Thiele-Small importance

[bigswede]

Hey.

I wanna build a bass reflex sub for my JWS woofer with Vas 122.37, and I´m using JBL Speakershop for calculating the box dimensions. But how important are the Thiele-Small parameters for the actual sound?? (I have entered more than just the Vas parameter into the program...)

JBL speakershop tells me that the optimum vented box is 216 liters big, and has a -3 dB point at 23 Hz. If I want a 100 liter vented box the program tells me that the -3 dB point will be at 36 Hz. The program also tells me that the optimum sealed box 124 liters big will have its -3 dB point at 50 Hz.

Can the difference really be that big? That my 12" woofer will have a -3 dB point at 50 Hz with an optimum sealed box???

[schizeckinosy]

Sure, those numbers sound perfectly reasonable. Remember that with a sealed box, there is plenty of useable bass below the f3 point, but with a vented, it mostly dissapears because the vent is not in tune for the lower frequencies and it unloads the woofer.

Cheers, Dan

[bigswede]

Ok. Thanks.

So, what would you recommend; building a sealed or vented box? I mean, 216 liters is quite big for a small living room (and my wife having a thing against big speakers)...

Do the advantage of a vented box justify the bigger size? After all my wife is not impossible to persuade :-)

[schizeckinosy]

You have to weight the tradeoffs. You have already seen the size difference. Vented boxes handle more power above the tuning frequency, but really low bass can damage them. I think they have measurably less distortion as well, and a flatter response, again at or above the port frequency. But, the port itself adds some distortion.

The sealed box has good all-around performance, and you don't have to worry too much about it unloading and killing itself. It also is much more forgiving of size differences. If I was going for a sub, I'd tend toward the sealed. For satellites, I'd go with whatever, leaning toward vented (my next pair are vented), especially if you actively filter out the subbass, or don't overdrive them.

Good luck, Dan

[schizeckinosy]

Vented boxes are generally more efficient, but with a big enough amplifier, you shouldn't worry about it.

Dan