Horn mouth termination

[Panomaniac]

In this thread
http://www.hostboard.com/forums/alte...ml#post1775827
I mentioned my experiments with soft termination of the mouth edges or "lips" of the Altec 811. Documented somewhat in this diyAudio thread diyAudio Forums - Horn Honk and Towels - Page 1

Basically I started out with rolled up beach towels draped around the mouth of the 811. The result was subtle, but audible. It sounded like something went away. In fact it did, a kink in the FR at ~1 KHz. The effect was twice as pronounced with towels top and bottom as with just one. The sound was overall smoother.

I also used some pipe insulation, the black foam stuff that I picked up at a local HVAC store. It worked well, but was hard to make pretty. Ended up with tubes sewn out of gray flannel stuffed with Dacron fiber.

The idea is to easy the impedance change from the horn mouth to free air. If the change is too abrupt, it causes diffraction artifacts that are audible. Using a lossy material to gradually easy from hard metal to soft air seems to work well.

I wanted to build a good rollover with chicken wire or hardware cloth covered with felt, but never did.

This tweak does make a difference and is worth the effort. Cost is very low. Have fun with it!

[Steve Mac]

I'll try to post a pic tonight....I used some felt fabric that I rolled up
and used...he he...some paper clips to hold it in place just to listen.
I need to repeat the test because my hearing was already used up
yesterday but I thought it deadened the sparkle and lowered the horn sound.
Maybe the key is to use as little as possible and maybe that's why I see it used
so sparingly in the applications that are out there. Obviously this is fairly
obscure territory to be wading in...

[gearfreak]

For more info you might do a search on 'HOM' 'Higher Order Modes' (IIRC). May be the same thing.

I believe it'd get you into a discussion of 'mouth reflections' and the signals bouncing around the horn, arriving out of time with the original.

Topics RE the UREI 604 monitors (?) with the foam CD horn might also apply.

[GM]

For more info you might do a search on 'HOM' 'Higher Order Modes' (IIRC). May be the same thing.

I believe it'd get you into a discussion of 'mouth reflections' and the signals bouncing around the horn, arriving out of time with the original.

Topics RE the UREI 604 monitors (?) with the foam CD horn might also apply.

HOMs are reflections inside the horn that comb filter with the direct radiation not unlike what happens when speakers generate early reflections off the room's various boundaries. Mouth generated reflections are both spurious (HOMs 'ringing' off the lips such as bell modes) and eigenmodes (standing waves) across it that cause reflections back to the throat (or at least the first fold), modulating it.

Dr.Geddes's foam insert quells the HOMs and the Tractrix, Jean-Michel Le Cl?ac'h terminations as well as Peavey's foam 'lip' or similar do a good job of dealing with the eigenmodes.

GM

[Panomaniac]

HOMs are a bit different and relate more to reflections generated in the throat of horn.

You should not lose sparkle with the soft lips, but you will hear the loss of something. That something is generally a kink in the midrange. Response should be flatter - and better of axis too - with the edge treament.

As long as you are not blocking the sound path and you hear less of something, that's probably a good thing. =) Live with it a bit to see what you think.

EDIT: OH, GM posted before me. He says it well.

[lowpoke]

Rather than trying to fix some kind of damping 'thing' around the horn mouth, would adhering a thin acoustically absorbent material like felt or suede even, to the last few inches of the actual horn surface give the desired result?

I've tried to illustrate what I mean in the attached image.

[Panomaniac]

I see what you mean and it's certainly a more elegant approach...

However it may not do enough. Easy enough to test. Just try a couple of big towels to start (2 per horn). Then judge your own improvements by that standard.

[gearfreak]

In this thread
http://www.hostboard.com/forums/alte...ml#post1775827
The idea is to easy the impedance change from the horn mouth to free air. If the change is too abrupt, it causes diffraction artifacts that are audible. Using a lossy material to gradually easy from hard metal to soft air seems to work well.

I will try it of course... but will also pose the question.

Is this approach valid for Manta Ray horns, or is the 'diffraction' that we'd be killing, part of the equation WRT how they actually achieve CD (or otherwise how they work)? Mine happen to be the smaller ones, probably 12" / side, still un-id'd officially.

I may try spray-starch as a reversible way to attach the (foam, felt, whathave you). Unsure of it's effect on paint, so I may test on something similar first.

[Steve Mac]

I will try it of course... but will also pose the question.

Is this approach valid for Manta Ray horns, or is the 'diffraction' that we'd be killing, part of the equation WRT how they actually achieve CD (or otherwise how they work)? Mine happen to be the smaller ones, probably 12" / side, still un-id'd officially.

I may try spray-starch as a reversible way to attach the (foam, felt, whathave you). Unsure of it's effect on paint, so I may test on something similar first.

I plan to use rubber cement for my work.

[Panomaniac]

Yes, but what about the paint? It seems that anything I used on the 811 horns took off the gold paint.

There are some double face tapes that aren't too bad. Made for photos and such....