I have decided to add a small full range driver to my quarter wave tapered line speakers which will replace the original tweeter and the xover will be lowered to 500Hz.
This involves adding a pipe from the front baffle to the rear panel to act as the mid tweeters enclosure. It needs progressive stuffing of course but the line is not very long (197 mm / 8") and that doesn't give much opportunity to soak up all the backwave.
I have no problem drilling the back panel of the speaker - nobody looks behind do they 🙂 So I thought why not extend the line. I could cut a 68mm hole in the back panel and install a black pvc curved 90 degree bend used on guttering systems. The pipe could be extended down the back of the floorstanding speaker to near floor level - that would give me a line just over 90cm / 3ft long.
This is just to soak up back wave of a mid tweeter so I presume it doesn't need tuning to Fs and no output at the end of the line is desirable.
Does that sound like a good plan ?
Does the line have to be tapered ?
Is it best in this case to leave the end of the line open ?
Many thanks .
This involves adding a pipe from the front baffle to the rear panel to act as the mid tweeters enclosure. It needs progressive stuffing of course but the line is not very long (197 mm / 8") and that doesn't give much opportunity to soak up all the backwave.
I have no problem drilling the back panel of the speaker - nobody looks behind do they 🙂 So I thought why not extend the line. I could cut a 68mm hole in the back panel and install a black pvc curved 90 degree bend used on guttering systems. The pipe could be extended down the back of the floorstanding speaker to near floor level - that would give me a line just over 90cm / 3ft long.
This is just to soak up back wave of a mid tweeter so I presume it doesn't need tuning to Fs and no output at the end of the line is desirable.
Does that sound like a good plan ?
Does the line have to be tapered ?
Is it best in this case to leave the end of the line open ?
Many thanks .
If driver is going to play from 500Hz and up than line lenght of 197 mm / 8" is enough.
Stuff it well, 20-25 grams per liter of polyfill, or half of that amount for figerglass.
Equally across the line lenght.
Stuff it well, 20-25 grams per liter of polyfill, or half of that amount for figerglass.
Equally across the line lenght.
We have successfully used similar length lines, but heavily tapered, successfully down to 250 Hz.
dave
dave
Nothing to be confused about, a tapered line (narrowing toward the terminus) has a lower fundamental than an untapered, so can be used to a lower frequency, assuming a similar (sufficient) Vb for purpose. Progressive damping density can be useful in avoiding excess in proximity to the cone.
Right, and works fine providing you don't require a high density in close proximity to the cone. If that's the case, it's usually a good idea to back off a bit for mid-TLs if you're running something like a small wideband, or you can get some mass-loading effects on the moving components. Hence the progressive-rate damping, especially since it tends to be most effective when located at Vmax. Likely window-dressing in many cases, but no harm in a few refinements since it doesn't cost anything but a little time, especially if you can take an impedance (& preferably HD sweeps) to check on behaviour.
I get it now 😀 (I think). I did not know the relevance of GM's post. Speed of sound ÷ 4 (quarter wavelength) ÷ 8 (length of my line in inches) means it will work down to 423 Hz.
The comment about equal amounts of stuffing along the line threw me as I assumed (correctly) that I should be stuffing it with increasing density as You go towards the back.
The pipe I have is 95mm internal diameter. I could extend the line through the back panel and down the back of the speaker. That would be in 64mm inside diameter black pvc pipe. Does that count as a taper ? I suppose it won't hurt to increase the length of the line it will surely give more chance yo soak up the backwave ?
The comment about equal amounts of stuffing along the line threw me as I assumed (correctly) that I should be stuffing it with increasing density as You go towards the back.
The pipe I have is 95mm internal diameter. I could extend the line through the back panel and down the back of the speaker. That would be in 64mm inside diameter black pvc pipe. Does that count as a taper ? I suppose it won't hurt to increase the length of the line it will surely give more chance yo soak up the backwave ?
Not off-hand, I just got back from my second sojourn to A&E in four days and I'm stuck on my spare machine. Ask GM if you would prefer to believe him, since we've both made this point for years (over a decade in my case, a number of decades in his). If you place high densities of damping material in close proximity to a drive unit, especially something like a small wideband, you can end up with it behaving sufficiently like a solid that the air mass-loads the moving components, and at extremes causing temporary deformation, or preventing it resonating as intended. This is a factor since most of these units rely on TL modes to produce a large portion of their BW rather than oscillatory.
