Vent length recalculation formula
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 27th May 2006, 01:31 PM #1 arjscott   diyAudio Member   Join Date: Apr 2006 Location: NZ Vent length recalculation formula I'm building a bass reflex enclosure that specifies a vent with a diameter of 65mm, and length of 75mm. I have some 62mm pipe so I need to recalculate the vent length based on the smaller diameter. I found two formulae in this post: http://www.diyaudio.com/forums/showt...466#post600466 The thread is quite old, so I decided to start a new one instead of replying in there. L1 = original length D1 = original diameter L2 = new length D2 = new diameter The first one is pretty simple, and L2 looks sensible (68.23mm) L2 = L1*(D2^2/D1^2) The second one (according to GM) takes end correction into account. I don't understand the result of this calculation because the length works out to 77.2mm. L2 = L1*(D2^2/D1^2)+(D2-D1)^2 I've tried entering several different diameters where D2 is smaller, and it always seems to work out longer. If the vent diameter is reduced, shouldn't the length be shorter? Can someone with a better understanding of vent calculations (and maths!) explain what is happening here? Am I repeatedly screwing up the calculation? Apologies in advance if I am missing something obvious.
 27th May 2006, 04:04 PM #2 Ron E   diyAudio Member     Join Date: Jun 2002 Location: USA, MN Length varies as area. If you double area, you double length. Area of a circle varies as diameter squared. L1/L2=(D1/D2)^2 L2=L1/(D1/D2)^2 L2=75/(65/62)^2=68mm If you wish to take end correction into account - which isn't necessary in your case - just calculate the vent length the normal way. Lv=2360*Dv^2/(Vb*Fb^2)-.73*Dv (dimensions im mm, volume in liters) __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. —Aldous Huxley
 27th May 2006, 08:02 PM #3 Notax   diyAudio Member     Join Date: Jun 2004 Location: Belgrade Lv=(Rt*Rt*C*C)/(4*pi*Fb*Fb*Vb)-1.7*Rt Rt=Dt/2 - half diametar (m) C - speed of sound (m/s) Fb - tuning freq. (Hz) Vb - volume of box NETTO (m3) All in SI metric, and you got Lv (m). Lt2=(Rt2*Rt2)/(Rt1*Rt1)*(1.7*Rt1+Lt1)-1.7*Rt2 In this case: Lt2=65.8 mm
 29th May 2006, 12:54 PM #4 arjscott   diyAudio Member   Join Date: Apr 2006 Location: NZ Thanks for the clarification, Ron E. Notax: That formula looks interesting but actually understanding it is beyond my mathematical ability! I've had some fun making calculations with it anyway.

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