Join Date: Dec 2011
Feedback Score: 7 reviews
Bass is about Sd, which is the effective piston diameter of a driver.
A 6.5 can get a little bit of air moving. Maybe even enough to get perceptible bass in a small room (as Z has noted). Actually, I know they can. I've built speakers like that. They're in my living room, and they sound pretty good (I used the now discontinued Vifa M18WO as my midbass).
For actual subwoofer output, from 50Hz on down to single digits, you need to move more air, and move it further. 8s are a good place to start for a room in your house. 10s are better. 12s are nice. Multiple 12s are really great. multiple 12s with big heavy passive radiators are awesome. My subs at home are quad long throw 12s (2 per box) (59mmX-max) with 2.5kg PRs, in boxes tuned to about 7Hz. They rattle the walls enough to make the drywall crack in the corners. I'm getting good at fixing it.
In a car, the rules are different. There's road noise, which goes up almost exponentially when you increase speed. There's also cabin gain, which gives you north of 6dB per octave starting around 100Hz and increasing as you go down in frequency. Cabin gain (the result of functionally being inside the outer chamber of a 4th order isobaric bandpass subwoofer) is what lets you get decent bass in a car with a single 10 or 12" woofer in the trunk powered by a few hundred watts.
If you want really good, tight, responsive, bass in a car, you want a sealed subwoofer, or subwoofers, with large Xmax, a flat BL curve (maybe something based on XBL^2 motor technology, or at the least, a very underhung voice coil), and Thiel-Small parameters that make it happy in 1-2cuft with a Q around 0.6. Tune the box so it matches the cabin gain in your car (in my 4dr, the gain is a little more than 6dB/octave and starts at 120Hz, and has a big bump at 58, and a big null at 29), and feed it 300-500W. Use a parametric EQ to tweak the peaks. You can't fix dips, those are nulls and are an acoustic issue. Move the sub around in the car to take care of them.