Help with design of this Altec mid bass horn

Hi audio friends

I'm wondering if you'd kindly look over the attached drawing.

I've designed this for a couple of reasons.

1. The measurements of the outside cabinet exactly meet my room restrictions.
2. I already have the Altec/GPA 416-8G woofers (8 ohm versions, brand new).
3. The design incorporates both bass reflex loading and front loading for efficiency.
4. The design picks up some other proven horns, eg the Altec 816 plus JBL similar versions (but these have quite weak bass below 80Hz)

http://www.greatplainsaudio.com/downloads/416_8C.pdf
New Page 2

I'm wondering what ideas people could add to make this work?

In particular -

1. desired flare rate. I'm using a standard 30 inch constant radius, as per flare on the Altec VOTT A5/7. The top and bottom sections are curved, the two side pieces are straight.
2. porting. I think I'll just suck it and see, using 3-4 smaller 3-inch ports, where I can close off and tune accordingly.
3. dampening. I'll start with fibre/wool filling for first attempt.

My target is for a good bass response from 70hz to 800Hz with approx 98-100dB, where these will be crossed over to an Altec 511 horn using Altec GPA 902 one-inch compression driver. Xover to be decided, but likely to be MiniDSP active to best tune the bass and give some EQ where needed down low. I will complement the horn with dual tapped bass horns from 30Hz to 100Hz later on.

Room size is largish...eg 25 feet x 20 feet, opeing to a larger dining area. Distance to listeing position is 19 feet.

Typical source is CD, driven by 300B SET...but as stated above, likely to run a separate SS amp suitable for the bass horn, eg Pass Labs F5, which I have sampled before and sounds great with the 416 in my existing large 620 cabinet.

Kind regards

Andrew

FYI, the ?standard? small VOTT flare radius is 30-31/32?, so 31? is close enough.

No throat area is spec?d so assume it?s the ?standard? 128?^2. The basic dims works out to a ~110 Hz mouth and axial length in full [4pi] space, so longer/lower if setting perpendicular on the floor. Loading the various specs into HR yields a ~127 Hz expo flare frequency. Didn?t work out its ?close enough? radius though, so no clue if it?s expo or a hypex.

For comparison, the VOTT horn is a ? size ~55 Hz horn or ~110 Hz actual, so while at a glance yours appears a larger version in comparison, it will have a more ?pinched? mids, i.e. greater directivity over a narrow BW, meaning there will probably be a hump in the mids response right where you want it ?smooth as a baby?s bottom?.

If this is your plan, then you?re better off overall shifting the horn forward to truncate it to Altec specs and adding a round-over termination out to the cab?s width, height ala Le Cle?ac?h to minimize reflections back to the throat, though only do it on three sides if ?mounted? perpendicular to the floor. That, or use a longer ~77.8 Hz flare to get to the 770 cm^2 mouth.

The way to have your ?cake and eat it too? though is to plot the 825/828?s area expansion every ?x? distance [of your choosing] and extrapolate it out until it meets your mouth dims to find the correct axial length, flare frequency and closest radius. Now it should perform like an Altec through the XO BW except with a bit more bottom end and directivity control.

As for reflex tuning, we come up with similar for an 80 Hz/4^th XO, so looks like you?re good enough for fine tuning in room.

GM

Thanks GM, appreciated.

Was on the tools today, developing a MDF horn section.

Throat is 20cm x 20cm (7.87 inch x 2) = 61.93 inches square.

Mouth is 76cm x 76cm (30.71 x 2) = 943 inches sq

Axial length is 40cm (15.75 inches).

Cabinet volume approx 250 liters, if I use my existing box...to be decided....as I'd have to cut it apart.

Flare is "line of best fit" based on 31 inch radius.

See attached.

GM, see attached updated drawing, showing how it would look IF I went with my existing cab.

You’re welcome! Looking forward to what performance you wind up with as it strays a bit too much from my experience in trying to get as much high SQ BW out of a truncated or compression loaded design, especially when factoring in an 800 Hz XO point.

GM

GM, if I went back to the drawing board (I still can), but still remained within my restriction of 40cm axial length of horn, what would be the largest square mouth area possible using a standard/proven flare?

Again:

"The way to have your ‘cake and eat it too’ though is to plot the 825/828’s area expansion every ‘x’ distance [of your choosing] and extrapolate it out until it meets your mouth dims to find the correct axial length, flare frequency and closest radius. Now it should perform like an Altec through the XO BW except with a bit more bottom end and directivity control."

Conversely, work out the Sm based on axial length and start with Altec's 128"^2 throat if you want the largest practical one.

GM

Thanks gm, I don't understand fully the first para. (btw, I'm using 902's.)

On para two, what is Sm?

I have fixed axial length, but no limit on size on mouth. Hence q re what lowest bass hz can I get depending on choice of flare....

Kindly

Main horn complete....I've added numerous layers of MDF, adding silicone a plenty for dampening. Baffle added, with throat 180mm x 200mm. Will now start finishing inside horn with putty and sanding.

putty filler added