flared ports are typically simply a patch for when the diameter of the port is too small in the first place.
otherwise you've got specific desires for the radiation pattern the flared end pours out into the room.
**edit**
any person good with gases can tell you the flared end can cause more or less resistance depending on the content of the air.
and if more or less resistance is what you are after, a change in port diameter can give more or less resistance.
the problem is how much resistance of the choked or more-free flowing is necessary?
when you only need a twinkle, you typically know somebody is trying to maximize the most from a speaker that was stubborn in the first place.
i don't claim to be good enough with gas to note if there is any possibility (without inducing) as to whether the flared end chokes or frees up some resistance.
but
what i do know is, there are changes in atmosphere pressure from one season to the next .. as well as one geographical location to the next .. meaning, if the speaker has a gap in quality that big.. it should be a reject in the first place.
it is rather a gamble to provide that much of a tiny difference to help the speaker, only to see all of those math calculations go to waste because you modeled the cabinet for the summer time when it is hot and now it is winter time when it is cold and the performance improvements have utterly disappeared (or possibly became opposite in execution).
otherwise you've got specific desires for the radiation pattern the flared end pours out into the room.
**edit**
any person good with gases can tell you the flared end can cause more or less resistance depending on the content of the air.
and if more or less resistance is what you are after, a change in port diameter can give more or less resistance.
the problem is how much resistance of the choked or more-free flowing is necessary?
when you only need a twinkle, you typically know somebody is trying to maximize the most from a speaker that was stubborn in the first place.
i don't claim to be good enough with gas to note if there is any possibility (without inducing) as to whether the flared end chokes or frees up some resistance.
but
what i do know is, there are changes in atmosphere pressure from one season to the next .. as well as one geographical location to the next .. meaning, if the speaker has a gap in quality that big.. it should be a reject in the first place.
it is rather a gamble to provide that much of a tiny difference to help the speaker, only to see all of those math calculations go to waste because you modeled the cabinet for the summer time when it is hot and now it is winter time when it is cold and the performance improvements have utterly disappeared (or possibly became opposite in execution).
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Good thoughts, thank you for sharing, this is the kind of conversation I hoped to spark.
I will contribute my views when this hangover goes away.
The semi-popular, if rarely understood, Big Vent Reflex (BVR) philosophy also implies there is more to vent geometry than classical BR theory would suggest. Even the very existence of the MLTL as a design concept (When is a BR really an MLTL? That depends...), and also the Onken designs indicate clearly the limitations of the point'n'click assumptions underlying many BR simulations and designs.
What does port Q mean in practice?
Plainly, most enclosures have a bit of everything in them.
I hope someone smarter than me (ie. anybody) can codify the borders between the various design philosophies and we can identify and model designs in toto, from every angle.
Port area, and port length, and port volume and port shape all have a secondary influence on performance. If they are not considered in design then they can have a Primary influence on performance, witness the sub with wildly chuffing ports.
Keep up the discussion, it might really go somewhere.
Rgds
blakkbeat
What does port Q mean in practice?
Plainly, most enclosures have a bit of everything in them.
I hope someone smarter than me (ie. anybody) can codify the borders between the various design philosophies and we can identify and model designs in toto, from every angle.
Port area, and port length, and port volume and port shape all have a secondary influence on performance. If they are not considered in design then they can have a Primary influence on performance, witness the sub with wildly chuffing ports.
Keep up the discussion, it might really go somewhere.
Rgds
blakkbeat
So apparently in horn theory it's all about (as far as efficiency) coupling the relatively high mass diaphragm to the low mass air effectively. By the same token if you have a high pressure wave coming out of a small port it is not going to couple as effectively to the air outside the port as a lower pressure higher volume wave from a larger port and much of that energy will be lost as heat in the voice coil and other losses in the box. I like the idea of a horn for the port, however for that to work effectively you would basically have to have a "full size" horn so one might as well horn load the front of the driver at that point and gain efficiency across the entire intended frequency range for said driver. What you are talking about sounds like a tapped horn to me.
Horn theory does not transfer directly to air in a port. The point of a horn is to couple a (heavy) diaphragm to (light) air. With a port, you are coupling air to air. No impedance transformation required.
