# Pressure build-up in a vented sub???

Discussion in 'Archived Threads 2001-2004' started by PeterOS, Jan 25, 2003.

1. ### PeterOS Extra

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Hoping some one might be able to help me out here. I want to find out what is the actual pressure build up inside a subwoofer, more specifically my Tempest Adire aligment. I want to do an analysis of how much the MDF moves under the pressure generated using ALGOR (its a Finite Element Analysis software).

I was on this website and thought I was sorted but what the hell is Xv???
http://www.silcom.com/~aludwig/Sysde...l_analysis.htm

Here a section of it. (the equation that goes with it would paste for me)

"A speaker cone with effective area SD moves sinusoidally with a peak excursion Xc. There is a vent of cross-sectional area Sv and length Lv. It is assumed that the air within the vent moves sinusoidally as a rigid mass with peak excursion Xv"

Any help would be appreciated. I want to do up a graph of the differt Freq's against the pressure. Pump these values into ALGOR and see what it gives me. I presume the biggest pressure will be at thee box's resonant freq.

If I get it done I will post the results for those who are interested. I will also be doing a analysis of the airflow through ported vents to see how much of a difference the vents make, these I will also post for those interested.

2. ### Pete Mazz Supporting Actor

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My understanding is the air in the vent is treated as a "solid" slug of air with its corresponding mass and moves much like the cone of a driver.

Pete

3. ### PeterOS Extra

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Just to be clearer, are you saying the the value for Xv is equal to the volume or the mass of the vent. The equations therefore being

vol = (pie * radius^2 * length)
or
mass = (pie * radius^2 *length)*density

Ex. For a vent of radius 3.75cm (diam 3") and length 28cm (11")
The value of Xv would be 0.00149 kg (using mass eqn)

Giving me a pressure for my 214L Tempest of

76.244kg/m^3

Anyone know if this sounds right???

4. ### Jonathan M Second Unit

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I would say that Xv refers to the amount that that "solid" slug moves - not the volume of the air in the slug.

5. ### Pete Mazz Supporting Actor

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This is his equation to derive pressure, substituting for X(v).

X(v) is measured in meters.

Pete

6. ### Jeff Rosz Second Unit

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i'll take a stab at clearing it up a bit. but im no mathmatics guru, so have a grain of salt.
Xv would have to be a distance. therefore as johnathan stated, it cannot be a volume or a mass. as far as i can see, the equation pete posted is frequency dependent (which makes some sense to me). the variable that looks like a W represents angular frequency. "thats just my opinion, i could be wrong"

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