Ports
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2000 Watt CAFz'r
Joined: Jan 2001
Posts: 2,026
In a port, what is more important, the length from end to end, or the volume the air passes through?
Here is what I mean
3"ID x 6"L port works out to 42.4 cubic inches.
4"ID x 3.375"L port works out to 42.4 cubic inches.
So, could I use a 4" port x 3.375"L with 3" openings on either end to attain the same tuning as a 3x6 port?
Since there is the same amount of volume between each 3" opening the tuning would be the same.
Am I wrong? If so, why?
Here is what I mean
3"ID x 6"L port works out to 42.4 cubic inches.
4"ID x 3.375"L port works out to 42.4 cubic inches.
So, could I use a 4" port x 3.375"L with 3" openings on either end to attain the same tuning as a 3x6 port?
Since there is the same amount of volume between each 3" opening the tuning would be the same.
Am I wrong? If so, why?
1000 Watt CAFz'r
Joined: May 2002
Posts: 1,649
..........to add to Dave's comment, there is usually a minimum vent area recommended so that you don't get port "chuffing" or whatever they are calling it nowadays. IF you go smaller, you may run into issues of port noise.
Thread Starter
2000 Watt CAFz'r
Joined: Jan 2001
Posts: 2,026
Originally posted by Dave_MacKinnon:
Volume has nothing to do with vent design.
Vent dimensions are calculated using the net enclosure volume, the desired tuning frequency and the chosen area for the vent. A larger area will dictate a longer vent for the same tuning frequency.
Volume has nothing to do with vent design.
Vent dimensions are calculated using the net enclosure volume, the desired tuning frequency and the chosen area for the vent. A larger area will dictate a longer vent for the same tuning frequency.
I've done plumbing work in the past and I know that if you double the pipe diameter you roughly quadruple the water flow through the pipe. And for every 100 feet of pipe there is X amount of pressure loss due to friction. When you double the pipe diameter, it will take roughly 400 feet of pipe to get the same x amount of pressure loss due to friciton.
The more that I think this through, the more I realize that my originaly logic is backwards. In the past to reduce friction in piping I would install a larger diameter pipe to significantly decrease pressure loss so that even though the water has travelled 1000' it has only lost as much pressure as it would have in the original diameter pipe over 300': essentially the bigger diameter means less friction/pressure loss. Hence when a larger diameter vent is used it neds to be longer.
Therefore in my earlier scenario, if I would go for a small opening with a big port, I would need just as much port length as if I was using the full size diameter to get the same airflow/pressure amount.
I don't know if I make sense, but it is become clear to me.
Thread Starter
2000 Watt CAFz'r
Joined: Jan 2001
Posts: 2,026
What difference does port length mean then if it has nothing to do with air resistence? Frequency wave lengths? If air resistence (in direct relation to box size) isn't the factor to port design what is the key factor? Why then can't we simply cut holes in a box and leave it at that?
I guess I am wonering what science is used in port physics?
[ January 09, 2004, 10:40 PM: Message edited by: maltesechicken ]
I guess I am wonering what science is used in port physics?
[ January 09, 2004, 10:40 PM: Message edited by: maltesechicken ]
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Aw, you are going to make me have to get my books out..
The air inside the vent acts like a piston and vibrates in response to the movement of the woofer’s diaphragm. The smaller the vent, the higher the air velocity and visa versa. Generally, the air velocity through the vent should be kept relatively low by using a vent of sufficient size. This avoids the problem of air turbulence noise in the vent.
Some of the vent’s sound waves lag behind those of the woofer, creating a phase shift. At and above the resonance frequency of the woofer, the vent’s sound waves have the same phase as those of the woofer so they reinforce each other. At the box resonance frequency, the vent also damps the woofer so that its diaphragm moves very little while the air velocity in the vent reaches a maximum.
I'll dig out my loudspeaker design cookbook this weekend and find some more.. I'm headed to bed..
The air inside the vent acts like a piston and vibrates in response to the movement of the woofer’s diaphragm. The smaller the vent, the higher the air velocity and visa versa. Generally, the air velocity through the vent should be kept relatively low by using a vent of sufficient size. This avoids the problem of air turbulence noise in the vent.
Some of the vent’s sound waves lag behind those of the woofer, creating a phase shift. At and above the resonance frequency of the woofer, the vent’s sound waves have the same phase as those of the woofer so they reinforce each other. At the box resonance frequency, the vent also damps the woofer so that its diaphragm moves very little while the air velocity in the vent reaches a maximum.
I'll dig out my loudspeaker design cookbook this weekend and find some more.. I'm headed to bed..
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