Advance SPL Topics - port volume
Pressure is a confusing term for a lot of people. People often use pressure and the force exerted by a high velocity fluid as the same thing. Think of a garden hose dispensing a stream of water. If the water shoots out a long way people generally say it has a lot of pressure. Reality is there is no pressure and it is all velocity.
Fluid dynamics tells us that a fluid flowing though a pipe/hose/duct has two components, velocity and pressure, and the two vary inversly to each other. If you make the pipe/hose/duct larger the velocity goes down and the pressure goes up. So by making a vent as large as possible to make the velocity though it as low as possible, yes the pressure within the duct will be as high as possible, but that pressure is gone once it leaves the vent.
Fluid dynamics tells us that a fluid flowing though a pipe/hose/duct has two components, velocity and pressure, and the two vary inversly to each other. If you make the pipe/hose/duct larger the velocity goes down and the pressure goes up. So by making a vent as large as possible to make the velocity though it as low as possible, yes the pressure within the duct will be as high as possible, but that pressure is gone once it leaves the vent.
50 Watt CAFz'r
Joined: Dec 2010
Posts: 89
Is this for SPL?
If is, you can simulate the best box ever, then throw in the garbage and build one on trial and error, because you are not simulating the box environment (car), the cabin gain (8, 10, 12dB) and its frequency and bla bla bla...
SPL in higher frequencies 55+ you use a small box and big port, high volume. If you want to build a PA box, half space loaded than yes, huge box, calculate port volume and watch air speed, cone travel and bla bla bla.
If is, you can simulate the best box ever, then throw in the garbage and build one on trial and error, because you are not simulating the box environment (car), the cabin gain (8, 10, 12dB) and its frequency and bla bla bla...
SPL in higher frequencies 55+ you use a small box and big port, high volume. If you want to build a PA box, half space loaded than yes, huge box, calculate port volume and watch air speed, cone travel and bla bla bla.
--SPECIAL ED--
Joined: Nov 2010
Posts: 77
ok bro wich part of testin did you forget to do yourself cus yore scoer lookz like you shoudda done more of it
50 Watt CAFz'r
Joined: Dec 2010
Posts: 89
For instance: big box sealed car bad SPL, leaking car, high SPL. venting = against rules
bla bla bla
nice sub, big amp, buuurp... 153. humm quiet... find a couple holes under the dash, seal them, fix stuf on door seals, 154.5dB. after a couple other boxes 155dB. passenger door is fuc$k$up, bend it with a 2x4 so it locks tighter, 155.5dB
Some people will stay on the 153 and complain about how hard is life and how expensive is the sport.
0 Watt CAFz'r
Joined: Oct 2009
Posts: 42
Based on my experience, I do believe there is a port volume/box volume ratio. I believe the ratio is highly affected mainly by the cabin size which is the main factor in the ratio combo. I have yet to have found that magical ratio number but do know it exists. It is obvious when you see the standardized bow sizes and port sizes that are usually the same in certain classes.
I also do believe that trying to obtain a 34mt/sec port velocity is what will generally help. going to small is like a human trying to breathe thru a straw, and going to big is like trying to breathe thru a 4" pipe out of your mouth, you will not be able to fill your lungs. Therefore the same effect happens ion boxes, to fast a port velocity and no back pressure, and to small to little back pressure.
These are my observations based on my experience, not saying its concrete science but its how I have come to see things, but then again nor is SPL its always new and surprising. Just my 2 cents!
I also do believe that trying to obtain a 34mt/sec port velocity is what will generally help. going to small is like a human trying to breathe thru a straw, and going to big is like trying to breathe thru a 4" pipe out of your mouth, you will not be able to fill your lungs. Therefore the same effect happens ion boxes, to fast a port velocity and no back pressure, and to small to little back pressure.
These are my observations based on my experience, not saying its concrete science but its how I have come to see things, but then again nor is SPL its always new and surprising. Just my 2 cents!
