Explanation for horizontal beamwidth of JBL 3252(N) ?

Hello,

According to JBL, the 3252(N) uses dual 15-inchers crossing over to a 1-inch horn at 2kHz. However, it is also shown to have a horizontal beamwidth of not less than 90* across the 1kHz-10kHz range. What could be a mechanism that enables this kind of beamwidth, considering the fact that a 15-incher at 2kHz is maybe 60* (-6dB) at best ?

Thanks in advance.

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DualTriode, looking at it I'd have the concern that maybe the horn vertical widens down near the cross because it is short vertically. This would not make for constant directivity. Compare it to the height of the woofer part.

newvirus2008, in some cases where the two are further apart at the cross than they ought to be, the overlap can blend their directivities to create something useful.

There's no data here to say quite how this was achieved.. there may be lobing involved.
 
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DualTriode, looking at it I'd have the concern that maybe the horn vertical widens down near the cross because it is short vertically. This would not make for constant directivity. Compare it to the height of the woofer part.

newvirus2008, in some cases where the two are further apart at the cross than they ought to be, the overlap can blend their directivities to create something useful.

There's no data here to say quite how this was achieved.. there may be lobing involved.
This is likely a 2.5 way speaker, only one woofer will be playing at or near the crossover frequency.

In terms of lobing the distance between the Centers of the active woofer at the crossover frequency and the horn is the key to lobing.

There will always be lobing. *

Depending on the slopes of the crossover, yes there will always be blending to a greater or lesser degree.

The plots in post #1 are consistent with my comments here and in post #5.

Thanks DT

* the theory of lobing assumes that the drivers, Woofer and Tweeter are point sources, they are not. The "null(s)" shown on the lobe simulations show as sharp and deep, they are not.
 
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I offered lobing to explain that DI can be smooth in a simple beamwidth plot and yet inconsistent in reality. There wouldn't be much point sourcing going on here. I'd like to see separate DI plots for the devices before making any claims. 2kHz is kind of high for a 15'.
 
.., in some cases where the two are further apart at the cross than they ought to be, the overlap can blend their directivities to create something useful.
There's no data here to say quite how this was achieved..
So, if it's a brilliant move using carefully chosen / crafted characteristics, are there any potential disadvantages to such a design ?
....there may be lobing involved.
Could it be the phase shift between the LF/HF (inherent / network induced) causing their vestigial lobes to combine and become useful once again?

This is likely a 2.5 way speaker, only one woofer will be playing at or near the crossover frequency.
Since the datasheet mentions the option of bi-amping (suffix N=network), I wouldn't think that'd be true.
 
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if it's a brilliant move using carefully chosen / crafted characteristics, are there any potential disadvantages to such a design ?
Even when the devices are carefully designed to match, they're never perfect and it's worth shuffling these compromises to get the best out of a system.

Sometimes it gets pushed to the limit. In this case the beamwidth plot looks promising, but I'd still like to see more data.

Could it be the phase shift between the LF/HF (inherent / network induced) causing their vestigial lobes to combine and become useful once again?
Yes, phase does play a part in crossing this way.
 
Hello,



There is nothing magic going on here.

This type of design with two woofers and a Constant directivity horn has been going on for decades, circa ~1980’s

The JBL 4435 with two woofers is definitely a 2.5 way design, one woofer only operates below 100hZ . Bi-amp would not make no difference, one amp for woofers and the other for tweeters. All the separation of woofers and tweeter equalization will still be accomplished with passive parts.



Thanks DT

4430 and 4435 Information

https://audio-database.com/JBL/speaker/4435-e.html
 
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I'd like to see separate DI plots for the devices ... the beamwidth plot looks promising, but I'd still like to see more data.
Unfortunately, the plots in #1 are all what's in the datasheet. 'Technical Manuals' with part nos. exploded drawings, network details etc. have also been stopped by the manufacturer.

