How to Tune Bass Reflex Speakersby Dan Eash
Bass reflex speaker enclosures contain vents to prevent rear traveling sound waves from interfering with forward traveling waves. Since bass frequencies -- and to a lesser degree midrange frequencies -- are omnidirectional, most sound waves travel forward from the speakers, but some travel backwards then forwards after bouncing off the back of the enclosure. The bass reflex speaker port tunes reflected sound waves to a frequency that extends the low-end response.
Choose the tuning frequency. If you want the frequency response of your speakers to peak at a frequency that's ideal for high volume playback, select a tuning frequency of 45 Hz or higher. The sound quality will suffer but your speakers will sound a lot louder. For better sound quality, extend and boost your bass and keep the rest of the frequencies flat by choosing a tuning frequency of 25 Hz or lower. If you want a little of both, good sound and high output, go for a tuning frequency of 33 Hz.
Determine the port location. The formula used to determine port placement specifies that the distance between your ports; the speakers in your enclosure must be within a quarter wavelength of your tuning frequency. If your tuning frequency is 45 Hz, its wavelength is one tenth of that -- approximately 5 feet -- and one quarter of 5 is 1.25 feet. A lower frequency gives you less room to work with. Check this distance with your tape measure to find out where to put the port. As long as you stay within the zone dictated by this formula, you can put the port anywhere on the enclosure without affecting frequency response. Mark the center of the port's location with an X. Make sure it's in the same spot on both enclosures.
Determine your port diameter. Look up the bass speaker size and Xmax rating in the user manual. Go to an online tuning tool and enter these numbers in the corresponding fields to get the minimum recommended diameter for your port. Use a tape measure to adjust the pencil compass to this port diameter and draw a circle on the enclosures with the X marks at the center. Drill a hole on these circles as a starting point for your jigsaw and then cut out the circles. Find or buy a PVC or cardboard tube with this diameter.
Determine the port length. Measure the internal height, width and depth of your enclosure with your tape measure and multiply the results to calculate the enclosure volume. Enter the tuning frequency and port diameter in the applicable fields in the onlne calculator to get the recommended port length.
Measure and mark thetubes to this length and cut them on the miter saw. To finish the tuning, push your tubes into the holes until they're flush with the front of your enclosures.