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MyNameIsNoob

Port Velocity and Power

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Hey guys. Been learning, reading and thinking alot ever since i was suggested to use a slightly larger port to help keep port velocity down on this box I made.. I was looking through some posts on this and read a reply referring to somebody having a very very high watt per cubic ft enclosure. That got me thinking about the relationship between increasing power and  air space relative to higher air speed.

My question, is there a ratio of power, air space and port area that will help manage port velocity?

For instance, a 4 ft3 box w 16 in2/ft port area on 1000 watts and the same box at 4000 watts. How much port area will a box with that much more power and excursion need to maintain the same air speed?

I have been looking through existing threads on this topic, but cant seem to find an answer. 

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to answer your question, you're going to need to plot the sub and enclosure design in something like BBP6 or Term-Pro. 

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8 hours ago, MyNameIsNoob said:

 

My question, is there a ratio of power, air space and port area that will help manage port velocity?

Yes, there is a formula that will help you keep your port velocity at reasonable levels.  ckeeler11 posted a link to the thread where I discuss the formula I developed.  

A couple things to keep in mind, tuning frequency has a big impact on how much port area you need, in addition to box volume and input power like you said.  Higher tuning frequencies need more port area than lower ones do.  

Excursion has pretty much no impact on how much port area you need however.  This is because the frequencies where port velocity is highest is generally around the tuning frequency.  This happens to be where cone excursion is at a minimum.  This is also why reducing port area is a almost completely ineffective way to try to reduce cone excursion.  

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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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Also of note: A larger port, as measured at the smallest dimension, can have higher velocity without producing noticeable chuffing. Oh yeah and they're more efficient too due to boundary layer behaviors.

Shit gets weird..... Goes a little over my head to be honest.

But yeah.. the whole port area per cubic foot thing is very much a rule of thumb, just to make things simpler for entry to intermediate builds so as to not complicate someone doing this stuff for the first time.

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Thanks yall, lotsa good useable info here. I did see those 2 posts n jus reread them to see if I looked past what I'm trying to figure out. I saw where it was said you dont need to double port area bc 50% will do, but what I like to know is why and how...

Is there a way to calculate the box volume and power to find out the pressure inside the enclosure?

And how will that pressure and port area affect velocity?

I'm jus trying to understand a lil better so I can make I formed judgement calls next time.

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18 hours ago, MyNameIsNoob said:

...Is there a way to calculate the box volume and power to find out the pressure inside the enclosure?

And how will that pressure and port area affect velocity?....

SPL output around tuning is going to directly correlate to box pressure.

There are four things that increase port velocity - increasing box volume, increasing input power, increasing the tuning frequency, or decreasing port area.  The opposites of those four will decrease port velocity.  

Probably the best thing for you to do at this point would be to get some box modeling software (WinISD is free and works well).  With that you can play around with box parameters and see what effects they have on port velocity.  


"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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On 4/7/2019 at 11:02 AM, Triticum Agricolam said:

SPL output around tuning is going to directly correlate to box pressure.

There are four things that increase port velocity - increasing box volume, increasing input power, increasing the tuning frequency, or decreasing port area.  The opposites of those four will decrease port velocity.  

Probably the best thing for you to do at this point would be to get some box modeling software (WinISD is free and works well).  With that you can play around with box parameters and see what effects they have on port velocity.  

Thank you sir. I jus got my laptop working again n put a few programs on it to start playing with, so I will be doing the youtube tutorial thing for a bit (killin way too much time - my wife says lol). I had WinISD a few years ago, but didnt use it enough to become proficient w it, but I have some free time lately so Id like to improve my knowledge a bit while I can. 


Are there any other articles or write ups on this topic that go into it a bit deeper?


I've read up on aerodynamics a bit, so I'm thinking alot of the same principles will crossover. I've been into car audio way too long to not know these things, so I'm jus tryn to step my game up. 
 

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12 hours ago, MyNameIsNoob said:

Thank you sir. I jus got my laptop working again n put a few programs on it to start playing with, so I will be doing the youtube tutorial thing for a bit (killin way too much time - my wife says lol). I had WinISD a few years ago, but didnt use it enough to become proficient w it, but I have some free time lately so Id like to improve my knowledge a bit while I can. 


Are there any other articles or write ups on this topic that go into it a bit deeper?


I've read up on aerodynamics a bit, so I'm thinking alot of the same principles will crossover. I've been into car audio way too long to not know these things, so I'm jus tryn to step my game up. 
 

Its not exactly light reading, but I've found this AES paper to be very informative: http://mariobon.com/Articoli_storici_AES/jbl/JBL_2002_AES_Reflex_Ports.pdf

If you need any help with WinISD just let me know.  There are a lot of older versions of it floating around, but if you get the newest version (0.7.0.950) its a lot easier to use.  


"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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