Verifying port tuning

[Triticum Agricolam]

Does anyone on here ever verify the tuning of a box after it has been built?

I've been using a Dayton DATS to verify the port tuning of some enclosures I've built. I've found that the port lengths numbers you get from Torres/WinISD/etc result in a lower tuning than desired. Its not significantly lower, but its enough to be measurable.

I was wondering if anyone else has verified tuning via impedance sweep, watching cone excursion, etc and what your findings have been.

[Joe X]

Temperature mostly but other factors as well will make results vary. Also building tolerances will introduce some error, Torres has port displacement calculation errors so I would leave that one out, it uses the JL Audio equation so better use that instead.

[Triticum Agricolam]

I've looked at a bunch of different equations and they all seem to give pretty similar results. As of right now I've been using the port length calculation show here: http://www.diysubwoofers.org/misc/portcal.htm

My problem with the JL Audio equation, and some of the others like it, is it doesn't include any end correction, so you end up with a result that's even longer/tuned lower.

[Triticum Agricolam]

I haven't considered the effect temperature would have before. It makes sense that colder temperatures would lower tuning, I'm going to have to experiment with that a little bit.

[Wicks]

I did a full impedance sweep on my sub box and found that I'm actually tuned to ~32Hz whereas on paper I'm tuned to about 37.

Check out my build log, posted the graphical plot of the sweep.

Very few calculators will be able to accurately take the vehicles cabin gain into effect.

There are MANY factors that will change your tuning number.

Windows down or up.

Doors open.

Number of people in the vehicle.

Temperature.

Etc. etc.

It's interesting to have an impedance meter and see how everything really affects your tuning.

The box calculators are handy but it takes a lot more to get to your true tuning number.

[Joe X]

That's way off Wicks there may be a calculation error there.

[Joe X]

The JL Audio equation does have an end correction dependent on the square root of the port area, it's the term containing 0.823 coefficient.

[Triticum Agricolam]

The JL Audio equation does have an end correction dependent on the square root of the port area, it's the term containing 0.823 coefficient.

Yeah, now that you pointed it out to me I see it. When I get a chance I'll put the JL equation into my spreadsheet and see how it compares to the one I have been using.

Wicks, I'm not surprised that you saw a difference of 5 Hz between your actual tuning and what you have on paper. I'm glad I'm not the only one doing impedance sweeps!

[Joe X]

A comparative study was made with 5 different tuning equations and if I remember right the difference was less than 2 Hz in all cases. Unhappily I don't have a link for it right now.

[Wicks]

That's way off Wicks there may be a calculation error there.

Everybody seemed to say that Torres was the best calculator so that's what I used.

What else do you suggest?

I was concerned that my tuning would end up higher then I'd wanted so I was pleasantly surprised to see that 32Hz number when I measured it in the car and got a real world number.