Sundown SA12 Ported Box

[Triticum Agricolam]

DISCLAIMER - What you are saying is contrary to years of personal experience and research. Don't be offended if I disagree and call into question what you say. You are going to need to explain your reasoning and provide proof if possible, I will be happy to give you the same courtesy.

The more port area you have the faster it unloads above and below tuning. similar to having too large of a box. when you increase the port area you increase the air volume within that port.

no programs really show this. the only thing they show is the "gain" from the port. increasing port area much like box size will give you and increase in efficiency at tuning but a narrower bandwidth as well.. I'm not going to say 33 is to much for the 12 but its pushing the limits and its just not needed that sub will never displace enough air to create any noise.

think of it like this. when you have 30 in^2 of port area vs 60in^2 of port area say both tuned to 30 hz your effectively DOUBLE the air mass within the port but lowered resistance and damping because it become easier to create a high and low pressure with the same amount of force. the actual volume has changed but the density hasn't.

another example. you have a tire. you fill it wit air until maximum expansion say 20 liters. and for that 20 liters you have 8 PSI for maximum expansion to occur. now you need 32 PSI. the additional pressure is simply compressing the air molecules making them tight and tighter getting closer and closer so it becomes harder and harder to put more in. same with the port. the smaller ports offer more resistance to compression and it naturally controls the cone more. the result is a narrower bandwidth and less output at its resonance..

Quite a bit of what you said there is wrong, I'm going to explain why one piece at a time.

"The more port area you have the faster it unloads above and below tuning"
Port don't unload at all above tuning. Once you get much above the tuning frequency the pressuring in the box is changing too fast for the air in the port to change direction. At the point its like the port isn't there at all and the box performs very similar to a sealed box of equal size. As far as larger ports unloading faster below tuning goes, that doesn't happen and it would be easy to test for using a box with external ports of different sizes but the same tuning and measuring the frequency response of the two. I can do this test if you desire.

"increasing port area much like box size will give you and increase in efficiency at tuning but a narrower bandwidth as well"

This is half right, but not for the reason you are stating. Increasing port area will increase efficiency and narrow bandwidth if port velocity with the smaller port is high enough that significant port compression is taking place. However at lower output levels, where port compression isn't an issue, there is going to be no difference in output or efficiency between different size ports. If port size alone changed efficiency/frequency response it would show up at low volume levels too.

"when you have 30 in^2 of port area vs 60in^2 of port area say both tuned to 30 hz your effectively DOUBLE the air mass within the port but lowered resistance and damping because it become easier to create a high and low pressure with the same amount of force. the actual volume has changed but the density hasn't."

When you double the area of a port you also have to double its length to keep tuning the same. So you are actually quadrupling the mass of air inside the port. So no, it doesn't become easier to create a high a low pressure with the same amount of force.

"the smaller ports offer more resistance to compression and it naturally controls the cone more."

By that logic a sealed box would have lower cone excursion than a ported box does at tuning since nothing offers more resistance to compression than not having any way for air to escape the box. This one is easy to prove too. Take any ported box and play a sine wave tone through it at the tuning frequency and observe the amount of cone excursion. Now partially (or completely) obstruct the port, cone excursion goes up. Try it yourself if you don't believe me.

[diegoclass2010]

... dont mind me guys... i like a good discussion of ports... carry on ...

[Jessica]

i have thought about both sides of all these things, so i am gonna sit back and read and hope i learn something.

[bassfreak85]

DISCLAIMER - What you are saying is contrary to years of personal experience and research. Don't be offended if I disagree and call into question what you say. You are going to need to explain your reasoning and provide proof if possible, I will be happy to give you the same courtesy.

The more port area you have the faster it unloads above and below tuning. similar to having too large of a box. when you increase the port area you increase the air volume within that port.

no programs really show this. the only thing they show is the "gain" from the port. increasing port area much like box size will give you and increase in efficiency at tuning but a narrower bandwidth as well.. I'm not going to say 33 is to much for the 12 but its pushing the limits and its just not needed that sub will never displace enough air to create any noise.

think of it like this. when you have 30 in^2 of port area vs 60in^2 of port area say both tuned to 30 hz your effectively DOUBLE the air mass within the port but lowered resistance and damping because it become easier to create a high and low pressure with the same amount of force. the actual volume has changed but the density hasn't.

another example. you have a tire. you fill it wit air until maximum expansion say 20 liters. and for that 20 liters you have 8 PSI for maximum expansion to occur. now you need 32 PSI. the additional pressure is simply compressing the air molecules making them tight and tighter getting closer and closer so it becomes harder and harder to put more in. same with the port. the smaller ports offer more resistance to compression and it naturally controls the cone more. the result is a narrower bandwidth and less output at its resonance..

Quite a bit of what you said there is wrong, I'm going to explain why one piece at a time.

"The more port area you have the faster it unloads above and below tuning"

Port don't unload at all above tuning. Once you get much above the tuning frequency the pressuring in the box is changing too fast for the air in the port to change direction. At the point its like the port isn't there at all and the box performs very similar to a sealed box of equal size. As far as larger ports unloading faster below tuning goes, that doesn't happen and it would be easy to test for using a box with external ports of different sizes but the same tuning and measuring the frequency response of the two. I can do this test if you desire.

"increasing port area much like box size will give you and increase in efficiency at tuning but a narrower bandwidth as well"

This is half right, but not for the reason you are stating. Increasing port area will increase efficiency and narrow bandwidth if port velocity with the smaller port is high enough that significant port compression is taking place. However at lower output levels, where port compression isn't an issue, there is going to be no difference in output or efficiency between different size ports. If port size alone changed efficiency/frequency response it would show up at low volume levels too.

