"siren mounting" a tweeter - diyAudio
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Old 17th October 2015, 04:45 PM   #1
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Default "siren mounting" a tweeter

Click the image to open in full size.

I wanted to reduce the depth of a horn, and was considering "siren mounting" the tweeter, the way that they due in sirens. Basically there's a reflector right in front of the tweeter and the output exits radially, then hits a reflector and is combined to exit the horn.

To get an idea of the 'starting point', I took three tweeters and I mounted them face down on a tabletop, then I measured their output from half a meter away. Basically to see how bad the response would be when it's firing right into a flat surface. But also to get an idea of the radial frequency response.

Click the image to open in full size.
Click the image to open in full size.

The worst performing tweeter was a Vifa ring radiator. I had high hopes that the ring shape would response well to radiating radially, but that wasn't the case. I think the problem is the cavity in front of the tweeter formed by the slight waveguide loading of the Vifa. The pic is a Vifa XT25 but I used and XT19.

Click the image to open in full size.
Click the image to open in full size.
The Celestion CDX1-1445 offered wider bandwidth due to the larger diaphragm, but also suffers from a big spike in the harmonic distortion

Click the image to open in full size.
Click the image to open in full size.
The SB Acoustics SB19ST had the lowest overall distortion, and also the lowest price.

The response on ALL of these are iffy, due to the reflector, but the SB19 is arguably the winner.

I'm going to try varying the height of the slot to see if that improves the response.
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Old 17th October 2015, 06:00 PM   #2
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Click the image to open in full size.

I wanted to try a couple of more things.

First, I wanted to see if varying the height of the slot changes the frequency response.

It does.

A larger gap smooths out the low frequencies at the expense of the high frequencies.
And vice versa.

So there isn't an 'optimum' gap; it will really depend on how high you want it to play.

In the graph, the green color is the largest gap, yellow is in the middle, and red is such a small gap, the tweeter diaphragm is nearly touch the opposite wall of the slot.

I wanted to see how a midrange would behave when mounted this way. The purple traces shows the frequency response of a 2" midrange mounted the same way. (face down, radiating radially.)
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Old 17th October 2015, 06:02 PM   #3
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Click the image to open in full size.
Here's the frequency response and distortion of the 2" mounted face down.
Obviously, you want to low pass it to eliminate that big spike, but it might be a viable option for a midrange horn where you had limited depth.
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Old 17th October 2015, 07:55 PM   #4
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I think it's great when people experiment, rather than do all the usual boring, mundane stuff.

We'd still be in the stone age if it wasn't for people like you. Keep up the good work
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Old 17th October 2015, 08:52 PM   #5
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Hi Patrick,

I know this is completely OT but elsewhere I came across this post by Tom Danley on how to deal with pipe resonances in tapped horns.
I thought you might find that interesting what with being one of the more experimental types around here.

RE: not offset ? - tomservo - High Efficiency Speaker Asylum
(there is a link within the link with pictures)


PS: I always enjoy the write ups of your experiments, I learned a lot from them.
Thanks and keep up the good work! ;-)
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Old 17th October 2015, 09:03 PM   #6
kessito is offline kessito  Netherlands
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Interesting! I have been contemplating this to, I think that the proper shaping of the reflector in combination with the right horn might work..
Maybe we could come up with a reflector/phaseplug that makes an "ideal" wavefront
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Old 17th October 2015, 10:39 PM   #7
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Quote:
Originally Posted by kessito View Post
Interesting! I have been contemplating this to, I think that the proper shaping of the reflector in combination with the right horn might work..
Maybe we could come up with a reflector/phaseplug that makes an "ideal" wavefront
I tried a bunch of different things and found that the reticulated foam that Geddes uses makes all the difference in the world.

I'll post the results later.

Basically the dips in the frequency response are due to higher order modes. Basically a significant percentage of the output is reflected at the edge, due to the sudden change in angle. So the reticulated foam 'soaks up' the HOMS and cleans up the response.
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Old 18th October 2015, 09:53 PM   #8
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I've only disassembled a couple of "siren" horns and an old hand-held "loud hailer" or "bull horn", but they all had the same layout: They were folded horns. No flat reflectors.
Like this in cross-section:
https://upload.wikimedia.org/wikiped...nimation_2.gif

Patrick, could you put up a photo of the internals of the one you examined?
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Old 18th October 2015, 10:40 PM   #9
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I wonder if the flat reflector is just the end where it folds? Is the first reflector the phase plug? If not, I am confused too.
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Old 18th October 2015, 11:45 PM   #10
Pano is offline Pano  United States
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I'm with Don on this one. All the paging type horns I've seen are folded horns, not flat reflectors. Aren't sirens like that too? Or maybe they use a flat reflector to amplify certain frequencies and harmonics for a louder output?
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