Geddes on Waveguides

[Omholt]

did you ever run polar measurment for the midbass horns

Yes. Vertical and horizontal of a scale model and horizontal of the full size.
Horizontally, the directivity is 70 degrees between 300 Hz and 600 Hz and a little higher above. At 150 Hz it's gone up to a little over 100 degrees. Below 150 Hz it starts to get considerably wider.

 

[POOH]

Yes. Vertical and horizontal of a scale model and horizontal of the full size.
Horizontally, the directivity is 70 degrees between 300 Hz and 600 Hz and a little higher above. At 150 Hz it's gone up to a little over 100 degrees. Below 150 Hz it starts to get considerably wider.

Thanks, I have similar size horns but need to wait until the spring weather breaks to get them outside to measure them. I like how you set the flare for being used on the ground.

 

[gedlee]

I'll wait for good polar measurements before discussing further. Beam width measurements are not very useful.

 

[NisseW]

I hade a mailconversation with Giacomo Bartolini at B&C Firenze Italy. He stated that the
exitangle of the B&C DE 250 is 14,8° (1/2 7,4°), not 13° as many contending in this thread.
This tolerans is perhaps small but givs a missmatch, as far as I understand.

 

[Dan_E10]

Actually both of those questions were answered further up the thread, but yes, I remove the screen as the foam does this same job. And Yes I retain the gasket material on the driver. These gaskets squeeze down to almost nothing.

Okay, dumb question here. Is there an easy way to remove the bug screen on the DE250 or DE500? Does it have to be cut out or does it require lots of disassembly? I've read it's tricky to align the diaphragm in the DE250, can the bug screen be removed without moving the diaphragm?

Dan

 

[gedlee]

I just cut it out with a sharp knife.

 

[Dan_E10]

Thanks for the response Earl. Looking at the screens, I wasn't sure if they were plastic or metal. Once it's cut out, is sanding or filing needed to get a smooth surface or do you take care of the smoothing between the the throat and the entrance to the waveguide mostly with clay or putty?
Dan

 

[gedlee]

The screens are plastic and you can just cut them out a little beyond the throat aperture. Only in rare cases would I sand, mostly just a little clay to fill the inevitable gap with the DE250. The DE500 has no gap.

 

[Kindhornman]

Earl,
From the spec sheets it is hard to see much difference between the DE250 and the 500 except for the power handling. How do you compare the two drivers, the FR seems to be about the same and I'm guessing from the specs that the two diaphragms are actually the same? The differences seem to be with the magnetic assembly only.

 

[Dan_E10]

The screens are plastic and you can just cut them out a little beyond the throat aperture. Only in rare cases would I sand, mostly just a little clay to fill the inevitable gap with the DE250. The DE500 has no gap.

I took a look at them last night and saw that, as you say, it's just a very thin plastic screen. I hadn't looked very closely at them before and was thinking they were similar to the Selenium D220ti's which have what appears to be a metal screen.

Dan

 

[gedlee]

The DE250 and the DE500 do not have the same diaphragm. The DE500 is metal, the 250 plastic. The 500 appears to have more BL and has a bit more output at lower freqs than the 250, but a bit less overall response at 10 kHz than the 250. One cannot passively match a de500 with a 15" driver because of this, but it can be done with the 250. The 500 does match a 10" better than a 250 and the 12" works well with either, but a little flatter with the de500. Not at all the same drivers.

 

[300Z]

Weird, the DE500 has better low frequency extension and worse top end but cannot be passively matched with a 15" woofer because of it?

 

[miero]

300Z, you can check the 1518 project that is using B&C 15NBX100, DE500 8 ohm and 18" OSWG:
- passive crossover: http://hifi.slovanet.sk/bb/download.php?id=25565
- drawing front: http://hifi.slovanet.sk/bb/download.php?id=25513
- drawing side: http://hifi.slovanet.sk/bb/download.php?id=25514
- measurement: http://hifi.slovanet.sk/bb/download.php?id=23431

Sources: hifi.slovanet.sk :: Zobrazi? t?mu - Projekt "1518"

 

[AllenB]

Weird, the DE500 has better low frequency extension and worse top end but cannot be passively matched with a 15" woofer because of it?

... I think because the 500 is too quiet at the top and wants gain to match the woofers being used.

 

[300Z]

Judging by the curves showed by B&C and assuming they were measured in the same horn I would say the response from both drivers is very similar.

 

[bwaslo]

I'd think Earl measured both in the same horn. Driver manufacturers sometimes aren't very consistent about telling you what horn they measure their CDs in.

 

[ivicai]

Selenium D220Ti

I took a look at them last night and saw that, as you say, it's just a very thin plastic screen. I hadn't looked very closely at them before and was thinking they were similar to the Selenium D220ti's which have what appears to be a metal screen.

Dan

Hi,

I have very good experience with the SELENIUM D220Ti driver, especially over 1.5kHz with HM17-25 horn applied....

regrads
ivicai

 

[Kindhornman]

Most compression drivers are tested on a terminated plain wave tube and not mounted on a horn. At least that is what I know from the CD designer that I know. Only after the driver has been built and tested will it be mounted on some horn and tested for on axis response and polar response, nothing really to do with the driver development though.

 

[300Z]

I just checked the B&C page and they say that both drivers were mounted on the ME 45 horn.

 

[gedlee]

Weird, the DE500 has better low frequency extension and worse top end but cannot be passively matched with a 15" woofer because of it?

When a driver is mounted to a constant directivity waveguide, its response will fall at about 6 dB/Oct above a certain point. If this response falls below the pass-band response of the woofer then either the woofer would have to be attenuated (to me, this is a very bad idea) or a higher output driver must be used. The 15" B&C has a few dB more efficiency than the 12" and the DE500 falls just below what is required for the 15", so the few dB more output from the DE250 is required.

The project posted above does not use a foam plug so in that case the DE500 would work with the 15" woofer. Add a foam plug and its no longer workable passively.

And yes I test all drivers on the same waveguide with foam plug installed. Different horns will affect different drivers differently so comparing my results to B&Cs is not possible.

Does anyone know how he obtained that nice polar map plot?