EON615 JBL woofer directivity control waveguide

[astrawso]

The JBL EON615 from their pro line uses a very interesting "waveguide" over the woofer, presumably to handle the on-axis beaming at higher frequencies for such a large driver. Directivity charts aren't included, but they list the coverage at 110 degrees. Looking at directivity charts for other 2-way 15" pro sound applications on the JBL site, there's the predictable narrowing before the woofer hands over to the hf driver.

Anyone have experience with this unit or something similar? Would it result in resonances that are unacceptable in a hi-fi situation but are fine for pro-audio and inaudible at those higher spl levels?

 

[TimVG]

It was used as far back as the 60s in certain "BBC" designs.

https://www.grahamaudio.co.uk/technology/slotted-baffles/

 

[astrawso]

It was used as far back as the 60s in certain "BBC" designs.

https://www.grahamaudio.co.uk/technology/slotted-baffles/

So is there some audible reason why these wouldn't be used anymore? Other than the obvious aesthetic reason and the move to narrow towers for a large majority of hifi.

 

[Juhazi]

Obviously no resonances in the passband of the woofer, xo is most likely LR4 around 1000Hz

Damn, xo is at 1,8kHz like QMuse noticed. I looked at the same datasheet with blurred eyes! But let's hope that the good people at JBL/Harman have checked distortion and CSD graphs! Speaker is for PA which means high spl!

Karlson slot was/is for similar purpose - to smoother directivity.

 

[QMuse]

Obviously no resonances in the passband of the woofer, xo is most likely LR4 around 1000Hz

1.8kHz.

 

[Kal Rubinson]

It was used as far back as the 60s in certain "BBC" designs.
https://www.grahamaudio.co.uk/technology/slotted-baffles/

I remember building these plates for the B139s in my TL speakers.

 

[Juhazi]

Sorry to say, but Kal's plate won't do anything benecicial to directivity! Diameter horizontally is same as the woofer, and vertically two openings have same total height as the woofer. Actually it only makes on-axis and vertical directivity even more ragged above 1kHz!
What was xo, somewhere around 3kHz, shallow slopes?

 

[QMuse]

Sorry to say, but Kal's plate won't do anything benecicial to directivity! Diameter horizontayll is same as the woofer, and vertically two openings have same total height as the woofer. Actually it only makes on-axis and vertical directivity even more ragged above 1kHz!

You're probably right, but it does look intriguing and a little on the artistic side, like all early-days attempts.

 

[HammerSandwich]

@Charles Sprinkle might have some insight here.

 

[Kal Rubinson]

Sorry to say, but Kal's plate won't do anything benecicial to directivity! Diameter horizontally is same as the woofer, and vertically two openings have same total height as the woofer. Actually it only makes on-axis and vertical directivity even more ragged above 1kHz!
What was xo, somewhere around 3kHz, shallow slopes?

The intended purpose was different. The plate was supposed to compensate for the tendency of the central part of the race-track diaphragm to be out-of-phase with the ends in the 500-1kHz range and, yes, the crossover was in the range of 3kHz in 2-way boxes like the one in the picture,

In my 4-way application, the crossover was 24dB/octave @ 200-400Hz and I was trying to improve the smoothness in the octave above. Didn't seem to make any difference but it's one of many experiments.

EDIT: Of course, if I had been concerned about dispersion from that driver, I could have mounted it rotated 90deg. At that time, we (or I) was not sensitive to the issue as we (and I) are now.

 

[thewas]

Here is a current implementation of such a BBC classic https://www.grahamaudio.co.uk/products/ls55/

 

[peanuts]

a 15" woofer will start to beam above 8-900hz regardless of what you put in front of it.

 

[QMuse]

a 15" woofer will start to beam above 8-900hz regardless of what you put in front of it.

Sure, but a 15" woofer is usually attenuated by XO for at least 80-90dB at those frequencies.

 

[thewas]

a 15" woofer will start to beam above 8-900hz regardless of what you put in front of it.

By placing such a slot through diffraction you "reshape" its radiation pattern making it wider, it's not really different to optics.

 

[astrawso]

a 15" woofer will start to beam above 8-900hz regardless of what you put in front of it.

Sure the driver will beam, but that's due to the wide cross section of the driver surface area at those frequencies. Playing around in Hornresp, it seems that a panel in front of the driver that's narrower than the driver causes the wavefront to diffract around the smaller opening, creating a smaller apparent driver with higher spl. There seems to be a limit though when a too small area deforms the wave.

