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Old 24th December 2011, 02:40 AM   #11
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Quote:
Originally Posted by j.michael droke View Post
Hi there RV: Could you elaborate on the effect and procedure for "derating the port"? regards, ...Michael
to start a definition: Derate: transitive verb: to lower the rated capability of (as electrical or mechanical apparatus) because of deterioration or inadequacy.

simple answer: In the case of a Helmholtz neck ie. port, you have airspeed vs. surface friction. Port output is limited by the free air (non turbulent) volume of the port. This is because the turbulent air (boundary layer) along the surface of the port walls becomes thicker the faster the air moves in relation to the surface. A high ratio say 1:16 (height:width) port has much more surface area than a low ratio port.

really simple answer: A narrow wide port will close up (compress/detune) at lower airspeed velocities than a square or round one.

revb.
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Old 24th December 2011, 02:12 PM   #12
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Sorry to say: after modeling I'd have to say to go for a different driver or completely revise your design/goals.
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Old 25th December 2011, 01:27 AM   #13
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Would there be any problems with this design?
15 liter effective volume (excluding reflex port etc) tuned at 35 Hz.
According to winisd alpha first port resonance would be at 378 Hz.
5,5 cm x 5,5 cm port with length of 45,44 cm making one 90 degree turn in the upper corner of the box.
Picture is not in scale.
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Old 26th December 2011, 09:38 PM   #14
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Quote:
Originally Posted by Juupelila View Post
Would there be any problems with this design?
15 liter effective volume (excluding reflex port etc) tuned at 35 Hz.
According to winisd alpha first port resonance would be at 378 Hz.
5,5 cm x 5,5 cm port with length of 45,44 cm making one 90 degree turn in the upper corner of the box.
Picture is not in scale.
I see no problem as long as you don't send more than 25w to the LF driver.

To use the LF driver with 75w, 35Hz tuning, the port should be 10cm x 5.5cm x 84.5cm.

keep in mind the air velocity maximums are dependent on the size of the port, check out Flare-it - Free Speaker Design Software it's a handy little program.
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