Would be nice if this was backed up with measurements and some kind of theory that related the pressure variations at enclosure walls with the movement of the air in the port. Personally I worry more about the resonances in the port itself, these can be quite persistent and troublesome in two way systems. Anyone with a sound measuring rig can comply to that.
If you have software to model a ported box that is based on the dimensions of the box and not only its volume, like TL-modeling software has, you will be able to find the best location for a port relative to the box's dimensions and the woofer's location. It's simply not true that locating a port near the top of bottom will be necessarily be problematic and locating the port in the middle can be very problematic.
Paul
Paul
I remember when I first read that ported enclosures should be lined rather than stuffed. I took it with a grain of salt then, and it remains a seasoned issue. Due to the damping being less effective at the lowest frequencies it is not unreasonable to deal with box modes without destroying the bass-reflex action, and when unsure, which is more important?
Ideally the default would be for the totality of the inside of the box, and all port dimensions to be within direct range of the driver (all acoustically small at reflex frequencies). The port location and size therefore, shouldn't be critical with regards to how effective they are as a port.psa78 said:
Sure. Thats exactly what I did over in the Monkey Coffin thread. The discussion was quite controversial. Go and have a look...
The pressure at the port throat drives the movement of the air mass in the port.
A port walks into a bar, and orders a glass of port...
Meanwhile, I think having symmetric flare at both ends, and no obstructions, is more important than square versus round, based on Button/Salvati/Devantier's AES papers from Harman/JBL research. This suggests an interesting port would be a slot port vertically along the back wall, with...jeez it's hard to describe. You'd have a wall coming down from the top, and one coming up from the bottom. They would be spaced apart, forming a port. You could make this symmetric.
Meanwhile, I think having symmetric flare at both ends, and no obstructions, is more important than square versus round, based on Button/Salvati/Devantier's AES papers from Harman/JBL research. This suggests an interesting port would be a slot port vertically along the back wall, with...jeez it's hard to describe. You'd have a wall coming down from the top, and one coming up from the bottom. They would be spaced apart, forming a port. You could make this symmetric.
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Note! Some amount of subjective opinion and personal viewpoints may be part of the following view, even though I strive to be objective:
My (somewhat limited) experience (it is just a hobby after all) is that for some certain designs slot ports are a very good choice, but it depends on what you want (IE design choices).
In some scenarios a more restrictive slot port can be more preferable to a less restrictive round port. The very general summary listed below is my personal findings where room influence is completely left out of the discussion, I've been mostly looking at port issues at high SPL outside in halfspace (1/pi), trying to discern differences.
It could be that in one specific case you would want the roll-off or resonant behaviour of the port to stretch over a slightly wider area, to help reduce movement of the speaker cone just a tiny bit before the calculated box FS (because perhaps doppler distortion can be a bigger concern in fullrange or 2way designs), and to get a slightly less agressive roll off below FS.
The "price you pay" for this is at at higher SPL will sometimes be a higher 2nd and 3rd order distortion where the port is more effective, which IMO can be a nice feature sometimes because if done correctly you get a sense of "warm" sound at certain volumes. Can be very engaging and fun, and a restrictive slot can actually seem a bit more natural since a lot of acoustic instruments also give off a lot of harmonics at higher output.
For clean and loud a round port with rounded edges, like Aero port or equivalent, is king. You get a somewhat narrow area where the port is very active and breathes free, for a big subwoofer this would often be better, since the woofer needs a bit of excursion to vent out heat, and the port kicks in to vent the box just below the max excursion of the woofer. Ironically, if the sound is very clean and undistorted it is disturbingly common to turn up the bass quite a bit more, since many people will feel there's something lacking in lower frequencies.
Take it anyway you want, this is just a hobby for me, and while I have made some measurements it is across several different computers, some of which have been reformatted and repurposed, I do not have time to hunt for distortion data that in some cases may be missing entirely (IE gone forever).
Sometimes I find a little bit of distortion can be a wonderful part of the experience, other times it should be avoided as much as possible.
Depends on the implementiation, useage and surrounding circumstances.
If you're looking for scientific sources I am not entirely sure where to find them.