Compressed air is heavier than uncompressed air so not only are you trying to squeeze a lot of air through a small area (compression) you are creating even more of an impedance mismatch than exists with a larger port. It is my belief that the air in the port should be looked at as a diaphragm unto itself and the smaller the diaphragm the less efficient it is especially the lower you go in frequency. Can you give me more reasoning for your viewpoint? If I am wrong I want to know.
Don Hills is correct, a port is not a diaphragm, it should not be viewed as one, unless you want to confuse yourself 😉.
Unlike speaker cones, which gain 3 dB in efficiency when their area is doubled, a small port area is just as efficient as a large port area.
However, the contained air volume in a small duct when driven with a large displacement woofer will become turbulent at high excursion, in addition to making "chuffing noise" it no longer will function as a Helmholtz resonator when the air is "blown out" or is moved too fast.
A relatively small port volume can serve a low excursion driver just fine, but put a high excursion speaker in the same box and and it will only function well up to the same excursion as the lesser driver.
This leads to a classic dilemma, large volume ports required for low tuning with long excursion drivers reduce cabinet volume, which reduces efficiency. Long excursion drivers tend to have more moving mass than shorter excursion drivers, also reducing efficiency, but not ultimate SPL.
Given a specific total cabinet volume (cabinet volume + duct volume) a compromise must be reached between efficiency at low volume (power and excursion) and efficiency at high power.
Art
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How is it that the air in the port is not a diaphragm? Every air particle acts as a diaphragm to the next air particle and so on. Granted, it is just a much lower density diaphragm than the driver cone but still a diaphragm nonetheless. The purpose of a driver's cone is to push air particles and create a pressure wave, the air particles in the port do exactly the same thing. I would bet the air in a small port is at least several times the density of the air outside the enclosure and if this is so then why would the same laws that apply to the driver diaphragm not apply to the air in the port?
Edit: I looked up the definition of diaphragm and you are correct in the strictest sense, but I think not as it applies to our discussion.
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The air in the port is most definitely not a diaphragm. It is an acoustic mass. A diaphragm, in the context of electrodynamic loudspeakers, is connected to a suspension and is directly driven by the voice coil.
Please refer to some basic documentation for further reference:
Engineering is the Best: January 2008
Small-Direct-Radiator-Loudspeaker-System-Analysis.pdf
If you feel otherwise, please publish a paper in a respected scientific journal so that it can be professionally peer-reviewed, as we would all like to see what it is that you're trying to get at.
That is a very broad generalization that seldom works except in meticulously designed systems with a specific purpose. It is not the case 99% of the time.
To sum up, assuming in all cases the enclosure is to be tuned to a specific frequency:
Port area too small: Flow will become turbulent at high driver excursions, causing audible "chuffing" and de-tuning the enclosure.
Port area too large: The required port length may become so long as to take up excessive enclosure internal volume, and resonances in the port tube itself may become problematic. (If you're porting an enclosure for a driver that goes up to 300 Hz, and the first port resonance is 200 Hz, you may have a problem.) Also, HF sound radiated from the rear of the driver may leak out of the port.
Between those limits, there are a range of sizes that will work.
Regarding efficiency, the main limitation is at the small end. The ratio of port volume to port wall area determines the Q. Imagine an enclosure with, say, a 3 inch port. Measure its Q. Now fill the port with drinking straws, cut to the same length as the port tube. Ignoring the slight reduction in area due to the thickness of the straw walls, the air mass in the port does not change. But the greatly increased air friction will greatly reduce the Q.
Port area too small: Flow will become turbulent at high driver excursions, causing audible "chuffing" and de-tuning the enclosure.
Port area too large: The required port length may become so long as to take up excessive enclosure internal volume, and resonances in the port tube itself may become problematic. (If you're porting an enclosure for a driver that goes up to 300 Hz, and the first port resonance is 200 Hz, you may have a problem.) Also, HF sound radiated from the rear of the driver may leak out of the port.
Between those limits, there are a range of sizes that will work.
Regarding efficiency, the main limitation is at the small end. The ratio of port volume to port wall area determines the Q. Imagine an enclosure with, say, a 3 inch port. Measure its Q. Now fill the port with drinking straws, cut to the same length as the port tube. Ignoring the slight reduction in area due to the thickness of the straw walls, the air mass in the port does not change. But the greatly increased air friction will greatly reduce the Q.
ah..
i remember using straws to try and match the impulse response of the room.
i think it helps if the speaker is already close in range though.
i remember using straws to try and match the impulse response of the room.
i think it helps if the speaker is already close in range though.
