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Adjusting tuning frequency of square ported box

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outkast View Drop Down
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    Posted: 14 July 2016 at 3:59am
I have a 18inch reflex bass bin with a square port, dimensions shown in image below. Internal box dimensions are 565mm wide, 615mm high, and 624mm deep, giving a net internal volume of 195 litres (excluding volume of driver and port).

From my calculations the box has a tuning frequency of 42Hz. I am wondering if anyone can tell me how I might go about changing the frequency - I would like to bring it down to 36-38Hz. Everything I found online had solutions for round ports only. TIA.


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fatfreddiescat View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2016 at 7:41am
Make the port longer or decrease the area of the port - ie line the sidewalls with extra ply.
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Meat View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Meat Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2016 at 11:57am
Don't trust the calculations. We're talking about fine adjustment so you need to be doing impedance measurements. Measure it. Add a bit of length to the port (or close the port up a bit). Measure again.

Calculations can get you where you want to be within about 5Hz but for the last bit you really need to be measuring.
Don't test the champignon sound
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Post Options Post Options   Thanks (1) Thanks(1)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2016 at 11:59am
WinISD is pretty easy to use and will calculate a square or round port as required.

But what matters is the port area and length. So in theory a round port of equal area to a rectangular one of the same length will perform identically.

However in real life, it is never that simple. Different shapes have different drag effects on air movement, and distance from enclosure walls plays a factor. Rectangular ports often share a wall with the cabinet making that effect even more pronounced.
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