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Author Topic: JBL - 2way vs 3way tops  (Read 2122 times)

Bob Leonard

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Re: JBL - 2way vs 3way tops
« Reply #30 on: May 28, 2014, 04:26:26 pm »

All of these "90 degree" 15+horn front loaded boxes suffer from extreme tonal variations as you move off axis due to self-cancellation at the upper end of the 15's passband.

I call bullshit on this one.
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Jens Droessler

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Re: JBL - 2way vs 3way tops
« Reply #31 on: May 28, 2014, 04:42:56 pm »


I call bullshit on this one.
Of course not all of them will suffer from those problems. A good engineered one will have the crossover point between 15" and HF horn right where the 15" is narrowed down to the horizontal directivity of the horn. But for 90 degrees this will usually mean big HF horn with big HF driver, so a low crossover point can be done.
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Chuck Simon

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Re: JBL - 2way vs 3way tops
« Reply #32 on: May 28, 2014, 05:05:24 pm »

JBL uses the same crossover point for the 725(90 degree) as it does for the 722(75 degree).
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Tim Weaver

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Re: JBL - 2way vs 3way tops
« Reply #33 on: May 28, 2014, 05:06:40 pm »

Of course not all of them will suffer from those problems. A good engineered one will have the crossover point between 15" and HF horn right where the 15" is narrowed down to the horizontal directivity of the horn. But for 90 degrees this will usually mean big HF horn with big HF driver, so a low crossover point can be done.

It takes big horns and low XO points to make it work. Like 500hz.

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Jens Droessler

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Re: JBL - 2way vs 3way tops
« Reply #34 on: May 28, 2014, 05:55:12 pm »

Oh, and also, not all 15" speakers are alike when it comes to beaming. Yes, beaming occures because of the diameter of the speaker vs. wavelength, so in theory all 15" speakers would start beaming at the same frequency. But in reality there are quite some differences. Mostly because speakers won't behave like a piston anymore on higher frequencies. So if not all of the cone is moving, the diameter argument isn't fully correct. Above 2kHz it's basically only the dustcap radiating any useful sound. So for those frequencies you could use the diameter of the dustcap for your rule of thumb calculations.
This issue is of course still present in multiple loudspeaker per passband arrangements, but won't be the only factor determining the directivity of that passband.
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Tim Weaver

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Re: JBL - 2way vs 3way tops
« Reply #35 on: May 28, 2014, 06:26:14 pm »

Oh, and also, not all 15" speakers are alike when it comes to beaming. Yes, beaming occures because of the diameter of the speaker vs. wavelength, so in theory all 15" speakers would start beaming at the same frequency. But in reality there are quite some differences. Mostly because speakers won't behave like a piston anymore on higher frequencies. So if not all of the cone is moving, the diameter argument isn't fully correct. Above 2kHz it's basically only the dustcap radiating any useful sound. So for those frequencies you could use the diameter of the dustcap for your rule of thumb calculations.
This issue is of course still present in multiple loudspeaker per passband arrangements, but won't be the only factor determining the directivity of that passband.



Directionality is not only determined by the diameter of the speaker, but by the width and height of the cabinet it is in too. If you put a 10" in a 24" wide cabinet it will behave pretty much just like a 15" in a 24" wide cabinet.

A little light Baffle Step Diffraction reading for ya....
http://www.salksound.com/wp/?p=42
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Ivan Beaver

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Re: JBL - 2way vs 3way tops
« Reply #36 on: May 28, 2014, 08:01:55 pm »


Any driver can beam if not properly processed, that's a given.
I am curious what processing can be done to change the "beamwidth" of a driver.

I am not aware of any-but there may be some-I would be curious as to how it would work.
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Jens Droessler

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Re: JBL - 2way vs 3way tops
« Reply #37 on: May 28, 2014, 09:10:56 pm »



Directionality is not only determined by the diameter of the speaker, but by the width and height of the cabinet it is in too. If you put a 10" in a 24" wide cabinet it will behave pretty much just like a 15" in a 24" wide cabinet.

A little light Baffle Step Diffraction reading for ya....
http://www.salksound.com/wp/?p=42
Baffle step has an influence on directivity as well, yes. But it will have a constant effect above the baffle step frequency. So a 10" speaker in a 24" wide baffle would behave identical to a 15" speaker in a 24" wide baffle in the horizontal plane... until the 15" will start beaming "on it's own", which will be earlier than the 10". It is still a difference between sound blocked and refelcted in its way by the baffle and sound being radiated by a source.
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Nils Erickson

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Re: JBL - 2way vs 3way tops
« Reply #38 on: May 31, 2014, 01:29:56 am »

I believe that both the 722 and 725 are 75x50 degree boxes... just in the "for what it's worth" dept.

Blah blah blah,
Nils
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Bob Leonard

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Re: JBL - 2way vs 3way tops
« Reply #39 on: May 31, 2014, 07:26:16 am »

I am curious what processing can be done to change the "beamwidth" of a driver.

I am not aware of any-but there may be some-I would be curious as to how it would work.

Ivan,
Beaming is the resulting off axis (side) loss of frequency, or on axis (front) enhancement of frequency  most noticeable in larger diameter higher powered speakers. Or so I was taught about 100 years ago. Beaming can and will be enhanced in a poorly designed cabinet. No cabinet can make up for lost detail, or lost output when listened to off axis (side). However, a driver that appears to be beaming can be tamed through the judicious use of a good EQ.
 
As the wave length becomes longer beaming at higher frequencies becomes more noticeable hence the reason many people feel a 12" driver has a lesser beaming effect than a 15" driver, and they are correct. Two examples of mechanical devices used to lessen the effect would be a "Beam blocker" sometimes used on guitar cabinets but applicable to live sound. And the circle of foam used by EV on the sx500+ positioned directly in front of the 15" driver. I bow to your expertise in these matters Ivan, however, in practice these methods can and will reduce or eliminate the beaming effect of larger diameter drivers. Of course nothing is a substitute for a properly designed driver in a properly designed cabinet. IMO.
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