Is there a way to tell if the port will be in phase with the woofers? I assume this would depend on the frequency it's ported at and length.
My concern is placement. I would like to place ports firing beside the woofers, but if the port is out of phase, would this cause cancellation and reduce output?
I assume port beside woofer would give most output due to coupling.. is this theory incorrect?
Port length has yet to be determined upon this answer. Could someone please supply a formula? experience or theory is welcome.
My concern is placement. I would like to place ports firing beside the woofers, but if the port is out of phase, would this cause cancellation and reduce output?
I assume port beside woofer would give most output due to coupling.. is this theory incorrect?
Port length has yet to be determined upon this answer. Could someone please supply a formula? experience or theory is welcome.
If the vent is long enuff to cause issues, what you ewally have is a transmission line/quarter wave enclosure not a bass reflex. Consider the wavelength of the frequency at which you are tuning as a standard against which you are measuring its length.
dave
dave
Hi deebeez'
In the past (pre 80's), speaker builders would actually construct a box, mount a driver of unknown parameters and pray for good result. Today, with the advent of "Thiele Small Parameters", you no longer need to guess if a particular design will work, or how well. Just download a software based, "speaker design program" like "winISD", "Bass Box Pro" or Ishtek etc... and follow their instructions. PS: you may want to "refresh" a little on speaker design theory as this subject can get very tricky!
If the port is "out of phase" with the front of the driver, it won't matter where you place it. Remember, these soundwaves (talking about bass), can run anywhere from five, six feet long(200hz), to upwards of 40' (25hz or lower), so it doesn't matter where you punch a hole in the box, provided you don't block the path entrance or exit.
Again, the waves are very long, so the port(s) will couple with the cones at some point near the box. Just maybe "read up" some on different bass /sub-woofer alignments e.g., bass reflex (ported), I.B.(infinite baffle), acoustic suspension (sealed), free air... Then try to find a simple software program to do the calculations for you... like the rest of us do. 😀 Have fun!
rigtec, best regards
thanks for the help rigtec, i'm just concerned with phasing though. Is port phasing relevant? <80 hertz. And if so how do I calculate?
Well,... You know that the "front wave" and the "back wave" of the driver are already 180 degrees out of phase "right out of the gate", so it can only get better. One purpose of a "Duct port", is to create a delay in mS (milliseconds) to give the front wave a "Head start" so to speak. Theory is...
When two 180 deg, "out of phase" soundwaves (of that same frequency) meet, before they can travel one 1/4 of their full length, they combine destructively and cancel each other out. That's a problem with very long soundwaves; like at 25hz where the wave is 40' long. If that wave comes in contact with another (180 deg. out of ph) soundwave @ 25cps before it reaches 10ft (1/4 wave), they will cancel out; hence the need for a baffle.
Anyway, you don't have to worry about that because you're using a box, and a baffle, and not going to use two "dissimilar" drivers (I assume) in that same box, or in the same room... right?? Ok 😎 I personally don't know how to figure out (in mS) how much delay is needed to correct or "Re-align" the front and back waves properly; that's for engineers. I just need to use a program that'll do all of that for me... but it's nice to understand the theory at hand as well!
Again, just read up some on some simple "Audio system theory". I've been studying "Pro-sound audio" since the early nineties. I actually have been aware of the difficulties of bass amplification since around 1972 because I'm a bass player, and it takes a lot of power to just be heard on stage. That is why I became interested in the subject. If you go on-line, look for a book by the name of "Sound check", published by "Hal Lenard" Publishing. You will likely find some other great reads there as well!
Cheers!
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Learn what your simulator is trying to tell you and forget about formulas and rules of thumb. Here's a bunch of pics to illustrate what my simulator is telling me.
Pic 1 - This is a simulation of a transmission line showing port output only.
Pic 2 - This is the same tl but now showing driver output only.
Pic 3 - I've told the software to sum these two separate inputs. The program wants to know how close the two sources are to me relative to each other.
Pic 4 - I've told the software the distance difference is 0 cm and this is the summed response. They are both on the same baffle and both 1 meter away from me. (This is the default path length difference in the Loudspeaker Wizard. If your program doesn't ask the path length difference it's probably assumed to be zero.)
Pic 5 - I've told the software the distance difference is +999 cm, so the port is 1 meter away and the driver is 1 meter + 999 cm away from me and this is the summed response. This pic is basically the same as pic 1 which shows the raw port output. The driver is quite far away and has very little affect on the summed response.
Pic 1 - This is a simulation of a transmission line showing port output only.
Pic 2 - This is the same tl but now showing driver output only.
Pic 3 - I've told the software to sum these two separate inputs. The program wants to know how close the two sources are to me relative to each other.
Pic 4 - I've told the software the distance difference is 0 cm and this is the summed response. They are both on the same baffle and both 1 meter away from me. (This is the default path length difference in the Loudspeaker Wizard. If your program doesn't ask the path length difference it's probably assumed to be zero.)
Pic 5 - I've told the software the distance difference is +999 cm, so the port is 1 meter away and the driver is 1 meter + 999 cm away from me and this is the summed response. This pic is basically the same as pic 1 which shows the raw port output. The driver is quite far away and has very little affect on the summed response.
An externally hosted image should be here but it was not working when we last tested it.
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"Bass reflex" is also known as "phase inversion", so discussion of phase is relevant, but typical port location generally poses no phase problems.
The port location should be within 1/4 wavelength of the loudspeaker location to work properly at Fb (box tuning).
As an example:
Sound travels 1130 feet per second. Fb=40 Hz.
1130/40 =28.5 foot wavelength
28.5/4 =7.125 (1/4 wavelength)
If the 40 Fb port is within 7 feet (2.15 meters) of the loudspeaker location it will reinforce properly.
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