JBL Tweeter "Fins"

[jacksonbart]

I have seen pictures of older JBL speakers (only seen it w/ JBLs perhaps there are others who do/did it) that appear to have "fins" I assume over the tweeters. Can some please explain the idea behind it. They typically are bent downwords. Just trying to learn what they do. Thanks

[WMcD]

Those were to aid in dispursion by means of time delay.

You'll note that there was sort of a cut-out in the fins at the center of the mouth and the fin section there was shorter.

You have to imagine a plane just in front of the fins.

At the center of the mouth, the sound pressure emerges from the mouth and into the path created by the downward slope of the fins. It is a relatively short path and any individual paths do not extend down too much. So there is not too much delay.

At the left and right edges of the mouth, the sound pressure emerges from the mouth and ito the path created by the downward slope of the fins. It is a relatively long path and the individual paths extend farther down. So there is more of a delay.

As a result, the sound from the center emerges with a short delay, and the sound from the edges emerge with a long delay. Overall this makes the wave front more circular.

At least that is what I read someplace long ago.

Gil

[D-MAN]

Hope this works... this is from http://www.lenardaudio.com/education/07_horns.html



The Lens. For circular and near circular horns, a lens was the most effective means to increase horizontal dispersion, without introducing lobe distortion, and minimal loss of efficiency. An acoustical lens is the equivalent of the optical lens.

A labyrinth of concave plates is put in front of the horn. Sound from the centre of the horn passes through unhindered. Sound to the sides of centre in the horn, pass through the lens labyrinth, increasing the distance traveled, and delayed in time. The sound waves are bent forming a wide horizontal dispersion. The lens improves horizontal dispersion as the frequency increases.

The lens is no longer used. It is large, fragile and expensive. Economic rationalism, not technical performance, was and still is its downfall. Also the lens function was not well understood. Its physical appearance does not give an intuitive understanding of its function. Many old 60s - 70s roadie sound engineers believed the lens directed sound downward to the audience, sitting in the front row. The lens was commonly referred to as a waterfall effect speaker. Ignorance and miss-understanding was and still is a problem throughout the professional audio industry.

[CaptnBob]

Yeah, what he said.

I vaguely recall the lenses tended to reduce the "throw" of a horn and make it less "piercing." They were rather fragile, too.

[jacksonbart]

Thanks for the replies and the sweet link.

[Mighty Favog]

Yup, the ones on my L300's have a small chip or two.

[jdm56]

Well, gee thanks guys. I've always wondered about those too. Glad you asked, JB! I remember seeing them on some JBL's when I wuz a pup, and thinking that they looked cool, but I never had a clue what they were for.

[sunnysal]

I still wonder why these went out of favor....they did a great job of minimizing high frequency beaming in the vertical direction, sounds good to me....anyone know? I'll take a guess, since compression drivers went out of favor and dome tweeters had less beaming problems at high frequencies this was left on the wayside. tony

[boom3]

I still wonder why these went out of favor....they did a great job of minimizing high frequency beaming in the vertical direction, sounds good to me....anyone know? I'll take a guess, since compression drivers went out of favor and dome tweeters had less beaming problems at high frequencies this was left on the wayside. tony

Not really...compression drivers are still used in pro apps, not to mention Klipsch and other lines. Horns are better understood now, thanks to work by people like Keele and Edgar, and others whose names escape me now. The curent crop of good horns controls directivity just fine without acoustic lenses. The JBL horns that used acoustic lenses were oval in cross section and would have done a poor job of controlling directivity by themselves. Dome drivers for direct-radiator systems are cheaper than decent tweeter and midrange horns and as with any direct radiator driver, compromises are less noticable than with horns someone is trying to make on the cheap.

In sum...technology has moved on...leaving acoustic lenses a curiosity of yesteryear.