I'm also not in possession of the above-mentioned loudspeaker. I happened to come across this one as I was looking for a means to cross a 12-incher at 2kHz. Thank you very much for your replies.

Even Turbowatch (post #2) may have had a valid point regarding the possibility of cone flexing.

There is nothing magic going on here. This .... has been going on for decades ...

Please note that the 4430/5 cross over at 1kHz which is a reasonable for a 15-incher (12 to 13" piston). However, that is not the case at 2kHz (see attachment below). Hence the question.
 

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Hello,

Interesting, I use that graphic tool by Clifford Hendrickson of Altec frequently.

That graphic tool is based on piston drivers. 100 degree H by 50 degree V Controlled Directivity horns are different.

My acoustics text shows the directivity of a 100 X 50 CD horn as the average of 100 degrees and 50 degrees. The effective directivity index of the horn is the directivity of a piston with 75 degrees coverage.

The frequency that a 15 inch driver has 75 degrees of coverage is right at 2000Hz, the same as the crossover frequency of this JBL 3252(N) speaker.

Thanks DT
 
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Then why does it show 90* at 2000Hz ?
Hello,
The diameter of the piston in a 15inch driver is close to 13.25 inches.

Print out the Altec graphic tool that you posted.

With a straight edge draw a new line between 12" and 14" closer to 14 than 12 (at 13.2").

Draw a line on the graphic tool at 75 degrees.

Circle the point where the two lines cross, the frequency is about 1700Hz, close to the nominal crossover frequency of 2kHz.

This speaker like all the other 2-way speakers is a compromise.

Even when the devices are carefully designed to match, they're never perfect and it's worth shuffling these compromises to get the best out of a system.
 
This speaker like all the other 2-way speakers is a compromise.
Ok, so a little more is squeezed out of the woofers, but just enough to approximately match the horn's beamwidth, resulting in a low-cost budget speaker. However, (like AllenB said) the given overall beamwidth, still 'looks' very promising.
Having only seen the pictures, the woofers appear to be bi-flex in nature, so could explain a lot.
And that 'little more' is possibly due to a curvilinear cone that flexes for the higher frequencies, also mentioned by Turbo in the beginning. I have heard of it before but hadn't considered it, mainly due to the 'ribs' seen on the cones, that are meant to improve rigidity and thereby a achieve 'more pistonic' motion (attachment from JBL Tech Note Vol1, No. 3A).
A typical 15" has breakup modes below 2kHz. This wouldn't only create some degree of inconsistency, but less ideal conformance with that chart. Lobing from the single driver could typically be a part of this.

And, there's also the possibility that the chart itself may not be a very honest one, as there appears to be little corroborative information to substantiate it. Besides, like DualTriode said, there should be no 'magic' at play, as this is a low-cost budget loudspeaker and not any flagship product of JBL.

Nevertheless, it would be nice to know if it involves a simple (but elegant) lobing trick. I've been looking around for other models with similarly high crossing (and promising beamwidth) as this one, but haven't been able to find one yet.

If nothing comes up, I'd settle for the cone flexing theory. (y)
 

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(like AllenB said) the given overall beamwidth, still 'looks' very promising.
I have been trying to give the impression that if you're doing it yourself, you could do better.. since you're looking to use this as an example for yourself.

Of course with good placement and some judicious EQ I'm sure most people would be fine with it. Throw in a ceiling absorber and you'd get a few more on board.
 
Hi forum members,
i think the dictate for the design here is budget and we have a special listening situation as of the hall of a cinema theatre with very long listening distances
for a correct match of the beaming behaviour between a 15 inch woofer and the horn like for instance a midfield monitor the horn must be much bigger and the cutoff between woofer and horn less than 700 - 800 Hz. A real pro midfield monitor with a 15 inch woofer like the SE-MF-2.1 model here: https://www.strauss-elektroakustik.ch/mastering-studio-monitors goes down with the cutoff to 400 Hz.
 
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