Are you saying that a larger port and corresponding increase in wall area (due to the increased length to maintain the same tune) is going to increase friction and turbulence thus reducing efficiency?
That is exactly what he is saying. Port "Q" is similar to driver "Q"---it is number that demonstrates the relative strength (or similarly, damping) of the fundamental resonance.
High Q speakers resonate a lot at Fs (possibly large bump in response), and low Q speakers resonate less (obviously it is all relative). The same kind of description can be translated to the port output (dB) at its fundamental resonance.
This is the reason circular ports are just the teensiest bit more efficient than a rectangular port of the same area---the rectangular port has more surface area inside the port, yet the opening area that determines the tuning is the same. The "more rectangular", that is to say the greater the aspect ratio between the L and W of the rectangular port, the greater the frictional losses become at elevated levels.
Do keep in mind though, that increasing a port's area lowers the air velocity, so just because there is more internal wall to create drag doesn't mean that there IS drag because the air velocity just dropped by making the cross-section bigger.
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that is one reason to say think of the port like a diaphragm of a speaker.
because somebody say the cone needs to be connected to a voice coil makes it a diaphragm isn't the truth, because passive radiators have a diaphragm too.
but
efficiency has lost definition, as i am also able to say efficient from a port can be equal phase.. equal q.. as well as the ability to shape the soundwave inside the speaker box as if to do something with the reflections & ripples inside the box.
**edit**
just remember the air is like a spring, and the size of the port has a velocity 'tune' or 'tone' that can equal the same pulse from the speaker cone.
**edit again**
the point is to avoid a muddy speaker with a feathered light output from the port, or vice-versa.
obviously, if the speaker is muddy.. a larger port (4 inches) is going to help the port output sound muddy to match.
just don't make the mistake of not stuffing the speaker cabinet to be listening to the dumb echoes inside the port that ruins the experience (causes fatigue).
because somebody say the cone needs to be connected to a voice coil makes it a diaphragm isn't the truth, because passive radiators have a diaphragm too.
but
efficiency has lost definition, as i am also able to say efficient from a port can be equal phase.. equal q.. as well as the ability to shape the soundwave inside the speaker box as if to do something with the reflections & ripples inside the box.
**edit**
just remember the air is like a spring, and the size of the port has a velocity 'tune' or 'tone' that can equal the same pulse from the speaker cone.
**edit again**
the point is to avoid a muddy speaker with a feathered light output from the port, or vice-versa.
obviously, if the speaker is muddy.. a larger port (4 inches) is going to help the port output sound muddy to match.
just don't make the mistake of not stuffing the speaker cabinet to be listening to the dumb echoes inside the port that ruins the experience (causes fatigue).
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Forgive me, but I find it preposterous to think that the smaller the port the more efficient it would be. There are a number of other factors, including the ones you mentioned Oscar, that would indicate a larger port would increase efficiency even though the overall wall area is significantly higher. I would not disagree that there are some significant practical disadvantages to a larger port but I think "practical tradition" and resultant views are outweighing common sense on the issue of efficiency.
I would have to say the air exiting a port is a diaphragm for all intents and purposes, whether or not it meets the strictest definition of the word.
I would have to say the air exiting a port is a diaphragm for all intents and purposes, whether or not it meets the strictest definition of the word.
while not connected to a voice coil, passive radiators have a suspension system too. You either didn't read what I posted correctly, or purposely chose to not quote it---I said "in the context of ELECTRODYNAMIC SPEAKERS.......". To say what I said isn't the truth is ignorant because I clearly specified the context. A passive radiator is not an electrodynamic speaker, therefore it is not necessarily not included in the set of all things that could be considered diaphragms. Read that a few times please.
dustman, you're overthinking every little thing. A smaller port does is not more efficient than a larger one because of the [hopefully] obvious reasons. When comparing two DIFFERENT SHAPED ports of equal cross sectional area, if one has significantly less internal surface area, it will be slightly more efficient and elevated power levels. I haven't read enough scientific journals to tell you the exact dB difference, it could be hundredths or tenths of a dB, but I do know that is an important aspect to consider when air speeds inside a port are elevated.
I would rather use the word "radiator" as it is used in Akabak. In Akabak, speaker cones, ports, PRs are all considered radiators [of sound] if they terminate into open atmosphere---and must be specified as such in the scripts.
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