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Old 15th August 2008, 08:30 AM   #11
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I also have to say thanks David, this angle has bothered me too for a long time.
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Old 16th August 2008, 03:18 PM   #12
GM is offline GM  United States
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Default Re: From Hornsrep to the drafting table.. How do I calculate this angle?

Quote:
Originally posted by steve71
I'm building a conical midbass horn loaded with an GPA 515G driver. It's based off of Erik's work at

http://www.volvotreter.de/pics/conic...n_setup_06.jpg

FWIW, due to the driver's rising response I highly recommend you re-think your design to allow a 1:1 CR and lower Fhm same as all the Altec factory horn designs as well as optimizing for 2pi space, especially if yours has a vented DC.

BTW, are these measured specs as I notice they are somewhat different than those published?

GM
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Old 16th August 2008, 06:43 PM   #13
steve71 is offline steve71  Australia
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Default Re: Re: From Hornsrep to the drafting table.. How do I calculate this angle?

Quote:
Originally posted by GM


FWIW, due to the driver's rising response I highly recommend you re-think your design to allow a 1:1 CR and lower Fhm same as all the Altec factory horn designs as well as optimizing for 2pi space, especially if yours has a vented DC.

BTW, are these measured specs as I notice they are somewhat different than those published?

GM

Thanks for the input GM.

I'll revisit the Keele equations (St & Fhm) and play around with horns rep's 2pi simulation and post up the results by tonight.

I'm not sure if the T/S parameters you're referring to are the ones in the link I provided (in this thread), but they are for the Electrovoice driver Erik used in his simulation. Here are the relevant T/S parameters I used (see attached pic). They were all taken off Altec/GPA's published specs. For Mmd (diaphragm mass) Hornsresp calculates 39 grams. I contacted Bill at GPA and asked him to measure the diaphragm mass of the 515G driver. He came back to me with 20.6- 25.2 grams (thanks Bill!!). IIRC anything under 40grams should allow an upper cut off (-3db) of 500hz according to HR, but I think the Keele equations suggests something around 250hz. Time to check my math.

I'm certainty no expert but I though the 515G was mated to the 288 compression driver which is used down to 500hz. Or was it just the 515B that was used in that configuration.
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Old 17th August 2008, 12:25 AM   #14
GM is offline GM  United States
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You're welcome!

I was referring to the specs in your HR sim.

Hmm, so you're saying his published specs are pretty far off. Good to know.

Also, did he give you the Le = 0.58 mH spec?

FWIW, Vd/Xmax = Sd = 22.4"/0.17" = ~131.7647"^2 = ~850.09 cm^2.

Anyway, all point source horn drivers tend to have a response that looks something like this, so mating the horn to it is like matching up two drivers with an XO, i.e. the horn's HF response rolls off such that it blends with the driver's rising on axis response. Obviously, you can screw around with adding active or passive EQ to flatten its response, but acoustic solutions to acoustic problems is always best.

GM
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Old 17th August 2008, 01:05 AM   #15
steve71 is offline steve71  Australia
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Quote:
Originally posted by GM
You're welcome!

I was referring to the specs in your HR sim.

Hmm, so you're saying his published specs are pretty far off. Good to know.

Also, did he give you the Le = 0.58 mH spec?

FWIW, Vd/Xmax = Sd = 22.4"/0.17" = ~131.7647"^2 = ~850.09 cm^2.

GM
I couldn't find a published spec for Mmd from GPA or from the old Altec pdf's, but HR can calculate it from cms, sd & fs. Thielesmall.com has a value of 58.42 grams for moving mass (which includes the voice coil I guess), but nothing for Mmd.

I got the Le = 0.58mH spec from the old Altec pdf. "Voice coil inductance = 0.58mH"

That same spec sheet shows a Sd of 848.4cm^2 which is basically the same as you've calculated.

I've worked through all the Keele equations and used them as a starting point for my 2pi HR simulations. I think the FR looks usable (good even), but I'd love a second opinion from someone more knowledgeable than myself. I'll post a link to the thread when I post it up tonight.

Once again thanks for the help GM!


[edit]

I just realized that Mmd is supposed to include the voice coil weight, so I put Bill to all that trouble for nothing

At least that explains why HR calculates a value of 39grams. Oh well better to find my mistake now rather than after I've built the horn.
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Old 17th August 2008, 03:22 AM   #16
steve71 is offline steve71  Australia
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Here's the link to the new thread I've started discussing some of GM's suggestions (ie larger throat area & lower fhm).

http://www.diyaudio.com/forums/showt...93#post1587293

If you get the chance, let me know what you think GM. It seemed to me that the larger throat rolled off the FR sooner (lower fhm) as you said, but playing with a smaller throat and smaller back chamber (ignoring Keeles Vb calculation for reactance annulling) seemed to yield a wider bandwidth. However as I keep saying, I have no practical experience with all this and I could be overlooking something simple.
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Old 17th August 2008, 02:15 PM   #17
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Quote:
Originally posted by steve71

I got the Le = 0.58mH spec from the old Altec pdf. "Voice coil inductance = 0.58mH"

That same spec sheet shows a Sd of 848.4cm^2 which is basically the same as you've calculated.

I've worked through all the Keele equations........

Once again thanks for the help GM!

[edit]

I just realized that Mmd is supposed to include the voice coil weight.........
You're welcome!

Yeah, I wasn't thinking, the vast majority of 515s I've measured were 16 ohm models with 1-1.4 mH, so 0.58 mH falls right in the middle if 8 ohms.

Not sure what .pdf you're referring to for Sd, but it's wrong based on other known data, though in the scheme of things it's close enough for our purposes since you're not using actual measured specs. Not too surprising though as we've found numerous errors in Altec's published data when sharing known data on the Altec forum.

Never heard of Keele before I got on-line and since I use Prof. Leach's proven math I haven't bothered to delve into it.

Yeah, I meant to ask what exactly the 26 g was for since no 15" diaphragm/VC assembly I'm aware of weighs < 1 oz, but was apparently paying more attention to the Olympics than what I was typing.

GM
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