Helmholtz Resonance (again)

[chairforce]

Hi All,

I got a question about the correct calculation of a Helmholtz resonator. The formulars for a tube are:

fo=(c/2Pi)*sqrt(C/V)

C= (Pi*d*d)
-----------
4*(l+0.8*d)

where c-> speed of sound
d-> diameter of the resonator tube
l-> length of tube
V-> Volume of the box.
C is substituted

(I added a small Maple V R4 sheet)


The second formular can not be applied to rectangle-shaped openings.
Does anybody know how to adapt the calculation to a squared opening?

I searched the internet and my library, but I could not find that information.

Thanks for your help!

 

[Flaesh]

(pi*d^2)/4=pi*r^2
can you find formula for square\rectangle area? 😉 ..

factor of 0.8 is empirical and approximate.. use square side or slightly more instead of d

 

[chairforce]

@Flaesh

I hoped for a different factor than 0.8, one that represents the square in a better way. If I cannot find a better factor I will do what you proposed.

 

[Flaesh]

and if port is wide and short, or near box side\floor, or rounded - this 0.8 seems greatly approximate..

 

[mjf]

hello.
i think both channels (round or square) should have the same volume.
if you use the same length you only have to calculate the the area to get the same volume.
transl:bei gleicher länge des kanales einfach die querschnittfläche so anpassen, dass sich wieder dasselbe volumen ergibt.
grüsse

 

[Inductor]

Correction factors from round to square (a/b):
0,988 for a/b=square
0,9 for a/b=3,5
0,8 for a/b=7
From: francis.audio.fr
http://pagesperso-orange.fr/francis.audio2/Bank_Pierce.gif

More here: Huesbscher
Bass-reflex : Calcul de l'évent'

 

[chairforce]

Thank you for your help!