo.

Anyway, unless they reduced the net Vb when they switched to the -8C, then in theory it's more underdamped than when loaded with the -8B, yielding some peaking at Fb. FWIW, I get a M19 net Vb = ~16,290.432"^3, and in a sim using MJK's Mathcad PORTED WS, reducing it to ~10,650"^3 net and increasing its vent length from 0.75" to 1.5" yields a <1 dB lower amplitude alignment, so in theory an inaudible difference, though of course due to several variables I doubt it will actually sound the same in-room as a -8B loaded M19.

That said, the M19 and other speakers that beam their HF below face level don't sound 'right' to me and most other folks who have had extensive exposure to live music or large cinema sound, so getting the HF horn up higher is desirable from both a technical and perspective POV IMO. Since the M19 is one of those rare speakers that accounts for XO delay, the horn/woofer spacing should be maintained, and due to the TL effect on vent tuning, its location becomes more critical with increasing aspect ratio, ergo if you choose to make it taller, then I will need to sim it to find the theoretically best location/size/length for whatever alignment you prefer.

GM

[yup]

GM,
Thanks for the very helpful information.

I reviewed my speaker location and tried negotiated with the wife and have come to the final conclusion that I can not make the box any taller.

I will take care to space the woofer the same distance from the horn as the 19. The port however is a different story. Since the box is more narrow I do not have as much room beside the woofer for the port. Will placing the port below the woofer be an issue? Would two round ones have any advantage?

Hopefully I can use the variable thickness port shims to adjust the final configuration from and underdamped one to an overdamped one and see which one suits my room and listening tastes.

Jeff

[GM]

Greets!

You're welcome!

I'm so glad I never had any audio SAF issues!

No, the cab's dims are 'square' enough that it shouldn't make an audible difference.

The only reason to use more than one vent would be due to space limitations unless you want to use a bunch of small ones to make an aperiodic vent (you don't).

GM

[yup]

One more question GM...

The 25 in^2 port (10 x 2.5) is only 1.75 inches away from the inside bottom of the box. Should I leave it here or move it down to where the bottom edge is the same as the inside floor?

And of course, why?

Thanks,
Jeff

[GM]

Greets!

You can move it if you want, just make it a bit larger since the extra area of one of its sides will lower initial tuning a bit. Really, extending it all the way across is what I'd do, then tune it with the top piece, which also acts as a brace: http://img149.imageshack.us/img149/5038/speaker21uk.jpg Just make sure to temporarily seal the joints with Blu-tac or similar, otherwise you won't get an accurate tuning till it's glued in place and can't easily change it.

GM