Cab building question about shelf port length...

[Bass Unique]

Bass Unique, yes, I did not consider that the effect of air friction could be strongly non-linear .

About the method to tune the box, at the box tuning freq. Z is minimum (althoug it is at or near the peak of the speaker in free air). And the location of min. and max. impedances is not changed by the fact I drive the loudspeaker with finite impedance, provided it is not reactive. Peaks are the order of tens of ohms, no more. The 10K of my source reduces marginally the "Q" in peaks, but must have zero effect in location of peaks (or valleys in the case of port tuning).

Thanks!

ok - although its below 10 ohms potentially up to 100 Ohms, then this means the ratio between source impedance and driven impedance varies typically between 100 and 1000. This order of magnitude through your measurement would affect the Q of the measurement slightly.

Also note, for a simple reflex box, that the driver box resonance would show as a relatively sharp peak (maximum back EMF from speaker and cone movement) and the port box resonance is a dip but much smoother and wider than the peak (reduced cone movement so more current).

Agreed, within small displacements, the tuning would not shift dramatically, especially if you are fairly close. I have been flaring ports and often get away with 30 m/s without excessive noise. (without flaring, less than 20 m/s).

 

[parapentep70]

I found this TB thread. Very interesting the "sugar test", imagination and common sense go a long way in engineering!

Will33, I think with very large port area shelf ports using the walls of the cab still need significant correction. I think the 66% (or 1.5x) factor works well as a first approximation. At least it worked in a few designs I analyzed.

The additional correction for my narrow ports was just below 10%, and I am not sure it is exclusively due to the narrow ports, there are many factors involved.

 

[Bass Unique]

I found this TB thread. Very interesting the "sugar test", imagination and common sense go a long way in engineering!

Will33, I think with very large port area shelf ports using the walls of the cab still need significant correction. I think the 66% (or 1.5x) factor works well as a first approximation. At least it worked in a few designs I analyzed.

The additional correction for my narrow ports was just below 10%, and I am not sure it is exclusively due to the narrow ports, there are many factors involved.

In addition to the shape, there is an "end effect" which is where the air just beyond the port (inside and out) is coupled to the air considered to be inside the port. This means the actual physical port length is shorter than the working port length.

For a round port, the effect is more predictable and generally considered to extend the physical length between 0.5 and 1.5 times the port diameter (various technical papers give different results). Shorter ports with greater wall friction will increase this effect as would shelf ports with a large aspect ratio. Anything more than around 6:1 in height/width ratio makes this difficult to predict so the actual working length will depend on a number of factors:

1) The shape and aspect ratio of the port width / height

2) The physical length to width aspect ratio of the port (minimises mis-tuning if port is longer and has port dividers)

3) The flare of the port - larger ports have little effect, smaller ones can increase the coupling effect.

4) If the port is bent / curved etc (with smooth corners) - this also lowers the Q of the port slightly widening the bandwidth... thats for another thread...

 

[T_Bone_TL]

4) If the port is bent / curved etc (with smooth corners) - this also lowers the Q of the port slightly widening the bandwidth... thats for another thread...

Zombie thread, arise! Did this other thread ever happen? I can't find it if it did, and this is an area I've become VERY interested in for practical reasons - a broader port response would do wonders for the power handling / Xmax issues of my speaker rehoming project. I'm ready to boil some PVC pipe and coil it if it will do that...