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Bill poster 16th March 2010 04:28 AM

Front Horn flare diameter questions
I was thinking of making a front horn design. Couple of questions from a newbie..
(please don't say don't bother etc)

Will a 180 hz spherical horn accoustically go up to 20 khz?

Is there any way of reducing the diameter of horns without losing low end(I want to go down to 180 hz with the flare but its bloody wide), ie lengthening the throat.

GM 16th March 2010 05:16 AM

As asked, not in this physical universe. A horn + driver's beaming HF response can do it on axis though.

A typical horn is no different from any other box alignment, it's gain BW is a function of its net volume (Vb), so as you shrink it for a given axial length, F3 rises.

Tapped horns OTOH utilize both sides of the driver's radiation to shrink it, but the trade-off is that it has no usable HF BW, so strictly a 2-3 octave band-pass (BP) alignment depending on the driver's specs and flare factor.


Bill poster 16th March 2010 12:31 PM

Thanks very much for answering that - I'm guessing an elliptical horn could be used to reduce the width of the speaker cabinet- is there any trade-off with using an elliptical shape?

GM 16th March 2010 03:14 PM

You're welcome!

Well, 'sound is round', so any reshaping of the waveform distorts it, though according Dr. Geddes an oblate spheroid expansion is the most benign. Other than his speakers though, the only way to get them AFAIK is to DIY.

That said, In another lifetime me and a few others compared round, square and several different aspect ratio rectangular WGs and came to the same conclusion as the pioneers of audio, i.e. for a given mouth cutoff, a square or rectangular one with the same area as a round one (perimeter > 1 WL) is 'close enough' to same-same with the caveat that the rectangular one's aspect ratio doesn't exceed ~ 1.0:1.273 where the short side is 1/4 WL of the mouth cutoff.

This by itself is grossly misleading though and doesn't really tell us much about what it takes to make a truly low distortion horn/WG as it's the throat and mouth transition that matters by far the most IME.

Anyway, the only elliptical looking horns I've auditioned sounded audibly worse than my square and low aspect ratio rectangular DIY units, but I blame it more on their abrupt mouth transition than its shape.


Bill poster 16th March 2010 08:09 PM

I think I get that- so basically if elliptical, round or square, the 'circumference area' of the mouth (for want of a better term) should be the same. If rectangular or elliptical the ratio should be within the one given in your post.

Now thinking of an upward firing square/rectangular horn (alas, a cylindrical shape would take up too much room), using a deflector of some sort. Possibly a one driver compound horn ('rear' horn floor firing, 2.5 metre length).

edit: ah.. but will the HF be flat ???

GM 16th March 2010 08:58 PM

For a true omni, only round will work.

It will if it has the right amount of rising on axis response and the phase plug is an acoustic mirror of it. No trivial pursuit, especially if the driver isn't designed for the app.


Bill poster 17th March 2010 03:03 PM

Ok thanks, you know your stuff and explain in a way that's understandable to a novice! (rare)
If I do an omni it will end up looking too much like the Duevels so will now do a front firer in a completely rectangular cabinet/wide baffle/ black lacquer and floor firing. If possible single driver compound horn, if 2-way the bass will be reflex

FINAL question (he says); back folded horn loading- if the line is long enough to reach 30 -35 hz or so, is there a stipulation on how wide/area the mouth needs to be?

AndrewT 17th March 2010 03:39 PM

in free space the circumference of the circular mouth should be greater than the wavelength of the lowest frequency. 30Hzwl=11.4m, mouth diameter >= 3.65m, mouth area = 10.5sqm
For half space loading you can halve the area. (horn sitting of the floor.)
For quarter space loading you can quarter the area. (horn on the floor and backed up hard against the back wall)
For corner loading divide the area by 8. This is the Pi/2 seen for eighth space.

Bill poster 17th March 2010 04:16 PM

yeah, quarter wave so I'm guessing the line has to be around 2.5-3 metres long.
I didnt realise that the cabinet has to be flat against the wall either, thanks.

GM 17th March 2010 05:29 PM


Originally Posted by AndrewT (Post 2122371)
in free space the circumference of the circular mouth should be greater than the wavelength of the lowest frequency.

Hmm, last things first: I believe you mean the effective circumference, ergo less than a WL of the lowest desired horn loaded frequency.......... ;)


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