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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.

On 5/8/2011 at 7:38 PM, Kranny said:
On 5/8/2011 at 7:35 PM, 'Maxim' said:

It hurts me inside when I read stuff like this and remember you're 15

LMFAO so true

:blush:

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Sticky this for sure. Great info, thanks for the write up


Thanks alaskanzx5 I might just do that
I can recone a sub myself. About a year ago I took 2 12" power acoustic mofos and made one sub. I took the magnet and the motor off of one and j b welded it to the other motor and magnet. I had to wind my own voice coil so it would work. After I was done hooked up to 2 boss 5000w amps. And shattered my back and all my side windows

came to this thread to recommend soundqubed and ct sounds. OP goes with soundstream.

oh lawd.

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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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"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

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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.


Owner of BigAss Ports

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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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"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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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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2007 Mazda 3; 5000K HID's, Kenwood Excelon KDC-X997, Infinity Reference 6.5 comps in front and coaxials in the rear doors, JL 320.4 four channel, Rab Designs built ported enclosure with an SA12, Kenwood monoblock, Redline Leater shift boot/e-brake boot/center console cover, JBR short shifter/shifter bushings/rear motor mount. Nothing special, just a daily driver/wife and kid hauler.

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1996 Mazda Miata: Kenwood Excelon HU, Alpine speaker in the doors, Clearwater (miata specific) headrest speakers, sounded awesome but no room for bass.

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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.


Owner of BigAss Ports

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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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Krakin's Home Dipole Project

http://www.stevemeadedesigns.com/board/topic/186153-krakins-dipole-project-new-reciever-in-rockford-science/#entry2772370

Krakin, are you some sort of mad scientist?

I would have replied earlier, but I was measuring the output of my amp with a yardstick . . .

What you hear is not the air pressure variation in itself

but what has drawn your attention

in the two streams of superimposed air pressure variations at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space

Everyone learns to render the 3-dimensional localization of sound based on the individual shape of their ears,

thus no formula can achieve a definite effect for every listener.

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Great write UP!!!! Thank you


No 1 But me paid for this stuff and NO FREEBIES so no brand names (No Free Advertisement )

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ill say it like i always say it. F##K the DD-1. I don't care if you or anyone buys one. (well i do, but that isn't my point) Get an O-scope in the very minimum and learn how to use it or you are just guessing your settings like every other noob. :) once you master the O-scope then step up to a DD-1 or even better a DD-1 Plus.


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


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