Polars Plots for Altec Multicells?
 

I'm using hornsrep to model a front loaded bass horn to cover 60-300/500hz.

I want the horizontal dispersion pattern to match the 1003 multi-cell at the crossover at around 400hz.

The 1003 is a 90x40 horn, but at what point does the horn loose pattern control?

There are three options.

(1) Find some published measurements - I'm sure I've seen them somewhere, but can't locate them now...
(2) Built some sort of turntable and do my own off axis measurements out doors
(3) Model the 1003 in hornsrep. Not sure if this is possible and I can't find thiele and small data on the 288 compression driver. Is there a speaker testing tool that I can buy that will measure the t/s parameters of a compression driver?

I've noticed, playing around with hornsrep, that it is mouth size that determines the point at which the horn looses pattern control. So can I just measure the size of the 1003 horn mouth and model a single exponential horn of that size to determine the horizontal beam width at the crossover point.

I know that the 1003 is billed as a 90 deg dispersion horn and a conical bass horn around 150cm length with a mouth area ~ 4,000cm^2 will give me a 90 deg horizontal beam at around 400hz and play down to 60hz.

Help! :D

I have seen polars of the 1003 before, and also could not find them.
As I recall, they are diffracting outward more than 90 degrees at 400 Hz.
Your own off axis measurements out doors would be best.
Your bass horn will likely be quite a bit deeper than the 1003, if you built 90 degree waveguides for the sides of the 1003 matching the LF horn you could control the dispersion and reduce diffraction.

What's wrong with #2? You don't need a turntable, just a dot, a protractor, and some string. You can probably get workable measurements indoors too, if you've got a room with a high ceiling.

From playing around with hornsrep the bass horn needs to be about 150cm long, not including the driver chamber. I like the idea of extending the 1003 - I just need some way to make it look acceptable. On axis it sound good playing down to 300hz so it would be nice to cross that low. That would also mean I could make the bass horn mouth a little larger, smoothing out the response a bit.


I've never taken these type's of measurements before, so it will be a good learning process, since I need to do measurement of the final speaker anyway.

The biggest problem I have is that the 1003's that I have are tar filled 150lb monsters. They're in the house at the moment and I need to relocate all my gear to the outbuilding (I'm turning it into a home theater) which is separated from the house by about 150 yards. And that 150 yards is buried in snow right now.

It would save me a ton of trouble if I could just model it in hornsrep or go off Altec's measurements. Beats waiting for the snow to melt. :D

I envision something like the sketch below, whether it looks acceptable would be a subjective opinion. The wings would definitely be a big help if you want to cross as low as 300 Hz.
Personally, I find looking at horns and raw speakers makes me constantly think of what could have been done better, so I have always have covered them with speaker cloth since 1981 :D.

Art

That looks good, I like that concept. But if I were to make the mouth of the bass horn that large, the beam width would be less than 90 deg at 300hz - or am I missing something?

steve71,
I think that Cal Weldon has the information that you are looking for. I see him on many of the multicell threads and I think he is a real Altec lover. Personally I can't handle the 1.4" drivers at that low a cutoff frequency but everyone has their own preferences. Talk to Cal and see what he says. Search for his name, he is on all the time.

Thanks, I will send Cal a PM if he doesn't chime in here. What horn were you using down to 300hz and what didn't you like about it? What crossover slope were you using? I'm using an active crossover with 24db/octave roll off, that might be the difference?

I haven't seen polar plots, but Altec did publish directivity plots for all the multis. I swear I had a copy but can't find it now. Should be online somewhere.

I could do it, but it would be a week or two before I'd get a chance.

Around 180 Hz.

GM