Calculating the tuning effects of a down firing port

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Hi,

I am retrofitting an existing enclosure, 23l or 1400 cuin, .7 cuft (approximate numbers) with a DCS205-4 (8" sub ). After running winisd is clear that the unit has to be down-firing to keep the port air velocity under control. Outside dimensions of the box are 15 x 11 x 11 " (38 x 33 x 33 cm aprox.)

Due to architectural constraints the sub will point forward and the port down.

Recommended winisd tuning is at 33.6 Hz. Resonance frequency of the driver is 31 Hz

According to winisd, if I increase the tuning to 39 Hz, I can use 3" (7.6 cm) x 14 " (36 cm) port.

The bottom of the cabinet is 10" x 12" (25cm x 31 cm) with 2" legs (5 cm). 3" ( 7.6 cm) downfire port with a flare be about 3/4" ( 2 cm). I understand that the down-fire will change the tuning frequency.

It seems that Polk audio had some spreadsheet to calculate the value (powerport spreadsheet) but it seems that it is not available anymore.

Since the tuning frequency will go down, the ports will need to be adjusted in length: e.g. if the frequency drops by 10 Hz, that has to be input in winisd as the new target tuning frequency so the port length can be adjusted).

My objective is to know how far the tuning will get (it seems that other guesstimates set the change up to 15 Hz down) so I can go back to winisd and calculate the length of the adjusted ports.

How can I calculate the change of the tuning frequency that a down firing port creates?

Thanks,Francis.
 
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You can correctly simulate down-firing ports using Hornresp. Hint: think about the port cross-sectional area and length as it expands towards the bottom edges of the cabinet.

Modelling it with the driver down-firing too would probably require a tapped horn layout, since the driver and port are combining before reaching the outside world.

Chris
 
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