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Re: "tapped" horn?

Posted by linasm (A) on May 4, 2005 at 16:08:14

In Reply to: Re: "tapped" horn? posted by chmed on May 3, 2005 at 18:10:44:

This might be interesting , the subwoofer I came up with a month or so ago.
I used it for a couple of shows by now, with Selenium 15SW1P driver in one and LowRider 15 in another, output is immense, and the reflex portion keeps the cone from large excursion where "normal" horn would be "reactance annulled" and would exibited excursion peak. If anyone interested, I can post graphs and sketches.
Here is the story:
I was trying to design the horn sub. Shortened horn when modeled in hornresp, shows a sizeable output peak once wavelength is around half of an effective horn length. So my thinking was to put the driver away from horn "throat" so that dip from the quarter wave reflection would cancel that peak.
Well, once done in a plywood, it indeed "canceled"... I've got a -16dB dip at a supposed kick drum "meat" frequency of 80Hz, missing a half wave pipe resonance of 120Hz, which also showed up, "uncanceled" and loud. So little disappointed ( I approximated that horn is about 65 % length of the pipe for reflected wave , was more like 85 % for this taper)I started fiddling with stuffing , to kill the Q of the dip. Dip got reduced , but efficiency elsewhere suffered. So I proceeded to shorten the horn
by putting in a new partition, some distance away from the "throat" . The horn itself is divided into two branches , from the driver down towards the "throat", so I shortened only one of them. Well, dip is still there, from the long branch, plus another one , now correctly placed to cancel peak , from the short branch. OK, so I shortened the other branch too . Now the dip is centered at 120 Hz, but starts taking away at 70Hz or so, and 40 Hz lost sensitivity too - obviously from volume loss and shorter horn. Desperate to do something to restore 40Hz , I take out the one of the shortening partitions and put in a bass-reflex style port from the "throat"of one of the branches, into the back chamber , against the "horn no-no's", just in case and before disassembling the whole thing.
This time spl plot got interesting. Not only curve was up 3db starting at 10 hz, but dip at 80Hz is completely gone ! So this port is "soft " enough not to reflect back , or maybe it's now acting as the new "throat entrance " from the "back " chamber which now acts as the front chamber ? And looking more carefully into low end response I noticed that maximum increase is at 15Hz. Port was intended to resonate at 35 or so! So now all exited I pull a port tube away , leaving just the hole in plywood, pull partition away from another horn branch and drill a port hole here too. New spl sweep and now 30Hz to 40Hz good 6db up from the original box, and it smoothly extends into 120 Hz, and then there is a deep dip at 150hz. I didn't have the test set-up calibrated, so for the reference data I run a ported known cabinet with the same driver , and it is 7 to 10 db lower from 30 to 100 Hz. ! All the measurements were outside, in quarter space enviroment with mike 3 meters away and on the ground plane. Then I pulled the box away from wall for more like half space measurement. Expecting to have a 6 db hit on the low end I was even more surprised that there was hardly any change , maybe -3db in narrow area around 40Hz, but that"s the original "cut-off" point for this horn. I also did run an impedance sweep, and here are three peeks, at 22Hz, 55 and 110, about 60 ohm, 50 ohm and 30 ohm, respectively ( quoting by memory, no graph in front of me).

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