A combination of the shorter (correct) port lengths, and acoustic foam (tall eggcrate) seems to have fixed the sound problems. Alvin burned out the fuses in the amp from having it too loud, but says the bass is nice and tight. I haven't heard it yet! We're going to glue the last pieces of foam (ran out of spray glue) and get some new fuses.

I also want to measure its actual frequency response curve to compare against the theoretical one.

Sonotube Subwoofer

I finally got another chance to work on the subwoofer that Alvin and I built years ago. When we originally built the enclosure we weren't able to test it very well, but Alvin has tested it and says it sounds pretty awful. I ran the numbers through WinISD Pro (using Wine of course!), and it seemed to confirm our existing design. However, it calculated a port length of 4 inches, and we had been using something like 8 inches before.

We haven't made any changes yet, but I came across a better design program, in the form of an Excel spreadsheet. It turns out that the frequency response curves we were using were for anechoic response only. While this would be fine for a recording studio, in a living room the curve will change. Our next step is to try out the sub and measure its actual response. Then, using the program, we can compensate for the room and find the flattest curve. We'd also like to find out what effect(s) the sonotube enclosure has vs. the standard rectangular enclosure.

Here are the Thiele/Small electromechanical parameters of the driver:
Name = Dayton PF460-8 18" Cast Frame Driver 4" VC, Parts Express #295-085
Dia = 18" (45.7 cm)
Qes = 0.36
Qms = 6.74
Qts = 0.342 (as calculated using Qes and Qms; spec sheet says 0.34)
Vas = 16.12 ft^3 (456.47 l)
Fs = 25 Hz
Re = 6.74 ohms
Znom = 8 ohms
Le = 2 mH
Xmax = 6 mm
SPL = 97 dB 1 W(2.83 V)/1 m
Pe = 500 W RMS, 700 W max

Design parameters:
Np (# of ports) = 2
Dv (port (vent) diameter) = 4" (10.16 cm), both ends flanged. A vent noise calculation recommends 7.67".
Lv (vent length) = 4". Overall length will be 5" including the flared ends (according to AeroPort instructions).
Vb (currently) = approx 317.1 liters. Need to re-measure, in case we did not account for the driver and ports.
Fb (WinISD) = 28.53 Hz