Reducing port area to lower Fb?

I've inherited two empty subwoofer cabinets (from the Tannoy VXS18 DR) that I'd like to use with the BMS 18N860 woofer.

BMS recommends, for the 18N860. a 200 L cabinet tuned to 26 Hz. I've made such a cabinet before and it works well.

Each VSX18-DR cabinet has a volume of 206 litres and has two 16" long triangular ports, each with a port area of 40 sq. in.

One friend did some calculations for me and reported that those specs represent a box tuning frequency of 34 Hz.
He added that blocking one port completely would lower the Fb to 19-20 Hz, much lower than I want.

Can anyone suggest a way to calculate how much to reduce the port area (not enough cabinet room to lengthen the ports) to achieve an Fb of 26-28 Hz?
Any tips greatly appreciated!

Thanks! JT
Port sharing 2 walls K=1.728

Cabinet Volume 206 liters

triangle port x2
Length 16" = 40.64 cm
Area 40 in2 = 258.06 cm2

estimated Fb = 32.3 Hz

according to my calculator. Virtuix Cad
Difference between calculators common because
different formulas and or same formula but different constant
for Speed of sound and assumed temperature.


With set length of 2 ports at 40.64 cm

Port Area 258 cm2 = 32 Hz

Reducing port Area

180 cm2 = 28 Hz
150 cm2 = 26 Hz

high end of scale 26.9Hz be 164 cm2
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Easiest thing to do on first blush is to damp the vents until aperiodic.

That driver works quite well, especially with room gain — in 206 litres. If you don’t have so much gain, just blocking one of th events gives a decent alignment (if 2 is 34 hz, 17 Hz tuning). 25 Hz is as high as i’d consider for vented.

But getting the minimum aount of bass ripple thruout the room, one needs to carefully place the woofers in the room. EQ is still often required. Sealed is the easiest and safest to EQ, tuned low enuff they are unlikely to unload is OK.aperiodic in-between.


Extending the length of the ports did occur to me. However, it appears that to achieve an Fb of 26 Hz, the port lengths would have to be extended from 16" to 36", not a realistic option. . .
Also, I'll be using these subwoofers in a home audio system, not for PA. . .
1. Install driver in the box
2. Wire a multimeter in series with the driver and an amplifier and set it to read AC current.
3. Using a tone generator (e.g. REW's free tone generator), hook it up to the amplifier and vary the frequencies sent to the driver at the same volume setting until the multimeter shows a maximum value. That will be Fb.
4. Alternatively, wire a small resistor (like 4 ohms) in series with the driver, and using a multimeter, measure the voltage across the resistor as you vary the frequency sent by the tone generator. It will hit a maximum value at Fb.

Of course if you have DATs or an impedance measuring jig, you can just use that to determine Fb. It would be at the lowest point between the two impedance peaks.
Can anyone suggest a way to calculate how much to reduce the port area (not enough cabinet room to lengthen the ports) to achieve an Fb of 26-28 Hz?

You could extend the ports with an adjustable right angle "L" bracket without reducing the port area. The bracket could be made to with the interior open end equal to the port area when extended towards the back of the cabinet, the slot port's proximity to the cabinet back will decrease tuning more than the equivalent length will indicate with most (all?) predictive software. As depicted, when the "L" bracket extender is about 2" from the back of the cabinet, port area would be about 40 square inches, same as you say the triangular port area is. From previous experiments using a similar approach, my guess is the Fb will drop to near 28Hz at that point.
Right angle port extender %22L%22 bracket.png

The "L" bracket could be moved from even with the end of the 16" deep port to very near the back, one screw per unit should be enough to hold in place for determining tuning, then add more screws and caulk the joints when the desired Fb has been achieved.
Fb28, FB37.png

The chart above is a 210 liter (approx net) cabinet using a 7.25 x7.25 x 9.75 deep port with a 13.25 diameter round flange on the inside, resulting in a 37 Hz Fb, the same port/flange at 12” deep (within a few inches of the cabinet back) dropped it to 28Hz Fb.
The interior port flange proximity to the cabinet back also appears to have eliminated the port resonances near 180Hz & 400Hz, had not noticed that feature before...

You can also determine Fb visually using a tone generator, the cone excursion will be be at minima at the impedance minima, a dot with a white or silver ink pen (Sharpie marker) makes the peak to peak excursion easy to see.

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