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.
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.
An externally hosted image should be here but it was not working when we last tested it.
The Celestion CDX1-1445 offered wider bandwidth due to the larger diaphragm, but also suffers from a big spike in the harmonic distortion
An externally hosted image should be here but it was not working when we last tested it.
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.
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.)
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! ;-)
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! ;-)
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.
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/wikipe....gif/200px-Reflex_loudspeaker_animation_2.gif
Patrick, could you put up a photo of the internals of the one you examined?
Like this in cross-section:
https://upload.wikimedia.org/wikipe....gif/200px-Reflex_loudspeaker_animation_2.gif
Patrick, could you put up a photo of the internals of the one you examined?
I think everyone else is correct. Sirens are folded horns.
I've screwed around with paralines, and found that minimizing reflectors helps a lot at high frequency. The Paraline has four reflectors in it.
If you use a dome instead of a compression driver, you can fire a tweeter straight down with just one reflector.
Clearly, the frequency response suffers, but if you have really limited depth it might be worth a shot.
Unfortunately, there's another problem that's introduced, which I hadn't considered:
Firing the tweeter straight down turns the output into a ring. For instance, with a SB Acoustics SB19, you get a ring that's about 3.5" in diameter. (Due to the width of the baffle.)
In my measurements, I ran into a problem I hadn't considered:
The pathlength difference in the ring rolls off the highs. IE, if you're looking DIRECTLY at the center of the tweeter, everything is fine. But go off axis even a little, and you get a pathlength difference because one side of the ring is further away from the other.
To fix this, you'd have to 'recombine' the output of the ring into some other shape. For instance, concentrate it into a ribbon shape, like the Paraline.
But that requires more reflectors, which becomes challenging.
If you used this for an omnipolar speaker it would probably work fine.
I've screwed around with paralines, and found that minimizing reflectors helps a lot at high frequency. The Paraline has four reflectors in it.
If you use a dome instead of a compression driver, you can fire a tweeter straight down with just one reflector.
Clearly, the frequency response suffers, but if you have really limited depth it might be worth a shot.
Unfortunately, there's another problem that's introduced, which I hadn't considered:
Firing the tweeter straight down turns the output into a ring. For instance, with a SB Acoustics SB19, you get a ring that's about 3.5" in diameter. (Due to the width of the baffle.)
In my measurements, I ran into a problem I hadn't considered:
The pathlength difference in the ring rolls off the highs. IE, if you're looking DIRECTLY at the center of the tweeter, everything is fine. But go off axis even a little, and you get a pathlength difference because one side of the ring is further away from the other.
To fix this, you'd have to 'recombine' the output of the ring into some other shape. For instance, concentrate it into a ribbon shape, like the Paraline.
But that requires more reflectors, which becomes challenging.
If you used this for an omnipolar speaker it would probably work fine.
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