Two port(s) or not to port(s).

[Jennice]

Lousy re-phrasing, but I need attention and help on port math.

Here's the deal. I've played with WinISD, using a Peerless SLS-8 in a ported box.

Due to port length at a decent diamater (7 cm), I thought os using 2 smaller ports (i.e., 2 ports half the diameter).

Now, WinISD makes the port length much shorter when using 2 ports than when using just one (same size). I thought it was a matter of port length/diameter that determined the resonance, but this doesn't seem to be the case.

Can anyone please enlighten me?

Jennice

[MaVo]

maybe you confused port diameter and area? half the diameter isnt half the area.

[theAnonymous1]

Originally posted by MaVo
maybe you confused port diameter and area? half the diameter isnt half the area.

Exactly. The area of a 7cm port is appoximately 38.48cm^2. Two 3.5cm ports would equal 19.24cm^2.

[Jennice]

I didn't confuse the two things. I'm just not aware of the factors at play (no pun intended)

Ok... If I understand you guys correctly, the resonancde is determined by port area and -length (and a handful of math ).

Now, then... Is it the total port area, or each single port area (when using two ports) that determines the port length?

Jennice

[AndrewT]

yes,
the total port area is used to determine the box resonant frequency (Fb).

[Collo]

also note that when using multiple ports, the length WinISD tells you is for EACH port, not the total of all ports....

[Jennice]

Thanks for the responses, folks!

I was looking into the option of more smaller ports (rather than one big port), as I was thinking it would reduce port length.

Seemingly, not everything has an easy solution...

Jennice

[theAnonymous1]

To reduce the port length you either have to increase the box volume or decrease the port area.

There is one last option; a passive radiator. That's a pretty big can-o-worms though if you don't yet completely comprehend how a standard port works.

[Jennice]

Right... I think I know the port basics, but another issue comes up:

Where should the port be? It seems that the vast majority of speakers have the ports at the rear, but why?

It's not handy if the speaker is near a wall, and the low frequency radiation is not really directive.

I have heard some claim that a port near the woofer would (partially) cancel the effect, but considering the wavelength of low frequencies, would location really matter?

Others claim it's rear mounted to make port noise (whisteling) less obvious, but isn't that a matter of doing the math?

Does anyone know if WinISD's port limit suggestion is any good? (it turns red at small port sizes)

Please comment, folks.

[Ron E]

Port location doesn't matter much for performance, but closer to the driver is theoretically better- if within 1/10 wavelength, it probably doesn't matter.

Front or back placement only affects out of band noise. Regardless of whether the port is designed correctly (flared, large enough) the port will have a train of organ pipe resonances.

I don't use WinISD, so I can't comment - other than that "WinISD pro alpha" is in general a better program than "WinISD beta".

If you have Excel 2000+, try Unibox, a much more reliable program.