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Hugh G. Rection

understanding ports, a lesson in dimensions vs efficiency.

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Glad you enjoyed it. This is one of the least known aspects of ports.

Exactly. All I've ever really looked at was port area.. And stopped there.

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Someone a lot smarter than me came up with a formula for determining the diameter of port that would have the same air flow characteristics as a rectangular port of known dimensions. Here is the formula:

EqualPort_zps758a7e47.png

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Someone a lot smarter than me came up with a formula for determining the diameter of port that would have the same air flow characteristics as a rectangular port of known dimensions. Here is the formula:

EqualPort_zps758a7e47.png

seems like it would take some very odd measuring equipment, and alot of time, to test and verify this. with the air in the port moving both directions, measuring velocity and pressure would take much different tools than if it were just moving one direction.

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seems like it would take some very odd measuring equipment, and alot of time, to test and verify this. with the air in the port moving both directions, measuring velocity and pressure would take much different tools than if it were just moving one direction.

Yeah its definitely not something that can be easily measured by most folks. I know a hot-wire anemometer is the right tool to measure port velocities, since those things don't care what direction the air is flowing, but again, not something most folks have laying around (myself included).

I posted the formula since it mathematically supports what you said in your original post and to try and give folks a objective method by which they can compare different shaped ports of equal area. I've got the formula plugged into Excel which makes comparing ports pretty quick and easy. I can post what the formula looks like in Excel if anyone is interested.

Edited by Triticum Agricolam
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seems like it would take some very odd measuring equipment, and alot of time, to test and verify this. with the air in the port moving both directions, measuring velocity and pressure would take much different tools than if it were just moving one direction.

Yeah its definitely not something that can be easily measured by most folks. I know a hot-wire anemometer is the right tool to measure port velocities, since those things don't care what direction the air is flowing, but again, not something most folks have laying around (myself included).

I posted the formula since it mathematically supports what you said in your original post and to try and give folks a objective method by which they can compare different shaped ports of equal area. I've got the formula plugged into Excel which makes comparing ports pretty quick and easy. I can post what the formula looks like in Excel if anyone is interested.

Just a thought, what about repurposing a used MAF/Mass Airflow Sensor from a car? Figure you can get one out of a junkyard fairly affordably, but it'd take some calibration to use it and make sense of the data based on the size of the port/sampling tube in the MAF.

Just thinking outside of the box.

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seems like it would take some very odd measuring equipment, and alot of time, to test and verify this. with the air in the port moving both directions, measuring velocity and pressure would take much different tools than if it were just moving one direction.

Yeah its definitely not something that can be easily measured by most folks. I know a hot-wire anemometer is the right tool to measure port velocities, since those things don't care what direction the air is flowing, but again, not something most folks have laying around (myself included).

I posted the formula since it mathematically supports what you said in your original post and to try and give folks a objective method by which they can compare different shaped ports of equal area. I've got the formula plugged into Excel which makes comparing ports pretty quick and easy. I can post what the formula looks like in Excel if anyone is interested.

Just a thought, what about repurposing a used MAF/Mass Airflow Sensor from a car? Figure you can get one out of a junkyard fairly affordably, but it'd take some calibration to use it and make sense of the data based on the size of the port/sampling tube in the MAF.

Just thinking outside of the box.

its an interesting concept, but i just dont believe that mass airflow sensors work in a way that would be useful for something like this.

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If you don't mind I would like to post part of my paper I've been working on ever so slowly.




The red numbers are the point in which the velocity can no longer go any higher from just changing port flare radius.




Now we can start out talking about port compression, this is what happens when you do not have the minimal cross-sectional area for the diameter of the port


Useablevelocityat30hz_zps8d4a571e.png

Table 1*


Now the graph above is measured at a frequency of 30hz, as you lower the frequency being played you lower the maximum amount of usable velocity. Let's use the 86mm port (3in) as a reference Notice as you increase the radius of the flare the higher the velocity of the air traveling through the port can have with no chuffing.However there becomes a point to where no matter the radius of the flare you can not achieve any higher usable velocity, and you will have chuffing at that point and with any larger radius. The maximum velocity numbers are labeled in red. You can see that at a 40mm flare radius you can have a velocity of 24m/s without any noticeable noise, but if that velocity rises to 25m/s you will have noise.This is called port compression, port flares allow a controlled change in velocity as air exits a port. This can prevent turbulence in the boundary layer, close to the port wall, from becoming audible. As the air velocity is raised a little further, turbulence within the straight section of the port becomes an issue.The reason a ported speaker works is because the air in the port causes a 180 degree phase shift in the air behind the driver. When the air comes out of the port, at least at the tuning frequency, it is in-phase with the front of the driver, giving a boost to the output. As air in the core of the port becomes turbulent, this phase shift decreases, thus reducing the in-phase contribution made by the port. The total output of the box no longer rises at the same rate as input power is increased, and the port is said to be compressing. Eventually the port acts as a mere hole in the box, short circuiting the acoustic output of the driver and allowing excursion to become uncontrolled.


The compression of an unflared port increases in a steady curve. A flared port "hangs on" for longer, and then quickly plummets. Ultimately both ports fail at the same level which represents completely turbulent flow throughout the port. Much like a car racer using slick tires, the trick is to use the extended capability of a flared port whilst making sure it's not pushed to the point where it "lets go."


Factors which come into play include air velocity, port diameter, port length, frequency and port flare radius.


This is a graph that represent the velocities at 20hz.


Useablevelocityat20hz_zps21c6fa9a.png

Table 2*








That is all I have written on this subject, I know there is more to add, but I just haven't done it.









There are of course situations where you want to use an internal flare with no external flare or both flared.



For high SPL reasons it is better to use no eternal flare with a large internal flare. With the large flare on the inside of the box it allows for the air current eddies to be allowed out of the internal box volume, while having no eternal flare protects the port from having eddies enter it from the outside.


There is more about it in another post somewhere.

Edited by Krakin

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Well this settles it, next box I build will have an areo port

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