Quick Port Question

I have one port tuned to 20Hz of length 45cm and opening 2 x 30cm.
R u sure?
For 20hz & port 45 cm x 30 cm, the box size will be 793.5 litres.
Is this ok?
Then if u want to put two ports of 45 cm length & 30 cm opening,
the length will be 111.945 cms. each, in one software or in other it is 115.53. Marginal difference. This calculation is based on above cabinate size. Or just give here .Qts, Fs, & Vas. for proper results.
I just used WinISD and I get 101.7cm for two equal ports of 2cm x 30cm
Or one port of 100.9 - 4cm x 30cm which has the added advantage of having less friction surface and giving less noise I would think.

Be carefull with WinISD! Simulate, for example, with 20 ports and a realistic length (take 100cm in this case) you will see the ports will get very small, yet the 'vent mach' seems to be very good. It's not in real live!
That's why you should use one port only in WinISD.
I already mailed to them a long time ago, no response and no update.
(This is WinISD beta)

Fs 23.5Hz

U r trying to go below resonance frequency.
I don't think this is a good thinking.
U should minimum aim at 27~ 30hz with this type of speaker.
If u want to go to 20hz, then Fs should be minimum 16 ~ 18 hz.
Fs lower the better.

I designed one in winspeakerz,
the box size is 32.7 litres.
The vent size is 95 mm x 767mm .
The port frequency is 27.9hz.
We can little improve upon this but linearity will be disturbed.

With winisd I got
Box is 42.31 litres
Port frequency is 26.4 litres
Port length 95 mm x 650 mm.

Ron E

2002-06-27 10:41 pm
Nex time you want design help, please post all parameters and your intended box size and tuning (if necessary). I assume the following specs are for your woofer?

Impedance:4 Ohms
Power:300 Watts
SPL 1W/1M 86dB
Le:1.30 mH
Vas:61.6 Litres
BL:17.46 T/M
Sd:468.7 cm2

According to the manufacturer, it has EXCELEENT <sic> build quality. ;) They also claim LARGE Xmax and 7.5mm is really about 1/2 what 12" subs are usually spec'd at days.

Calculated sensitivity is:
88.03 dB/2.83V
84.68 dB/1W

If you want 20Hz F3, you can get it by making an 80 liter box and tuning it to 20Hz with a 4" diameter x 27" long flared port. This will have about half the power handling above tuning of a ~42 liter box tuned to 26Hz with a 4" diameter x 31" long flared port. Louder beats deeper any day, believe me. The smaller box will handle 225 watts above tuning, the larger one only 125 before Xmax is reached. This is based on 8 ohm amplifier ratings since my software actually calculates volts to Xmax.

Which you choose depends on your expectations. Because your woofer has a relatively small Vas (and resultant Vb) it can be a challenge to tune to a low frequency and be able to fit a vent in the box. If it doesn't fit, you must use a Passive radiator.

I'd use a PR and the smaller box, personally. An F3 of 27 Hz is not bad at all.