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

GM, could you pls clarify how 128 sq inch throat is worked out. The sq root of 128 is 11.3 inches. When looking at A7 and other cabinets, the driver cut-out is around 8 inches by similar....hence my 18cm x 20cm actual.

Do you count the total area at the begining of the horn (including driver cutout) as the throat?

Quote:

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Originally Posted by atilsley

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

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I don't know how to explain it any simpler other than you looking at the 210's drawing showing its expansion over 'x' increments.

Sm = mouth area.

St = throat.

You're making a hybrid, hence don't know what the expansion rate the sections actually break down to without you working it out, so could only hazard a guess based on Sm, which I don't know that either.

Anyway, I see you've made them, so now moot.

GM

- - - Updated - - -

Quote:

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Originally Posted by atilsley

GM, could you pls clarify how 128 sq inch throat is worked out. The sq root of 128 is 11.3 inches. When looking at A7 and other cabinets, the driver cut-out is around 8 inches by similar....hence my 18cm x 20cm actual.

Do you count the total area at the begining of the horn (including driver cutout) as the throat?

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What does SQRT of 128" have to do with anything?

The 'standard' truncated Altec horn has a St = 8" x 16" = 128"^2/driver, which is approximately the driver's Sd. Remember, a truncated horn is nothing more than a section 'sliced' out of a full size one.

The cutout OTOH is the low pass filter that blocks off a portion of the driver's forward radiation, creating a low pass filter chamber, which ideally increases in net Vb, i.e. a smaller opening with increasing XO point as clearly shown in this early A7-8 [825 cab] for an 800 Hz XO point [fig. 5], which was replaced pretty quick with a HF horn baffle as a better overall performing solution: http://www.lansingheritage.org/image...ign/page04.jpg http://www.lansingheritage.org/image...ome/page09.jpg

GM

Thanks again GM.

I asked about sq root as I was trying to understand height and width of horn mouth....so I now understand the horn throat is more than just the driver cutout.

My horn's throat, therefore will be smaller, 18cm by 20cm...close to altec standard for driver cut out.

Flare is ' line of best fit' based on 31 inches.

Axial length is 40cm.

Sm is 68cm x 68 cm (26.7 inches 2).

Volume of cabinet 260 liters.

We'll just see how it sounds. Under $80 of materials to experiment with an idea...the cost of having my cake while eating it too...!

I can run some eq and correction for any dips/peaks anyway.

With this type of (115hz) horn, what would your preferred high xover point be, given 511/902 in use?

Kindly

Testing now....bigger sound presented, but not more punchy...maybe the drivers are just so good on their own? I've not heard an Altec bass horn before...so unsure how to assess.

OK, I can report that something special has happened...the horns work and sound great!! Back chamber is 60 liters, sealed and damped. Response is much tighter than in the big 620 cabs.


I'm very surprised by the bass depth, especially that these horns are 115Hz cut-off. I suspect I'm getting good bass to 70Hz...I will check this with my test CD.

With any DIY cabinets I think measurements are in order ! Otherwise you won't know what you are actually producing with them. Nice Job !

Quote:

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Originally Posted by Altec Best

With any DIY cabinets I think measurements are in order ! Otherwise you won't know what you are actually producing with them. Nice Job !

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I concur...a mate and I will be testing on Fri. Stay tuned.

Great project, Andrew.
So good to see the experimenters diving in and creating something new. Did you model the sealed box that you wound up with, or did you just take a guess at the dims?

Quote:

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Originally Posted by Alien_Shore

Great project, Andrew.
So good to see the experimenters diving in and creating something new. Did you model the sealed box that you wound up with, or did you just take a guess at the dims?

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Thanks...I like trial and error.
I figured the back chamber would be on the large side...so I can always fill up some space inside if necessary. It's well damped also.

Here's a reading taken from 1m. Bass depth is quite good for the size of the horn...but ragged reading from 400Hz.

The 902/511 is quite flat through to 14,000