This is a sweet design, as well as being super power efficient! Tho I think that I will need some more power before reaching xmax. Thank you so much for the insight!
Below is a simulation for the 21SW152 in a vented-box enclosure with Vb = 350 litres, tuned to Fb=19Hz. This design also includes parametric EQ consisting of −1.7dB at 52Hz with a Q=1.1. The low-frequency cut-off is F3=16.8Hz, which is commendably low, and makes this enclosure worthy of being called a subwoofer. I have used two slotted vents, each of area Av=220cm^2 and length Lv=62.2cm. The enclosure produces 115dB maximum SPL for a peak power input of 400 watts. At a nominal input power level of 300W re 8Ω, Xmax for the driver is only exceeded below 16Hz. Adding a 12Hz second-order Butterworth high-pass filter can keep the driver displacement in check below Fb, without affecting the response in the passband too much.
It seems things are not fully sorted out, but I would highly recommend LaVoce San214.50 over 21SW115, and San215.30 over 21SW152. 21SW152 is a lovely driver, but look at its coil overhang length. It is barely a subwoofer driver, and for fair distortion outcomes, it will be excursion limited to 13-ish mm. LaVoces are fine to 15-16mm for lower price, often with stronger motor being happier in smaller enclosures. I had B&C 21DS115 in ~130l, and I think those LaVoce's will be quite happy in ~150l net box volume. All of them. 21SW115 not that much.
Below is a simulation for the LaVoce SAN214.50 21-inch driver mounted in a vented-box enclosure of net volume Vb=150litres, tuned to Fb=39Hz. This is a fairly standard alignment, which trades off passband efficiency for low-frequency extension. It would seem that this is not really "subwoofer" performance.
Of course, we can get more low-frequency extension by detuning the enclosure to Fb=25Hz and adding a 2nd-order high-pass peaking filter set to 22Hz and Q=2.00. This provides +6dB of boost at 22Hz. The response is much more subwoofer-like, with F3=22.1Hz, which is way better than the previous F3=40.6Hz. Of course, the penalty we pay is that the maximum SPL in the passband has been reduced.
Of course, we can get more low-frequency extension by detuning the enclosure to Fb=25Hz and adding a 2nd-order high-pass peaking filter set to 22Hz and Q=2.00. This provides +6dB of boost at 22Hz. The response is much more subwoofer-like, with F3=22.1Hz, which is way better than the previous F3=40.6Hz. Of course, the penalty we pay is that the maximum SPL in the passband has been reduced.
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