Bass reflex enclosure port fine tuning/Wallin jig

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dch53

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Joined 2015
Hi. I've just completed a pair of vented compound loaded bass boxes.

The port diameter and length are as recommended by the design software but I'd now like to check the port resonant frequency and fine-tune the length.

I've tried rice on the cone and eye-balling cone excursion with frequency but I'm not confident in the results so I'd like to try measuring the impedance vs frequency.

I've seen references to impedance measurement and the Wallin jig but the original link to the jig appears to be broken.

Does anyone have an updated link explaining how the jig is used please?

Thanks.
 
vacuphile

vacuphile

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Joined 2011
Just connect your speaker through a 10 Ohm or so resistor (not at all critical) to your amp and measure the voltage across it. Your box tuning is the frequency where this voltage is the highest.

This should be the same frequency you found with the rice test, which is an excellent way to do it in and by itself.

Just remember that with a vented enclosure, the driver cone moves the least at the tuning frequency. This is what you see with the rice grain test. This translates into the least back EMF and thus the lowest impedance for the driver. This is what you measure by placing a resistor in series. The larger the resistor, the more profound the effect will be.
 
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Speaky

Member
Joined 2015
Wallin stopped doing his jig quite some time ago. Vacuphile pretty much nails it, though. The main difference with the Wallin jig was calibrated resistor selection to improve the voltage divider precision.

If you're just using it to find resonance, the resistor value is irrelevant with a good quality amp.

[I invented the jig and can dig up a schematic if you really want]

Mark
 
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dch53

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Joined 2015
planet10 said:
Do keep in mind that as the wick is turned up )or under the dyanmics of the music) most loudpeakers T/S parameters will change so there is no such thing as perfect tuning, just a compromise.

dave
Click to expand...

Thanks for that. The valley in the predicted impedance vs frequency response was quite broad so I probably won't be able to identify the port resonance exactly anyway.
 
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AndrewT

R.I.P.
Joined 2004
before you change the port, measure the two impedance maxima either side of the port resonance minimum.

Some speaker designers said that the port tuning that gave equal maxima led to best Bass sound.

I tried that but found too much bass for my taste in what "natural" voice and other bass sounds (like a car door slamming, or a truck ticking over on film sound tracks) sounded like.
 
GM

GM

Member
Joined 2003
AndrewT said:
Some speaker designers said that the port tuning that gave equal maxima led to best Bass sound.
Click to expand...

This dates from the time when to get any [mid]bass to speak of required the cab be tuned to the driver's in-cab Fs due to the amp's high output impedance and why they often sounded 'boomy' if the port wasn't critically damped. Now that the driver's in-box Qts dominates, you can tune it however you like with normally only minimal port damping, if any.

GM
 
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