Building Your Own Speaker Enclosures - Part 2

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Part 2 in a 3 part series - covering enclosure dimensions, tuning the cabinet and port size.

Before we start figuring these aspects of your enclosure, you will need the following information about your speakers...the Q(or, Qts) as you needed to figure the enclosure volume, and the "Free air resonance" (usually listed as "fo" or "fs").

With the table below, you can get the tuning factor from your Q rating, then multiply the tuning factor by the "Free air resonance" of the speaker. The result of that calculation will be the optimal tuning frequency of the enclosure.

QTuning Factor
.21.6
.251.34
.31.17
.351.05
.4.95
.45.86
.5.77
.55.7
.6.64

Now that you know the optimal tuning frequency, you can check the tables below to figure the length of your 2-inch or 3-inch port. In bigger enclosures, such as for sound reinforcement, or big home installations, I highly recommend using the 3-inch port to increase the air flow of the speaker, thereby increasing the efficiency and performance.

Port length for a 2" diameter port
Volume
Cu. Ft.
Tuning Frequency
20 Hz 25 Hz 30 Hz 35 Hz 40 Hz 45 Hz 50 Hz 60 Hz 70 Hz 80 Hz 90 Hz
0.50 7 5 3/8 3 1/4 2 1 1/4
0.75 5 5/8 4 3 1 5/8 7/8
1.00 5 1/2 3 7/8 2 3/4 1 7/8 7/8
1.25 6 4 2 3/4 1 7/8 1 1/4
1.50 7 5/8 4 3/4 3 1/8 2 1 3/8 3/4
1.75 6 1/4 4 2 1/2 1 1/2 1
2.00 5 3/8 3 1/4 2 1 1/4
2.50 7 4 2 1/4 1 1/4
3.00 5 5/8 3 1 5/8 7/8
3.50 4 5/8 2 1/2 1 1/4
4.00 3 7/8 2 1
5.00 2 3/4

Port length for a 3" diameter port
Volume
Cu. Ft.
Tuning Frequency
20 Hz 25 Hz 30 Hz 35 Hz 40 Hz 45 Hz 50 Hz 60 Hz 70 Hz 80 Hz 90 Hz
0.50 8 3/8 5 3/8 3 3/4 2 1/2
0.75 10 1/4 7 7/8 4 3/4 3 1 3/4 7/8
1.00 9 5/8 7 1/8 5 3/8 3 1 5/8 3/4
1.25 7 1/4 5 1/4 3 7/8 2 7/8
1.50 8 1/8 5 3/4 4 2 7/8 1 1/4
1.75 6 5/8 4 1/2 3 1/8 2 1/8 3/4
2.00 8 3/8 5 1/2 3 3/4 2 1/2 1 5/8
2.50 6 1/4 4 2 1/2 1 1/2 3/4
3.00 8 4 3/4 3 1 3/4 7/8
3.50 6 1/2 3 3/4 2 1/4 1 1/4
4.00 9 5/8 5 3/8 3 1 5/8 3/4
5.00 7 1/4 3 7/8 2 7/8
6.00 5 3/4 2 7/8 1 1/4
7.00 4 1/2 2 1/8 3/4
8.00 2 3/4 1 5/8
10.00 2 1/4 7/8
12.00 1 3/4

Now, you should have your port dimensions all figured out, so, what do you make it out of and where do you mount it?

A port can be made from wood, plastic, paper, or metal...theoretically. In the real world I highly recommend against metal, ESPECIALLY in high powered applications due to the vibration factor. A vibrating port can be really irritating to listen to. The best material I have used is typical plastic plumbing pipe available at any hardware store. It commonly comes in 2 and 3 inch sizes and and is easy to work with.

Where to mount the port is a relative question, I have worked with speakers that have them on the front, back, bottom, and sides, front and back being the most common. If you have the room on your face plate I recommend putting it there, trying to get the port no closer to the woofer than the port is in diameter. And be sure to glue it in or under-cut the hole a little so it has to be squeezed in very tightly to eliminate vibration.

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