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The subwoofer is a Dayton UM15-22 using a SPA500DSP plate amp. I selected a sealed enclosure design, with the amp sitting outside the enclosure. Listening is about 50% music and 50% home theatre.

To start with, yes, I know that Dayton recommends a sealed enclosure cabinet of 3.1 cf (about 87 litres).

However, when I first put the numbers into WinISD – and assuming I have done this correctly – it comes out with a recommended enclosure size of 0.1952 m^3 (or 195 litres), at a Qtc of 0.704. As an aside, WinISD suggests that at 87 litres enclosure, the sub will have a Qtc of 0.884.

As a cross check, using the calculator on diyaudioandvideo, it comes out with a sealed box size of 171 litres, at a Qtc of 0.707.

Speakerboxlite suggested 127 litres.

A couple of other online calculators came in around the 180 litre mark (including two that appeared to be a copy of diyaudio).

After looking at the disagreeing numbers, I built a box with a target useful volume of 171 litres. Given the amount of bracing inside the box, its internal volume is probably closer to 155-160 litres. In addition, it has approximately 30 litres of fibreglass insulation.

From a sound perspective, I am happy with the result. I have been working through various types of music and it is interesting to note the differences in source material between my old subwoofer and the new subwoofer. The most pleasant surprise was Robbie Williams, Beyond The Sea from Finding Nemo soundtrack. That kick drum, while not loud, has amazing presence.

Anyway … I started to get concerned with my construction when playing Blade Runner 2049. While the sound was amazing and shaking the windows, the mount of driver excursion had me a little worried. By eye, I estimate that cone movement was around +12-14mm. And in the scene in Rogue One, when Darth Vader’s Devastator appears at the end of the move, the amp went “click” and the sound cut out for a sec. Whoops.

So, I went back into WinISD to look at the calculated cone excursion a bit more carefully. And this is where it starts to get interesting.

- For a volume of 195 litres, WinISD calculates the cone excursion will reach 19mm at 20Hz and 210W, with 103db, and about 106db at 25Hz.
- For a volume of 130 litres (my build of box 160 litres + 30 litres fibreglass), WinISD calculates the cone excursion will reach 19mm at 20Hz and 270W, with 103db, and about 106 db. (Note: I have assumed that insulation can simply be taken as a reduction is speaker volume.)
- For a volume of 87 litres (Dayton’s suggested box siz), WinISD calculates the cone excursion will reach 19mm at 25Hz and 410W, with 106db.

As an aside, WinISD calculations suggest that while the driver is rated at 600W RMS, the box size would have to be around 60 litres (Qtc of 0.998) to keep cone excursion within limits (at an impressive 117db at 60Hz).

So, in hindsight, I can see where I went wrong. In chasing the best theoretical Qtc, I overlooked driver excursion limits.

Getting back to my build, I see a couple of possible actions.

- Do nothing. However, respect the limits of cone excursion at 240W. Particularly for material that potentially contains subsonic material
- Install more fibreglass. Adding 30 litres (box volume of 100 litre) increases the power limit to 350W before cone excursion limit is reached. More importantly, it does flatten out the sub-20Hz response curve so there is less chance of a large subsonic signal causing driver excursion.

So, my questions are;

A) Do my calculations seem correct?

B) Will adding more fibreglass solve this problem? (I really want to avoid doing stuff like adding sand or closing off part of the box.)