I've looked at the Slot Loaded Open Baffle stuff, but that didn't tell me what the slot does to the TS parameters of the driver.
I'm curious about the effect of mounting woofers behind a slot, or a grille of perforated steel with open area < cone area. Assume a well stuffed sealed box, to avoid an extra layer of complexity (adding in rear wave effects).
Am I right in thinking this adds air load to the woofer + diffraction effects, thus:
-pushes up Qts
-reduces Fs
-increases low frequency efficiency
-reduces HF bandwidth
I'm curious about the effect of mounting woofers behind a slot, or a grille of perforated steel with open area < cone area. Assume a well stuffed sealed box, to avoid an extra layer of complexity (adding in rear wave effects).
Am I right in thinking this adds air load to the woofer + diffraction effects, thus:
-pushes up Qts
-reduces Fs
-increases low frequency efficiency
-reduces HF bandwidth
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None of the above for a typical slot.
The slot makes the dispersion at the top of the woofer range.
http://www.eminence.com/pdf/Delta_Pro_15A.pdf
The response rises on-axis as the dispersion narrows. Using an aperture plate (slot) will spread the dispersion out and knock off the response rise.
You can try different slots out of cardboard and see how they work.
The slot makes the dispersion at the top of the woofer range.
http://www.eminence.com/pdf/Delta_Pro_15A.pdf
The response rises on-axis as the dispersion narrows. Using an aperture plate (slot) will spread the dispersion out and knock off the response rise.
You can try different slots out of cardboard and see how they work.
There are slots with practically no length, like the JBL posted above, and long (~30 cm) slots like those used in open baffles. The long slots add an air mass to the cone. The effect is amplified if the slot has a smaller area than the cone, such that the air vibrates with a greater displacement than the cone. At higher frequencies the 1/4 wavelength resonance comes into play, limiting usability to subwoofers. I measured a reduction in fs of about 20% when I added a 25 cm long x 7,7 cm diameter tube to a 10" PA woofer.
The above is not significant for slots like the JBL uses, where the length is almost zero.
@ MemX: yes the JBL slot does not operate at subwoofer frequencies, only at the top of the woofers bandwidth.
The above is not significant for slots like the JBL uses, where the length is almost zero.
@ MemX: yes the JBL slot does not operate at subwoofer frequencies, only at the top of the woofers bandwidth.
Good info y'all - thanks.
I realised I could test it myself with 30 minutes work.
My ugly test box is 'ported', meaning incomplete. Currently it is loaded with a 12" (on a removable front plate), and is 'tuned' to 30Hz (the peaks are 18Hz and 42Hz, the low part of the saddle is 30Hz). Onto this travesty of a box I screwed an even uglier frame of wood scraps, so that its open area was about 70-80% of the cone's area.
Using DATS, the before-and after values were virtually the same.
I added a sheet of perforated metal over that open area. Ditto.
The impedance curve does change shape / shift downwards in frequency marginally, and Qts does go up a fraction - but the change was less than the variation I measured between the set of 12" drivers.
In short: a short slot does very-close-to-nothing to the Thiele Small parameters.
I realised I could test it myself with 30 minutes work.
My ugly test box is 'ported', meaning incomplete. Currently it is loaded with a 12" (on a removable front plate), and is 'tuned' to 30Hz (the peaks are 18Hz and 42Hz, the low part of the saddle is 30Hz). Onto this travesty of a box I screwed an even uglier frame of wood scraps, so that its open area was about 70-80% of the cone's area.
Using DATS, the before-and after values were virtually the same.
I added a sheet of perforated metal over that open area. Ditto.
The impedance curve does change shape / shift downwards in frequency marginally, and Qts does go up a fraction - but the change was less than the variation I measured between the set of 12" drivers.
In short: a short slot does very-close-to-nothing to the Thiele Small parameters.
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