Polars Plots for Altec Multicells?

steve71

Member
2007-11-12 6:01 pm
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.
 

steve71

Member
2007-11-12 6:01 pm
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.

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.


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.

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
 
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 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
 

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steve71

Member
2007-11-12 6:01 pm
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

Member
2007-11-12 6:01 pm
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?
 
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?
Bass horn beam width at 300 Hz will be largely dependent on the horn throat angle, not the mouth angle.
My PA uses optional waveguides which extend the 26.5 wide 90 degree horn to 53 inches wide.
The dispersion remains a uniform 90 degree pattern, the waveguides extend pattern control about an octave lower.
 
Cal,
Sorry if I made your ears burn. I thought that you were the man with multi-cells! Have you done a sweep across the cells or just an individual cell? I though that they use to have this information in my old AES Journal papers on loudspeakers but my old copy is somewhere in storage. If I remember correctly they are not nearly as smooth as some seem to think, on average they may be good at dispersion but on a smaller scale I want to remember that they are rather rough in a polar response sense.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
On axis it sound good playing down to 300hz so it would be nice to cross that low.
Having "Been there, Done that" you'd have to define "nice".
To sum up a few years of work and pulling my hair out:
  • Q: Why would you want to cross that low?
  • A: Because I want to cover as much as I can with the horn.
  • Q: Why with the horn?
  • A: Because it sounds better than the woofer.
  • Q: Does it really? Or do you just think it does?

I found that it does not. That is probably not entirely the fault of the horn, the compression driver is important, too. Even with the robust Altec phenolic drivers like the 291 "Giant Voice" running that low does not sound great. Remember that at some point output will drop and distortion will skyrocket. You want to stay far away from that point, unless you are using very steep crossover slopes, which have their own problems.

After a lot of experiment, measurement and listening, I've found that 1.4" drivers are just better sounding crossed at ~600Hz or higher. Although they will play lower on the right horn, you do need the extra octave (or more) below the crossover point to stay out of high distortion. Even if they will play down that low, you probably don't want to use it for high fidelity.

Honestly, most good 15" woofers sound better in that range than the compression drivers. Placing the crossover point where it actually sounds best, not where we hope it will sound best, is best practice.
It took me years to learn that. :D
 
Pano,
I second that opinion on the crossover point of the 1.4" compression drivers and even more for a 2" for other reasons. I didn't want to get into a verbal joust so I kept my mouth shut. In pro-audio PA applications this is why I originally developed cone driver mid-range horns to cover this region. The sound of a compression driver that low at any kind of level is just not a nice sound. Besides the fact that you could easily overload the compression driver and watch a whole PA worth of diaphragms go up in smoke back in the day with passive crossovers. From what I remember the aluminum diaphragms did sound a bit better than the phenolic ones but would break easier in use than the phenolic. We developed 10" cone driven mids just to take this out of the equation and to improve the mids so much over just 15" horn bass bins and a horn top.