 

[mixsit]

Wow. I noticed this from JBL but never gave it a thought.
We have a similar mod here in the guitar amp/speaker world -where the extremely narrow dispersion in the upper frequencies from the speakers used is a known PITA and the bane of consistent tone spread from the stage.
A fellow named Jay Mitchell came up with a great solution, subsequently called The Mitchell Donut.
In this case a foam sheet is used to attenuate highs coming off the cone everywhere but the center.
Destructive high interference from across most of the cone is attenuated, and the driver now acts like a small source up top'.
It works great :>)

https://www.google.com/imgres?imgur...hUKEwj58_HQhLDoAhVtATQIHXWABOwQMygAegUIARDgAQ
http://blog.thegearpage.net/?page_id=443

 

[AnalogSteph]

His results would seem to suggest that some experimentation with the size and shape of the center hole may still be warranted; perfectly round anything tends to exacerbate resonances, and I would not be surprised if the remaining irregularities in the 3-4 kHz region had something to do with that. It also isn't necessarily a given that a flat inner surface is optimum. Both just make for ease of manufacture. I would want to try various cone shapes, bowing either inward or outward, with foam applied to the surface.

The shape of JBL's baffle looks like one of these Fourier transform nearfield - farfield transformation thingies.

You may have seen another form of acoustic modifier on some speakers of the late '70s in particular: an "acoustic lens" in front of the tweeter. Panasonic/Technics were particularly into this technique, to the point where e.g. the SC-PM01 mini stereo from the 2000s still uses one. This allows broadening the dispersion of relatively large tweeters, a cone tweeter in this case, though the usual slanted plate type only works in one direction AFAICT (which however may be adequate in practice, as we typically want wider horizontal dispersion than vertical).

 

[astrawso]

Wow. I noticed this from JBL but never gave it a thought.
We have a similar mod here in the guitar amp/speaker world -where the extremely narrow dispersion in the upper frequencies from the speakers used is a known PITA and the bane of consistent tone spread from the stage.
A fellow named Jay Mitchell came up with a great solution, subsequently called The Mitchell Donut.
In this case a foam sheet is used to attenuate highs coming off the cone everywhere but the center.
Destructive high interference from across most of the cone is attenuated, and the driver now acts like a small source up top'.
It works great :>)

https://www.google.com/imgres?imgurl=http://www.stratopastor.org.uk/strato/amps/prii/speaker/foamdonut/P5110819.JPG&imgrefurl=http://www.stratopastor.org.uk/strato/amps/prii/speaker/foamdonut/foamdonut.html&tbnid=0EMow9ju4MrBpM&vet=12ahUKEwj58_HQhLDoAhVtATQIHXWABOwQMygAegUIARDgAQ..i&docid=wQL07I4H1VpvsM&w=1600&h=1200&q=jay mitchell donuts&ved=2ahUKEwj58_HQhLDoAhVtATQIHXWABOwQMygAegUIARDgAQ
http://blog.thegearpage.net/?page_id=443

Foam! What a great idea. I wonder what the measured differences in a solid aperture vs an aperiodic aperture would be. I know that geddes uses a particular foam to alleviate some of the high end harshness of pro compression drivers. I also guess that you could use multiple layers of foam with a different absorption property to vary the apparent aperture

Looks like RAAL uses foam on their 140-15D ribbon tweeter-mid, presumably to attenuate the vertical beaming typical of longer ribbons.

 

[astrawso]

His results would seem to suggest that some experimentation with the size and shape of the center hole may still be warranted; perfectly round anything tends to exacerbate resonances, and I would not be surprised if the remaining irregularities in the 3-4 kHz region had something to do with that. It also isn't necessarily a given that a flat inner surface is optimum. Both just make for ease of manufacture. I would want to try various cone shapes, bowing either inward or outward, with foam applied to the surface.

The shape of JBL's baffle looks like one of these Fourier transform nearfield - farfield transformation thingies.

You may have seen another form of acoustic modifier on some speakers of the late '70s in particular: an "acoustic lens" in front of the tweeter. Panasonic/Technics were particularly into this technique, to the point where e.g. the SC-PM01 mini stereo from the 2000s still uses one. This allows broadening the dispersion of relatively large tweeters, a cone tweeter in this case, though the usual slanted plate type only works in one direction AFAICT (which however may be adequate in practice, as we typically want wider horizontal dispersion than vertical).

Interestingly, playing around with these filters in Hornresp shows a slight broadening in directivity (at 3000hz) and also moves the acoustic center down towards the woofer. Could this also help to deal with challenging driver center to center combinations? Maybe at expense of phase relative to the woofer.

 

[tuga]

Here is a current implementation of such a BBC classic https://www.grahamaudio.co.uk/products/ls55/

View attachment 55088

These are from the BBC LS5/5 white paper (attached).