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Subsonic Filters attentuate signals at a given frequency point to keep the speaker within it's mechanical limits. With a ported enclosure, below the tuning point, the excursion of the speaker rises dramatically.
Contributed by Stefan Hinote
his article is intended for those that understand the basic characteristics of different enclosure alignments say sealed and ported, know how to glue&screw; wood together, but need help calculating the volume, port size, port tuning, and various displacements. For this example I’ll be using a single 12″ SSA Dcon. Information can be found here: www.soundsolutionsaudio.com I’ll be shooting for around 2.5ft^3 NET volume, and 30hz port tuning. My space restrictions for the enclosure are: 30″ width, 16″ depth, and 15″ height in the vehicle. On the SSA dcon page, the displacement of the driver is listed as: 0.14ft^3. I’ll be using 3/4″ MDF for the panels, and a double baffle for the speaker mounting panel. With the measurements: 30″ width, 16″ depth, and 15″ height, you must subtract twice the wood thickness from each length, and account for the double baffle in order calculate the interior volume. The double baffle will be the 30″ x 15″ piece. This means an additional 0.75″ will be removed the the 16″ depth So the interior measurements are: (30 – 0.75*2)(16 – 0.75*2 – 0.75)(15 – 0.75*2) = 28.5*13.75*13.5 = 3.0615234375 in^3 Convert to cubic feet by dividing by 12^3 or 1728. 3.0615234375 / 1728 = 3.0615234375 ft^3 We’ll round that volume to 3.06 ft^3, now subtract the displacement of the driver, which is 0.14 ft^3 in our case. 3.06 – 0.14 = 2.92 ft^3 Next we need to find out how big of a port we should use. If the port is too small, the velocity of the air in the port being excited will be too high, and it’ll cause “port noise” and even “port compression.” Imagine stepping on a hose, the speed of the water increases, and only so much water can flow through. There is much debate as to how much vent speed/port speed (velocity of air from port) is acceptable, as a higher vent speed in an enclosure for a subwoofer may not be noticed, but for some midbass cabinet enclosures it may. For this article, I’m going to shoot for under 20 m/s For calculating vent mach please see article: blog.stefanhinote.com or use calculator: portsgalore.stefanhinote.com By using the above link(s), I’ve found a 4″ x 6″ port yielded 24 in^2 port area with ~17 m/s vent speed with 30hz tuning and approximately 300watts for power. Should be perfect. Now we need to calculate the port length to attain 30hz tuning. blog.stefanhinote.com Notice the Vb is in cubic inches, so our 2.92 ft^3 will be multiplied by 1728 to convert into cubic inches. 5045.76 in^3. The formula also asks for the port in radius, so think of our 24in^2 port area as a circle. Since the area of a circle is πr^2, we can find our “r” by: dividing 24 by π, and square rooting. I came up with 2.7639 for r. Plug in 2.7639 for r, 5045.76 for Vb, and 30 for Fb, and I got 20.65 in for port length. Now here’s the problem. The port area is 24 in^2, and the length is 20.65in, thus the volume of the port is: 24*20.65/1728 = 0.286ft^3. That means the net volume of the enclosure just went from 2.92 ft^3 to 2.63 ft^3. Since the tuning of the enclosure was based on the net volume, the tuning must have changed since the port displaced 0.286ft^3 volume. Depending on the size of the port, the displacement may not enough be enough to drastically change tuning, but it may… If you write down 20.65 in for a target port length, and start adjusting the tuning and volume inputted into the formula until the port length matches the original 20.65 then you just accounted for port displacement, and your golden. When adjusting the tuning and volume, the tuning would have to be raised, and the volume would be reduced. Alternatively, you can use this calculator that automatically adjusts the values until correct tuning is achieved: portsgalore.stefanhinote.com Be sure to change port location if your using a port external of the enclosure. If your using circular ports, you can also use: portsgalore.stefanhinote.com So with the above calculator, I plugged in 4″ port width, 6″ port height, 2.92ft^3 volume, and 30hz tuning. I got 22.65 in for port length. and 2.61 ft^3 enclosure volume after port displacement. External dimensions of enclosure: 30″ width x 16″ depth x 15″ height double baffle for speaker mounting panel (30″ x 15″) Port: 4″ width x 6″ height, and 22.65″ for length Tuning: 30.5hz Net volume: 2.61 ft^3 Driver facing rear, and port on the side.
Contributed by Stefan Hinote