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In the Forum: Horn-Loaded Speakers
In the Thread: Deep End DIY - Australian take one Macondo.
Post Subject: Free first order crossoverPosted by anthony on: 8/30/2017
I realised this morning that I have done a "not-so-standard" thing in using leftover fibreglass batts for stuffing the tubes.  The batts that I have on hand are high density batts, not the low density type of fibreglass that is usually used for stuffing speakers and it is having an interesting effect.  Looking at the spec sheet of these particular batts they start absorbing at about 100Hz and really get efficient at about 250Hz and they are having effectively acting as a low pass filter for the channel.  Completely accidental of course.

Think of if this way.  The backwave from the driver is goes back into the tube and because it is a subwoofer the low frequencies push straight through the fibreglass and some of them are reflected back to the driver and some of them pass through the walls of the tube and into the room...there is no way to stop this and it is what it is, but the more rigid the tube is the less sound that will pass through it.  Because the rear of the Cannons will be placed against a wall the sound that passes through the tube wall and the sound that wraps around the baffle (or no baffle in this case) is reflected off the back wall with a 5.8ms delay or phase shift.  However, the high density fibreglass inside the tube starts attenuating the backwave at about 100Hz and I have reason to believe that almost all of the 200Hz+ backwave is absorbed (the resonance of the pipes is about 190Hz and this is completely removed).  So we have no attentuation of the backwave in the tube at 20Hz, a bit starting at 100Hz and full-on attenuation at 200Hz...that is a low pass filter with the knee set at about 100Hz which is just perfect for this application because this is approximately where the line level low pass filter will be set.

As I mentioned earlier I just used the fibreglass that I had on hand, but it does a good job of removing a lot of out of phase energy above 100Hz which I think is a good thing, while not affecting lower frequencies.  So I think that I have just found an unintended benefit of the tube or Cannon topology.

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