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View Full Version : 2370, 2380 Bad? Why? Help me understand.



BeDome
01-01-2014, 02:13 AM
Can someone explain this to me.

I see a lot of discourse toward these plastic horns. I am wondering why.

Honestly. What is wrong with this design? They have great dispersion characteristics for their size, short throw - ideal for small rooms, absolutely NO audible resonance within their intended passbands, very similar from one to the next AND JBL made them for us.

Is it just because they are of the "newer variety" (and are not like the lenses or something) that I see so much bad press or am I missing something?

Are baby cheeks so much better (polar response, wider dispersion, tell me) or is it more than that. Why does it seem that these easy to work with flat front Bi-Radials get put down so often?

more10
01-01-2014, 06:52 AM
I use the 2370 now and I am quite happy with them.

I believe the diffraction slot is problematic.

Theoretically the diffraction slot will result in different wave propagation centers vertically and horizontally. Horzontal center is the slot, the vertical center is further back. This means that the sound source location is diffuse. They are not as bad as the smith horn in this regard.

I believe we are more sensitive to horizontal wave propagation than vertical propagation, so the small difference in focus might not be important at all.

However diffraction slots are considered evil. Buttcheeks also have diffraction slots.

Mårten

badman
01-02-2014, 10:39 AM
The diffraction slots create a secondary source along their edge- which radiates not only into the horn's pattern, but also back down the throat. There are accordingly stored energy and HF reflections in the throat- Geddes calls these "HOMs", and uses a foam filter to reduce them, along with a horn with reduced diffraction (softer transitions via the oblate spheroidal profile). A sanding of the hard diffraction edge would allow for a tradeoff between HF dispersion consistency and a reduction in these artifacts. One can also use a foam plug- aquarium 30-40ppi filter foam is about right, so long as they don't fill the whole horn (patented by Geddes, but a small throat plug is not covered due to prior art).

The 2370 and 2380 ALSO have a harsh lip at the baffle termination, moreso in the vertical than horizontal. This again acts as a secondary source with similar issues to the first. This can be helped by properly terminating them- an open-cell foam extension to the profile, proud of the mounting plane helps this quite a lot. Layered felt can also be used.

http://www.enjoythemusic.com/diy/0412/foaming_at_the_mouth.htm

fpitas
01-02-2014, 12:07 PM
"Bad" is relative. They're compact and flat-front, with a wide horizontal dispersion. Great for PA use in clubs, etc.

frank23
01-04-2014, 02:24 PM
They beam very much at high frequencies, and they have an elevation in their response that is not easy to compensate for. The 70 and 80 are both also not full biradial like the 60 and 2344. The 70 and 80 beam vertically, as I said, quite a lot at high frequencies.

BeDome
01-14-2014, 08:24 PM
I remember reading Geddes' paper many years ago, but you made me go find it and re-read it.

I get it, somewhat, now. But, still, how much of the measured reflection mentioned is audible at low volumes?

Thanks for your responses. You gave me much more to delve into.

Should I assume that it was to foreshorten the horns in physical size that JBL made the decision to use the constricting diffraction slot design?

Can I also assume, since the "bad" effects of slot loading a horn throat are frequency related, that some of the old "central compression chamber leading to a small slot, then out through an expanding throat" W bins used for bass were less harmed by this shortcut?

BeDome
01-14-2014, 08:29 PM
They beam very much at high frequencies, and they have an elevation in their response that is not easy to compensate for. The 70 and 80 are both also not full biradial like the 60 and 2344. The 70 and 80 beam vertically, as I said, quite a lot at high frequencies.

Please define "beam very much at high frequencies."

Mine are cut off at around 7KHz, then deferring to slots. I honestly hear a decent spread of all freqs when using pink noise around the room.

Are you saying that I am missing something?

As far as beaming vertically, since they are used in a living room with a vertical angle of sweet spot of about ten or less degrees, I doubt that beaming plays as much of a game as aiming, right?

more10
01-14-2014, 09:31 PM
The diffraction slots are used for better horizontal dispersion. You should have good audibility from all places where you can see the slot. The vertical dispersion is is much lesser than the horizontal because there is no horizontal diffraction slot.

There are horns that sound better. Tractrix and LeCleach horns should sound better since they have no diffraction slots.

ivica
01-15-2014, 12:49 AM
There are horns that sound better. Tractrix and LeCleach horns should sound better since they have no diffraction slots.

....But Directivity would increase with the frequency, so optimum listening place would become narrow.

regards
ivica

more10
01-15-2014, 08:11 AM
I agree with Ivica on this.