Port length vs air movement distance

Circlotron · 7th May 2009, 12:04 PM

Lets say we have a port of 100mm length and the air in it is moving 10mm p/p. All is working as it should. Now some teenager comes along and cranks up the volume a bit and we have the port air moving 300mm p/p, several times the length of the port.

The question is, does the port air mass behave in exactly the same way in both instances as far as box tuning is concerned, seeing that the air within the port at one moment completely leaves the port at another moment to be replaced by other air?

aznboi3644 · 8th May 2009, 12:32 AM

once you exceed a certain port velocity the port compresses and the enclosure then acts as a very leaky sealed enclosure.

Ron E · 9th May 2009, 01:44 AM

Define exactly the same - it obviously isn't if the excursion is 30x as great. As far as T/S theory goes, the port acts like a rigid piston. In real life, there is a whole lot more going on.

Say your port is tuned to 40 Hz - then it moves 300mm in 1/4 cycle so that is 160*.300 = 53.3 meters/sec - this is well beyond the realm of port compression, which is classically expected to start being significant at around 1/2-1/3 this velocity.

What's the motivation for the question, or was it just a thought experiment?

Circlotron · 9th May 2009, 02:34 AM

Just a thought experiments. I pulled the numbers out of my own port.

chris661 · 16th May 2009, 01:22 PM

Quote:

Originally posted by Circlotron
Now some teenager comes along and cranks up the volume a bit

What are you trying to say about teenagers???

EWorkshop1708 · 22nd May 2009, 03:52 PM

Quote:

Originally posted by chris661

What are you trying to say about teenagers???

I agree.

ROFL Age Stereotypes!

I blew some big speakers in my 20's, but not in my teens, LOL

djk · 23rd May 2009, 05:02 PM

In practice, at high levels the ports start to 'chuff', the tuning frequency shifts upwards, and then it starts acting like the leaky box.

chris661 · 25th May 2009, 06:48 PM

I agree there - I messed around with a small sub (before blowing it with several times more power than it's made for - courtesy of dad's PA amp) and, even just by listening, you can hear the port tuning change as volume changes. This, however only happens past a certain point, which must be the dist. moved by air >/= length of port.

Which means we need longer ports for higher volumes...

djk · 26th May 2009, 10:32 AM

"Which means we need longer ports for higher volumes..."

More port area is needed (which means they are longer). At some point a PR becomes needed.

7th May 2009, 12:04 PM	#1
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	Port length vs air movement distance Lets say we have a port of 100mm length and the air in it is moving 10mm p/p. All is working as it should. Now some teenager comes along and cranks up the volume a bit and we have the port air moving 300mm p/p, several times the length of the port. The question is, does the port air mass behave in exactly the same way in both instances as far as box tuning is concerned, seeing that the air within the port at one moment completely leaves the port at another moment to be replaced by other air? __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

9th May 2009, 01:44 AM	#3
Ron E diyAudio Member Join Date: Jun 2002 Location: USA, MN	Define exactly the same - it obviously isn't if the excursion is 30x as great. As far as T/S theory goes, the port acts like a rigid piston. In real life, there is a whole lot more going on. Say your port is tuned to 40 Hz - then it moves 300mm in 1/4 cycle so that is 160.300 = 53.3 meters/sec - this is well beyond the realm of port compression, which is classically expected to start being significant at around 1/2-1/3 this velocity. What's the motivation for the question, or was it just a thought experiment? __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan* Science is a way of thinking much more than it is a body of knowledge. --Carl Sagan

9th May 2009, 02:34 AM	#4
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	Just a thought experiments. I pulled the numbers out of my own port. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

23rd May 2009, 05:02 PM	#7
djk diyAudio Member Join Date: Feb 2001 Location: USA	In practice, at high levels the ports start to 'chuff', the tuning frequency shifts upwards, and then it starts acting like the leaky box. __________________ Candidates for the Darwin Award should not read this author.

25th May 2009, 06:48 PM	#8
chris661 diyAudio Member Join Date: Feb 2008 Location: Lancashire Blog Entries: 7	I agree there - I messed around with a small sub (before blowing it with several times more power than it's made for - courtesy of dad's PA amp) and, even just by listening, you can hear the port tuning change as volume changes. This, however only happens past a certain point, which must be the dist. moved by air >/= length of port. Which means we need longer ports for higher volumes... __________________ "Throwing parts at a failure is like throwing sponges at a rainstorm." - Enzo My setup: http://www.diyaudio.com/forums/multi...tang-band.html


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8th May 2009, 12:32 AM	#2
aznboi3644 diyAudio Member Join Date: Dec 2007	once you exceed a certain port velocity the port compresses and the enclosure then acts as a very leaky sealed enclosure.

26th May 2009, 10:32 AM	#9
djk diyAudio Member Join Date: Feb 2001 Location: USA	"Which means we need longer ports for higher volumes..." More port area is needed (which means they are longer). At some point a PR becomes needed. __________________ Candidates for the Darwin Award should not read this author.

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