Q: First off, I love the mag. I'm getting a Cerwin Vega Stroker Pro 15 and building a box to the recommended 3ft3 tuned to 25Hz. I used this port calculator I found online (http://www.psp-inc.com/psp-inc.com/cgi-bin/port_length_calculator.cgi) and came up with two 4" ports. The calculator says the length should be 33.2". Is that for both together? Would I have to half that for each or is that the length for each port?Thanks for your time,Kevin Davis
A: Yes, that would most likely be each port 4" in diameter and nearly 3' long. Think about that for a minute. Why would the port be so long? The alignment of the cabinet involves the speaker, port and box volume together as a three-legged chair (sorry for another analogy). If all the legs aren't the same general length, the chair is a mess. One leg can be a bit longer or shorter and the chair (or stool) will still stand, but you can't be far off.
When a cabinet is undersized, the ports usually become extra long to compensate as the design tries to reach a deep enough f3 point. This is the start of an unbalanced chair. If you want the ports to be shorter, your cabinet volume will increase - already at 3ft3. Maybe a non-optimum port is better than a cabinet the size of a Toyota. Sound quality will also suffer as the amount of ripple increases, since the port tuning frequency is getting too far away from the box tuning frequency to properly blend.
Ports are proportional. If you were to increase to a 6" port, it would be even longer, while a 3" or 2" port would be shorter. Here's where another common mistake is made. If you think you can save a few bucks by using a shorter port know that as the port diameter get smaller the velocity of the air vibrating in the port increases to the point where you can hear a "ripping" sound as the maximum velocity is exceeded.
I generally get uncomfortable when the port length is more than four times the diameter, so a 16" long port would be acceptable in my books. This design is over double my personal limit. But that may be all right for a speaker that was originally designed to recreate an earthquake in the '70s movie Earthquake, which was recorded in "Sensurround." This wasn't really surround sound, but was a separate bass track recorded to shake the theater apart during the scary scenes. It was later used in the war movie Midway.
Now, here's something to think about: I built one of these cabinets around 15 years ago to use in the classroom as a demonstration. With every new group of students, I would plug the Stroker directly into the AC wall socket, shaking the dust out of the rafters and generally startling the rest of the staff. It was great fun! This cabinet works for what it was designed to do - create massive amounts of sound and cause everyone's jaw to drop. It was never meant to sound good, and it sure lived up to that reputation.
I haven't had a chance to look at the port calculator you found, but I recall similar numbers from Cerwin Vega. When dealing with really unusual speakers, or "mission specific" speakers, it's a good idea to follow the manufacturer's recommendations for cabinet designs. After all, they've worked out the best options and built proof of concept units to back up their assumptions.