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Old 16th September 2013, 09:19 AM   #1
eqjive is offline eqjive  United States
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Join Date: Jan 2013
Default Help with formula - Maximizing Performance from Loudspeaker Ports?

Can anyone clarify this formula from the AES paper 'Maximizing Performance from Loudspeaker Ports'? (see attached screen shot). I'm looking to create a spreadsheet (as the paper suggests possible) to determine tuning frequency of heavily flared ports (NFR .5 - 1).

It is formula number 11, page 14 of the paper 'Maximizing Performance from Loudspeaker Ports' or page 32 of the J. Audio Eng. Soc., Vol. 50, No.1/2, 2002 January/February

I'm having some trouble figuring this one out... what are the units to be used if speed of sound is given in meters, what should throught area, diameter, volume etc be?

Can anyone give an example using the formula so I can understand where I am getting confused?

Thank you!!

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Old 16th September 2013, 09:48 AM   #2
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK

I'd uses cm, cm/s, cm squared and cm cubed for calculations.

Convert C to cm/s and Litres to cm cubed.

Really doesn't matter as long as your consistent.

What you uses to input parameters can be converted
in the calculations, so use whatever input format
you like like C in m/s and Vb in litres.

Note that big ports will reduce the actual Vb.

rgds, sreten.

If its going to end up part of something more complex
standardise on m, m/s, m squared and m cubed for
all calculations and convert the input / output format.
There is nothing more practical than a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 16th September 2013 at 09:58 AM.
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Old 16th September 2013, 09:49 AM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I get the impression that everything is in metres.
regards Andrew T.
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Old 17th September 2013, 01:11 AM   #4
OscarS is offline OscarS  United States
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Join Date: Jan 2011
As reten said, so long as you're consistent you can use different units.

The main idea is to realize that since c is in units of "distance/time", and frequency is "1/time", you need to cancel out the "distance" unit in the numerator that "c" contributes. That means that the outcome of the square root function has to simplify down to "1/distance". Sure enough, Aeff= distance˛, and the product of Leff*Vb = distance^4. Distance˛/distance^4 reduces to 1/distance˛, the square root of which leads to "1/distance".

So as long as the units for distance are consistent, the formula will churn out the correct numbers.
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Old 17th September 2013, 12:50 PM   #5
eqjive is offline eqjive  United States
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Join Date: Jan 2013
Thank you!!! With your help, I figured it out.

- EqJive
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