Port size calculations for 2x18" reflex sub

Ok, i think i got it. Assuming I've got it right about the partition (see last post), I entered the following info:

Edited by stx - 20 August 2014 at 8:26pm

Yes you're right about the partition, if there is a complete divide between the halves of the cabinet then treat them as 2 separate singles. Be aware that there will be some performance difference in real life if the ports terminate next to each other, but this is where the small differences between models and real world results come from. Effects due to ports finishing near other cabinet walls also make a difference.

As said by Chris the response is a bit peaky, a slightly lower frequency could get a flatter response and a little more LF extension. But there is not usually much point in getting obsessed with modelling the perfect result, port effects etc as mentioned will always create small differences and this is why you just take a good sensible guess, make it and measure it to see exactly what is going on. Once you've made a few bins and measured them you start to see trends and where the 'fudge factors' need to be inserted for factors like port length.

I agree with Chris, simulate a response you like, take 10% off the length, and build then measure (impedance sweep, cheap and quick).

Just a small point here about the "peaky" response. Come on lads, did you take notice of the scale?

Edited by TONY.A.S.S. - 21 August 2014 at 10:49am

Yh, tbh, i was kinda thinkin +/- 0.5db was good..... but i do see that if you get the figures right you can get the sim bang on the 0 line. But as i'm not building from scratch i don't have as much room to move. And i have no spec for the 'chinese' drivers i'm replacing.

Edited by stx - 21 August 2014 at 12:42am

Oh thats great because it seems that dropping the volume gets rid of that peak, and i haven't taken into account the vent volume, or the speaker basket and magnet. 

Hah, I didn't notice the scale at all. Yeah that's not really a peak to be concerned about then.

The tuning frequency is the point at which the port becomes a Helmholtz resonator. What this means is that at this frequency the air in the port is moving rapidly backwards and forwards and the driver is not moving much at all. Its like 2 resistors in parallel, except the resistance of the port is a function of frequency and it is at a minimum at the tuning frequency Most of your SPL is being contributed by the port at this frequency. Beneath it then the driver becomes unloaded and excursion rises quicker than it would for a sealed box. But you gain a bit of extra LF extension which people often prefer (and a larger group delay).

If you look at the impedance curve of a vented box you see two peaks. The port tuning frequency is the minimum value between these 2 peaks. Check your simulated impedance in WinISD.