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Old 8th February 2007, 03:07 AM   #1
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Default stuffing/lining a bass horn

This is something I haven't seen covered before.

Is there any benefit in stuffing or lining a bass horn?

I'm talking about a bass horn that would be used below 100 Hz, down to possibly as low as 15 or 20 Hz.

I understand that when the walls are parallel, resonances are formed. Also, with any driver there are harmonic distortion products that are out of the desired passband. Objectionable 3rd order HD for 100 Hz would be @ 300 Hz for example. Lets say the inside of the horn had lining. It would not reduce output of have any effect in the desired passband, but could absorb distortion products while the waves travel through the horn.

Could stuffing have a different effect than lining with open cell foam?

My thoughts were to put dacron at the start of the horn and then line the first metre or so. I'm thinking of a ~6m horn.

I have heard of Earl Geddes stuffing an oblate sphereoid horn for a CD to absorb "higher order modes" - the logic being that it will attenuate reflections more than the direct travelling sound waves, the reflections being the main cause of coloration in horns. In essence I'm applying a similar logic to this to bass horns, except it seems perhaps even more applicable since the desired sound won't be attenuated.

Thoughts anyone?
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Old 9th February 2007, 11:48 PM   #2
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I think that foam or some sort of damping material on the parallel walls of a folded bass horn could help reduce the resonance effect you speak of. Although with a folded bass horn, I think I would be even more concerned with the horn section lengths between the bends.
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Old 10th February 2007, 01:31 AM   #3
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Surely by its very nature a well designed folded subhorn will attenuate higher frequencies /resonances ?

Rob.

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Old 10th February 2007, 04:48 AM   #4
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I think the only way it could be beneficial is to absorb higher frequencies that somehow leaked into the horn. Otherwise, in a true horn it's only going to reduce efficiency by turning some of the energy into heat. If it's operation extends into the range where it has some TL action, then I guess there could be some resonances that damping might help.

Dr. Geddes Summa is not a horn, just a waveguide, and that foam plug obviously attenuates some of the HF content. That's why the speaker only extends to 10khz. A bass horn is a totally different animal.
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Old 11th February 2007, 04:11 PM   #5
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Hello,

for a bass horn,
to reduce frequencies above 100 Hz,
it is useful to take "Weichfaser" softfibre,
very cheap most 12-13 mm one side paper,
a sound reducing board.

if the press chamber and the first meter
is inside softfibre you can reduce the
first "Schallschnelle" over 100 Hz and
the 1+2 wave optimum over 300 Hz.

look my horns.
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Old 15th February 2007, 06:47 PM   #6
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Afaik,

the geometry of the throat section can be adjusted to attenuate frequencies above 300Hz.

Otoh, a good xover works too.

Slight felt damping material on the surface of bends in a folded horn will help to attenuate very much out of band energy, but the throat can do that by itself.

The HOM that Geddes refers to seem like non-issues in a typical bass horn, plus he loses gain in his waveguide design in favor of polar response (he claims) which seems less favorable in a bass horn design.

From what I have seen, most "bass horns" really do not go particularly low, since the mouth size for 20Hz. (for example) is utterly huge. Even those designed for 40Hz. often have insufficent mouth size to keep the response smooth and flat.

Imho, if you don't absolutely need the "extra efficiency" in most situations a bass horn doesn't buy you much if anything. Plus you get a very very large propagation delay to overcome (assuming such things matter to you).

_-_-bear

PS. Oh, "stuffing" the horn turns it into sort of a transmission line... which probably will pull down the HF components quite a lot actually.
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