DIY 2380A - Help needed

[GCT]

Hi

Just bought some 2445jpl's to go with my 2241 subs. I would like to build some 2380A flares from thin DM and fix to a solid wooden throat. Could anyone help with dimensions or even plans? I could approximate the flare curves from the PDFs given by JBL but to get the throat right looks like the difficult bit. If anyone has the 2380A flares and would like to help out with a few measurements/photos, that would be great.

Any help at all would be very much appreciated.

Cheers
GCT

[pos]

Sorry I cannot help with the plan.

Is it for home use?
The 2380 is not the best horn for home use, with its rapidly narrowing vertical directivity.

If you have the tools and knowledge to make your own horn, why not try to copy a better one, like this one (close to a TH4001)

[maxwedge]

Is it for home use?
The 2380 is not the best horn for home use, with its rapidly narrowing vertical directivity.

I don't see any problem, just keep them at ear level, aka point them at your head!
I use 2385's and they sound great.

I have 2380's too and photos but good luck copying the CD horn profile with out templates ect.

[GCT]

Thanks for the replies.

No, it's for PA use. I know they can cross at 500Hz but will probably want to go higher to extract more power. Hopefully they will compete with 1200w of bass per side, but also have the option to cross lower for a better sound at lower volume.

Anyone know what the slot size is and how we get from the 2" circle to the slot in the throat? What relationship has this slot size on the cutoff frequency of the horn?

Cheers
GCT

[GCT]

So what horn would you suggest as a model? I saw the 2380A horns that went with the 2445 in a PA stack - original JBL design so I thought. What are the advantages or choosing another horn type?

Cheers

GCT

[jf65]

One of the best is the Iwata horn, fully explained here: http://www.asrr.org/biblioteca/Revue...ATA/IWATA.html for the vertical dimensions and how to make it. In french but with many photos and drawings.

here are the horizontal dimensions http://horn.hebfree.org/atel/atel.html

I made a pair , you don't need many expensive tools and the result is guaranteed.

[pos]

What directivity pattern and frequency range are you looking for?

[GCT]

Don't really know. It's for PA so 90 40 is what I thought standard. I will use an electronic crossover and want maximum power (on occasions) so depending on the application ( and how it sounds) I want to be flexible on crossover frequency. I will use the horns with various configurations of speakers. Lowest is 500Hz for the 2445 but I haven't heard it yet to really know whether that's worth considering; arriving this week!

So that's my idea. Open to suggestions.

Cheers
GCT

[pos]

If you do not seek for a particual direcitivty pattern, then you only have to make sure the directivity of the horn and the midbass driver are the same at the crossover point. So it depends on the midbass driver dimension and maximum crossover frequency.
Another question you have to ask yourself is how high the horn will have to go.
The 2380 has a diffraction slot and can maintain a wide enough directivity up to ~12khz, which is really good for a 2" horn. If you plan to use a supertweeter above the horn then you can choose a simpler design (without diffraction slot) that will be easyer to replicate.

[GCT]

Thanks for the info.

Could you explain a bit the directivity thing please. How do you get the directivity the same at xover in both woofer and horn?

Are the bullets normally just run from the horn with a condenser in series?

Thinking no bullet for now. So I think a slot is necessary. What's the relation of slot size to frequency and distance from slot to throat?

I have someone who can calculate the throat shapes and send me but I don't have any dimensions to give him yet. The 2380 seemed an easy one to build if I can get the important dimensions. So I'm still investigating possibilities.

I have a 2241 on the bottom end in a front facing reflex cab flat to 40Hz. Looks like it's OK to 1K or 1.2KHz where it's down 5dB 45º off axis.
Cheers

GCT

[pos]

The directivity of a cone driver is mainly dicated by the cone diameter (and shape in a smaller extend). It starts with a 360° directivity down low, then when the wavelength becomes small enough compared to the baffle width it becomes 180° (the baffle acts like a 180° horn), then as the wavelength becomes small compared to the cone diameter the directivity narrows (and lobs appear if you go really high).
You can consider that the cone has a ~90° directivity when the wavelength equals the effective piston diameter of the driver (~ diameter with half of the surround).
It is a good practice to choose a crossover frequency where the directivity of the cone and the horn are the same. In practice what should really be avoided is to have a directivity that narrows and then becomes wider again (cone driver is crossed too high). The opposite situation is less problematic (you get a faster 180°/90° transition).
In any case, the shallower the crossover slope, then more blend in directivity pattern you get.

The slot in a horn can be explained in a similar way: if used alone the compression driver can be considered as a cone driver with a 2" diameter. The horn reduce the directivity to a given one (and constant with frequency in the horizontal plan for the 2380) exaclty as the baffle does for a cone driver. But as soon as the 2" diameter becomes bigger than the considered wavelength it become the main directivity factor (the horn can only reduce the directivity).
So the slot is used to reduce the apparent diameter of the compression driver, letting the horn control (reduce) the directivity higher in frequency.

[GCT]

OK this is interesting. Thankyou.

So a 28mm slot would control the directivity to about 12K if I have understood correctly?

They normally give a cut off frequency for a horn. What dimensions determine the lowest frequency?

Cheers

GCT

[badman]

OK this is interesting. Thankyou.

So a 28mm slot would control the directivity to about 12K if I have understood correctly?

They normally give a cut off frequency for a horn. What dimensions determine the lowest frequency?

Cheers

GCT

The pathlength and the size of the mouth (depth, height, width)

[pos]

Yes basically when you look at a horn profile the part close to the throat controls the highs (and the smallest it is the higher it will control) and the part toward to the mouth controls the lows (and the bigger it is the lower it will control). The profile of the horn from throat to mouth dictates the directivity pattern from highs to lows. With that in mind you can understand why a conical horn gives a constant directivity with frequency, whereas a tractrix or exponential horn narrows its directivity with frequency.

[GCT]

Side issue:

Having understood directivity a bit better I can see my sofit wall 3 way system is crossed over too high to the tweeter. At what frequency does the wave leave the 180º pattern compared with speaker diameter?

(I wondered why my nearfields were better for panning)