"when you have 30 in^2 of port area vs 60in^2 of port area say both tuned to 30 hz your effectively DOUBLE the air mass within the port but lowered resistance and damping because it become easier to create a high and low pressure with the same amount of force. the actual volume has changed but the density hasn't."

When you double the area of a port you also have to double its length to keep tuning the same. So you are actually quadrupling the mass of air inside the port. So no, it doesn't become easier to create a high a low pressure with the same amount of force.

"the smaller ports offer more resistance to compression and it naturally controls the cone more."

By that logic a sealed box would have lower cone excursion than a ported box does at tuning since nothing offers more resistance to compression than not having any way for air to escape the box. This one is easy to prove too. Take any ported box and play a sine wave tone through it at the tuning frequency and observe the amount of cone excursion. Now partially (or completely) obstruct the port, cone excursion goes up. Try it yourself if you don't believe me.

Id love to see some test.. when ever you have time..

[Triticum Agricolam]

Id love to see some test.. when ever you have time..

I'm actually in the process of doing a bunch of port compression testing, I've still got more I'd like to do but here is what I've got so far: http://www.stevemeadedesigns.com/board/topic/204068-port-compression-and-optimization/

Here is the measurement of a pair of 3" ports vs a 4" port, both tuned the same:

Granted, there isn't a lot of difference in port area between the two, but their frequency response is identical. If it would be more convincing I can make a port for my test box using two 5" ports and show you the frequency response doesn't change, even when using three times as much port area.

[bassfreak85]

Id love to see some test.. when ever you have time..

I'm actually in the process of doing a bunch of port compression testing, I've still got more I'd like to do but here is what I've got so far: http://www.stevemeadedesigns.com/board/topic/204068-port-compression-and-optimization/

Here is the measurement of a pair of 3" ports vs a 4" port, both tuned the same:

Granted, there isn't a lot of difference in port area between the two, but their frequency response is identical. If it would be more convincing I can make a port for my test box using two 5" ports and show you the frequency response doesn't change, even when using three times as much port area.

your changing volume and depending on the subs mechanical properties and the enclosure size it will also affect it.. it is much more noticeable with woofers that higher lower CMS/RMS as many woofers are mechanically over-damped.

do this. start off with 1/4 sd and then go to 1/2 then 3/4 and all of SD.. but don't chance box size or tune.. another thing to note is if the alignment is already "over damped" you see much milder results from the port's damping shift. what i mean by this is. if the woofer is in an already "small" ported box it is going to take a-lot of variance to notice it( by ear)and like i said the subs CMS/RMS mms and QES has a lot to do with its frequency response both above and below tuning. its is clearly apparent as you can see some subs peaky much harder at tuning than others and have a much faster roll off

BTW you where right doubling cone are to keep the same tune if quadrupling the compliance.. BTW its going to be most notable as the ports pass-band get narrower and peaks harder.. for example the smaller port will give you a shallower roll off above tuning and the higher frequency with be reinforced therefor be louder.

BTW if your not doing it freeair(not in a car) you might as well not even do the testing..

[bassfreak85]

BTW what driver tune and box is that? i have a prototype coming in.. i be doing some testing as well.. i just dont want to rip my car apart.. i need a test car.. lol

[Triticum Agricolam]

BTW what driver tune and box is that? i have a prototype coming in.. i be doing some testing as well.. i just dont want to rip my car apart.. i need a test car.. lol

If you read my other thread I linked to were I did the testing you would know the specs of the box and driver.

I was not changing box volume with the different ports, the ports are completely external. The box was built specifically for this testing. All tests were ground plane measurement done outside in the middle of a field. I've put a lot more thought into this testing than you are giving me credit for.

So, do you have any evidence to support you claim of different size ports changing the frequency response?

[bassfreak85]

i few years ago i built some prototypes and was testing drivers to find the ratio of mass to CMS to B/L i wanted and how the vented alignments affected the drivers. IE trying to build a high efficiency low distortion ported box driver that lower group delay and phase shifts and minimize impedance swing(rise/fall).

i learned that the much stiffer suspensions could gain more output in a given alignment as port area increased. the stiffer spiders/lower mass drivers kept gaining as port area increased where the softer spiders started to fall off after gaining sharply. i could also note that high frequency response suffered as they was less cone control(poor mechanical damping) and then soon after started to exceed linear throw,(like you said the driver acts free air to an extent above tuning)i started off with a MMS of 340 grams and a CMS of .12 mm/N down to .06. at one point the spiders where too stiff.output suffered so instead of increasing box size i increased port area and gained but the the softer spiders stated to act like they where unloading.. initially the softer spider drivers gained and fairly quick then it reached a diminishing return. granted that was about 60% of the drivers SD and way over kill(15" BTW) i did notice a change is more than about 40.. the f3 was shallower and the peak got harder. similar to increasing the box size.. IE the larger port acts like a softer suspension. compliance increases as port area goes up.

thank about the transmission line and how they work.. its a linear ratio of compliance between the enclosure and woofer with a chamber equal to the driver's FS in wavelength. anyone know of Bart is still around? i never tried that dual chamber dual tuned series vented alignment he played with.. shit was crazy and the concept was cool

[bassfreak85]

BTW the graph im seeing on you other post conforms my theory. the dual 3" ports with 2in^2 of port area peaked harder and fell of faster.. do 3 4" ports against the 2 3"s ports that's 37.7in^2 now double that( right around half the SD) and let me know.. if im wrong ill stand corrected but i testing showed what i said..

and thank you for handling the subject like a man instead of a kid..

like i said i have a prototype coming in very soon and plan of getting it tweaked for production and i will preform and test anyone here wishes me to do.. its what i like to do!