The tools you need to tune your enclosure are a multimeter, a software-based frequency generator and a high-power (>5 watts) low impedance resistor, ideally between 4 to 8 ohms. Somthing like a 10W wirewound resistor would do just fine.
Tuning - checking the resonance frequency
At the resonance frequency, the impedance will either peak at its highest level (sealed enlosures), or dip to its lowest level (ported and bandpass enclosures). 6th order bandpass enclosures will have two resonance frequencies, one for each volume.
To determine the resonance frequency of the system, set the multimeter to current measurement, then hook it up in series with the subwoofer to your amplifier, then use the frequency generator to drive the subwoofer. At the resonance frequency, the meter will read the lowest result (sealed enclosure) or the highest result (ported and bandpass enclosures). In the case of the ported and bandpass enclosures, If the measured resonance frequency is not the same as the predicted resonance frequency, adjust the port lengths accordingly. In the case of the sealed enclosure, the resonance frequency can be adjusted by adding or removing stuffing material from the enclosure.
A more accurate version of the above method involves using a resistor in series with the subwoofer system, and connecting the multimeter (set to voltage mode) across the resistor. In this case, the meter will measure the lowest voltage at the resonance frequency of a sealed system, and the highest voltage at the resonance frequencies of the ported and bandpass systems.
Source:
The Box-Building FAQ
