# Tapered vent question

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#### Puppetnation

Hi folks,
after a couple of hours before my registration I'm here, asking for your help.

I'm building a pair of bookshelf loudspeakers, 2-way bass-reflex.

System Fb is 50 Hz, I've made calculations resulting in a certain lenght/certain diamenter vent.

I had to buy a specific vent made for small cabinets, featuring dimensions to fit into my boxes, but once arrived a new problem has born: this vents are non cylindrical but slightly taperes, a true truncated-cone!

Declared as 6,6 sq.cm (on the smallest end) but actially larger at the opposite flared side.

Is there any formula to calculate the right vent lenght, considering if cutting it will result in a diameter increase?

thanks
Gianmaria

#### Svante

Puppetnation said:
Is there any formula to calculate the right vent lenght, considering if cutting it will result in a diameter increase?

Probably, but since it is hard to get it exactly right from calculations/simulations only, one should make the vent a bit too long. Then measure the result and then shorten the vent length until the Helmholtz frequency is right.

#### Puppetnation

Here's the drawing attached....

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• br-30hp.jpg
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#### Inductor

Puppetnation said:

Declared as 6,6 sq.cm (on the smallest end) but actially larger at the opposite flared side.

Is there any formula to calculate the right vent lenght, considering if cutting it will result in a diameter increase?

Has you might have found by now, if you consider the bigger side diametre you will have enough length as opposed to cutting it from a larger tube (smallest end) and not considering it as you are doing (larger at the opposite flared side).

On your drawing the left inner part as the (main) diameter, for an adjustable length that you can calculate with the software.

Just to say, use the other tube side.

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