Horn throat geometry: round to rectangular transition

[MaVo]

Hello,

i would like to build a constant directivity waveguide of oblate spheroid form for beeing used with a 1 inch compression driver. The ultimate goal is to construct my own unity clone one day.

My problem is, that a compression driver has a round exit while i doubt i can build a round horn in unity size. So there has to be a transition between round and rectangular.

My questions, all in the attempt to get the best sound quality, are:

- What is the best geometry of the transition?
- How long should the transition be?
- Does someone know which exit angle the BMS 4552ND has? I want to match the angle which the horn has at the throat with it.

Thanks for reading so far, i would be happy about some help

[SunRa]

Hello MaVo, what are you using for calculating the oblate spheroid waveguide?

[AndrewT]

Hi,
the exit angle from the throat is determined by the expansion rate of the horn.
If you have the correct expansion rate (maybe doubling the area every 100mm along the horn), then that will be independent of the angle machined into the compression driver exit throat.
I would go from round to octagonal then to square or rectangular.
The segments making the octagonal shape can be tapered from true eight equal sides diminishing to zero sized segments as you reach square.

[FrankWW]

MaVo, I should ask BMS directly what the exit angle is.

[MaVo]

Originally posted by SunRa
Hello MaVo, what are you using for calculating the oblate spheroid waveguide?

I found a excel sheet in an other thread about waveguides. Earl Geddes posted there too, i think it was based on his equations. I think it was called "geddes on waveguides" or somethink like that.

Originally posted by AndrewT
Hi,
the exit angle from the throat is determined by the expansion rate of the horn.
If you have the correct expansion rate (maybe doubling the area every 100mm along the horn), then that will be independent of the angle machined into the compression driver exit throat.
I would go from round to octagonal then to square or rectangular.
The segments making the octagonal shape can be tapered from true eight equal sides diminishing to zero sized segments as you reach square.

According to Earl Geddes, in a OS waveguide one can match the waveguide throat expansion angle to the one of the compression driver. That was also stated in the above mentioned thread. This should reduce the HOM generation of the waveguide. Then the angle rises to the real waveguide angle, which is kept constant for the bigger part of the waveguide.
I think you described a conical waveguide, which is rather identical with a OS waveguide except for the throat design. In a conical waveguide, angle matching is not possible since the expansion is constant all the way to the throat. With that, one gets a discontinuity at the throat, which generates HOMs.

Concerning the throat shape, i think in similar lines as you. When one morphs a square and a circle of equal area into another, the inbetween shape should be somethink like an octagon.

Originally posted by FrankWW
MaVo, I should ask BMS directly what the exit angle is.

yes, i should do that.


Thanks so far for the answers. If someone has experience with building such waveguides, i would love to hear about it.

[Brett]

Originally posted by MaVo
yes, i should do that.

Please post their response.

[tinitus]

I have been through the same thoughts and concluded that I would use some kind of filling/putting or whatever you may call it ... actually I might use a lathe to make the throath in hard wood doing the transition to the squared flare, and let filling do the rest ... mouth edges will be rounded and with felt underlined ... could be tricky to get smooth though ... a full scale drawing will help a lot

[Brett]

Why not turn the throat until the flare becomes stright sided and then make a 12 section conical. It's make the transition much easier.

Also if you used the came throat section, you could use a router and make the conical section fron laminated sheets of MDF. Yourkville have a successful round Unity.

[MaVo]

Originally posted by Brett

Please post their response.

I just send BMS a mail, but since its weekend, it will take some days until they respond.

[MaVo]

Tinitus & Brett, your ideas got me thinking. Maybe a round horn would be the way to go.

The throat could be done as you suggest, by drilling it out of wood.

For the the straight part of the horn, one could make a wooden rectangular case for the horn, then fill in the round shape with a base of polyurethane foam and a layer of epoxy or something like that. Terminating the case with a round wood cutout that resembles the horn mouth and throat areas on both sides would help doing the foam / epoxy part.