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mcsoul
08-11-2008, 11:22 AM
Is it the total area of all the interior surfaces of the port?
WxH + WxH + WxH + WxH for the port walls

Or is it the area (size) of the port opening?
WxH

What is the rule of thumb to prevent port distotion.


To many hits on my search; none on the first page
matched, sorry.

skmfkr
08-11-2008, 11:30 AM
port height x port width

WrenchGuy
08-11-2008, 11:34 AM
To add to the question. What Parameter determines how much port area?

mcsoul
08-11-2008, 11:51 AM
"Area of" is usually HxW, those are the parameters.
The question is how they apply to the port itself
when discussing "port area" in terms of preventing
distortion. In this case, a vent port.

Edit:
Wait a minute I get your question, I believe it is the
size of the driver and possibly the usable Xmax in a
ported application and given box volume.

But hey I'm not sure, that's why I'm asking.

skmfkr
08-11-2008, 11:56 AM
To add to the question. What Parameter determines how much port area?

you mean how much port area is needed? i think its like 12-16inches for every cubic foot

mcsoul
08-13-2008, 12:02 PM
The classic method for determining port size is the port that results in a velocity of 5% of the speed of sound (0.05 Mach) or less, but this rule is often broken to allow the ports to fit in the box. 5% of the speed of sound is ~17 meters/sec, or about 55 feet per second.

I derived the following equation based on the assumption that all the sound at Fb comes from the port, and have found that it works better than any of the rule of thumb equations out there.

Sp_min=0.24*10^(SPL_max/20)/(Fb*up_max)

Sp_min = minimum area of port in cm^2
Fb = port tuning frequency
SPL_max = max desired SPL at Fb
up_max = desired maximum port velocity in m/s

Note that the equation above gives the port velocity at Fb, and the actual peak in port velocity is just below Fb, but it usually isn't too much higher, perhaps 10%, so you could add a fudge factor....

You must select realistic values to get a useful result. up_max must be less than ~30m/s to get a valid result. The equation is quite sensitive to the SPL_max parameter - reducing by 1dB may reduce area quite a lot. The thing that is missing in this equation above is Xmax - you could very easily use an SPL in the formula that is way more than the driver could realistically handle. Modeling with a decent program to begin with to figure out an accurate SPL to enter is a good idea. Or just put in a value that you think is the loudest you will listen. I think that above 100dB, you probably aren't going to hear any port noise anyway due to masking from all the other frequencies....

For an example, I have a 12" woofer tuned to 30Hz at home and I am using it with a 2-7/8" (7.3cm) diameter port for an area of pi*7.3^2/4 = 42 cm^2

I can rearrange the formula to get an idea of max SPL at Fb for a given area and port velocity.
SPL_max = 20*log(Sp*Fb*up_max/0.24)
Assuming a max velocity of 17 meters/second:
20*log(42*30*17/.24) = 99dB
This is the SPL where port velocity equals up_max.

Once you have settled on a diameter - you may calculate port length with:
Lv=2360*Dv^2/(Fb^2*Vb)-0.73*Dv
Dv,Lv are diameter and length of vent in millimeters and Vb is volume in liters.

Something I found in research on port design.