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Back when I was using LMS/LEAP I could model driver excursion in a given enclosure. I would consistently see well controlled excursion above port tuning, minimal excursion at tuning, and the driver quickly unload below tuning. It's another factor to consider when developing ported systems.
The larger the cross-section of the port, the longer the port needs to be for a given tuning. This forces other design considerations such as enclosure dimensions, internal reflections, driver/port placement and the like. Everything is a trade-off.
While it's true that a larger port will reduce the pressure build-up within the port, it will also allow greater excursion below tuning than a smaller port at high volume. One of the "tricks" often seen in stage gear for example is the use of small ports for large drivers. At lower volumes the port functions normally. At higher volumes the port is unable to pass the higher pressures generated by high volume and essentially shuts down. At these pressures the enclosure now functions similarly to a sealed enclosure. This provides greater control over driver excursion at higher volumes.
Contrary to popular belief, port velocity is secondary to the the pressure build-up at the ends of the port. Relieve that pressure and the volume of air in the port moves more freely through the port.
Port design is essential to a good system and is all too often done poorly.
Nothing is free. Everything is a trade-off.
The larger the cross-section of the port, the longer the port needs to be for a given tuning. This forces other design considerations such as enclosure dimensions, internal reflections, driver/port placement and the like. Everything is a trade-off.
While it's true that a larger port will reduce the pressure build-up within the port, it will also allow greater excursion below tuning than a smaller port at high volume. One of the "tricks" often seen in stage gear for example is the use of small ports for large drivers. At lower volumes the port functions normally. At higher volumes the port is unable to pass the higher pressures generated by high volume and essentially shuts down. At these pressures the enclosure now functions similarly to a sealed enclosure. This provides greater control over driver excursion at higher volumes.
Contrary to popular belief, port velocity is secondary to the the pressure build-up at the ends of the port. Relieve that pressure and the volume of air in the port moves more freely through the port.
Port design is essential to a good system and is all too often done poorly.
Nothing is free. Everything is a trade-off.
Since the diffraction takes place at the port ends and is the primary reason for the pressure build-up at the ends of the port, using a flare, radius, or even cutting the inside of the port tube at an angle will help relieve that pressure as the air transitions the port end. Properly done, the air column within the port will then be able to move freely at velocity. It's not the port velocity that's the problem in a properly designed ported enclosure. It's the diffraction at the ends of the port that brings the bulk of the audible issues.
It's funny to hear a listener comment how they can feel the air coming from a pair of speakers with 2" ports at around 10' away, but never hear them. I've heard someone say it.
The argument that round ports are better due to smaller circumference versus area only holds if the velocity of air becomes high enough to cause turbulent flow (chuffing noise and compression which results in harmonic distortion). See:
Port Flares - Evaluation of noise
If you aren't listening at loud volumes and violating the max air velocity the port is capable of, all shapes of ports effectively perform the same.
If you are listening at loud volumes there are three solutions to reducing turbulence
-Add flares to the ends of the port
-Use a circular port as it gives the highest air velocity capability per unit area
-Make the port physically larger
If you don't care about the physical size of the enclosure, building larger rectangular or slotted ports can be a lot less complicated than having to source or custom build very large flares and pipes.
Keep in mind however that very long ports also cause pipe resonances up in the midrange frequencies, so if you're building a ported enclosure for a midwoofer instead of a subwoofer, you might want to keep your ports smaller and shorter to prevent noticeable resonances in the midrange frequencies.
Port Flares - Evaluation of noise
If you aren't listening at loud volumes and violating the max air velocity the port is capable of, all shapes of ports effectively perform the same.
If you are listening at loud volumes there are three solutions to reducing turbulence
-Add flares to the ends of the port
-Use a circular port as it gives the highest air velocity capability per unit area
-Make the port physically larger
If you don't care about the physical size of the enclosure, building larger rectangular or slotted ports can be a lot less complicated than having to source or custom build very large flares and pipes.
Keep in mind however that very long ports also cause pipe resonances up in the midrange frequencies, so if you're building a ported enclosure for a midwoofer instead of a subwoofer, you might want to keep your ports smaller and shorter to prevent noticeable resonances in the midrange frequencies.
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