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24th September 2007, 08:06 AM  #1 
diyAudio Member
Join Date: May 2003
Location: N. Cal

Formula for pipe Q
Hi All,
I had read somewhere that there's a formula which relates pipe length to circumference and cross sectional area for determining the Q of a pipe. Can anyone provide the formula or point me to a link? I tried to Google this, but was unsuccessful. I'm evaluating a design for a Hegeman style subwoofer, but need to adjust the pipe dimensions to better fit a larger driver. I'd like to find out if there are any constraints I'll need to contend with. 
24th September 2007, 05:37 PM  #2 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Greets!
The basic material Q I'm familiar with is = (SQRT SoS)/net po, so a 1 m long pipe with a 0.1 m dia = (SQRT~342)/((1*(0.1^2*pi/4)*1.21) = ~1946. Then there's a pipe cutoff Q = Fc/F as discussed here: http://mmd.foxtail.com/Archives/Dige....11.05.03.html GM
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24th September 2007, 06:11 PM  #3  
diyAudio Member
Join Date: May 2003
Location: N. Cal

Quote:
SoS? net po? where does 1.21 come from? And is the above formula for the resonant frequency or the cutoff frequency? 

24th September 2007, 08:02 PM  #4 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Speed of Sound and depending on who you ask/read, it will vary a bit, but FWIW in case you want to use MJK's TL simming program: 342 m/sec, the one I used in the example
po = weight of air, which varies with temp and humidity, which MJK uses 1.21 kg/m^3 Neither, it's the quality factor of the air mass 'slug', which is what you asked for by description, but I doubt it's what you think you need to accomplish your task. Not knowing the design details of the sub I can't help further beyond saying that scaling rarely works well without knowing how to scale everything, including the driver's specs.
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