I have built a 200 liter box with an 18 inch driver.
I built a closed box and messasured frequency respone to see how it follows theory.
Now i compared this response to the same construction but with one 4 inch hole(vent ?) in it, and later on i tested it again with two 4 inch holes.
The tuning on the box with 2 holes appears at 29.3 Hz, that gave an increased response about 15 dB at that frequency and it lifted the whole band from that point towards 123 Hz which i have as dB 0 in all response tests.
Now when i have a look at the "one" hole response i can see that tuning is about 22 Hz, it turned out to lift the response in 22 Hz +- 2 Hz region but the amplitude upwards is between the closed and the 2two hole response curves.
Now my question is:
Do i get more amplitude if i tune the box to 22 Hz with "two" tubes than i got with one simple hole ???.
How do one calculate port Q and SPL ?
No ventnoise calculations needed. i dont care at this point, its an experiment.
/Mike
I built a closed box and messasured frequency respone to see how it follows theory.
Now i compared this response to the same construction but with one 4 inch hole(vent ?) in it, and later on i tested it again with two 4 inch holes.
The tuning on the box with 2 holes appears at 29.3 Hz, that gave an increased response about 15 dB at that frequency and it lifted the whole band from that point towards 123 Hz which i have as dB 0 in all response tests.
Now when i have a look at the "one" hole response i can see that tuning is about 22 Hz, it turned out to lift the response in 22 Hz +- 2 Hz region but the amplitude upwards is between the closed and the 2two hole response curves.
Now my question is:
Do i get more amplitude if i tune the box to 22 Hz with "two" tubes than i got with one simple hole ???.
How do one calculate port Q and SPL ?
No ventnoise calculations needed. i dont care at this point, its an experiment.
/Mike
As long as the vent is not seriously too small and thus restricting flow massively, the response will be the same however you tune to a frequency, be it 1 port or 3.
Simply make your 2 tubes (they must be identical in length) longer to tune down to the same frequency (22Hz) as your single tube.
Simply make your 2 tubes (they must be identical in length) longer to tune down to the same frequency (22Hz) as your single tube.
Hi, I have seen a well respected design ( WAD KLS3) using staggered port tuning to widen the port enhanement region. According to the designers, the staggered design suited a wider range of rooms. If both ports are adjustable (sliding tubes) you can experiment at your leisure. I found in my room that a single port about 30% longer than the correct design gave the best balance across the bass range. I have not tried two port
regards
Andrew T.
regards
Andrew T.
Thats interesting because ive always read that 2 different ports sum to one port-meaning a longer port and shorter port sum to being an 'inbetween' port-meaning you dont get this 'enhanced' 'widerange' port effect. Bandpass designs/Dual tuned ported do this-with assocated worse transient response.
Cheers
Cheers
That's correct, more or less. The fb of the box is determined by the total mass of air contained with the ports, irrespective of how many or how long they may be. The notion that different ports will enhance anything is pure piffel; there is only one fb with a single chamber box. A mutiple chamber box is a different story.
Ok, so now i have a slight peak on resonance frequeny at approx 30 Hz with a good response all the way up to 123 Hz. Allmost flat ! 28-130.
If i tune those two holes with tube down to the 22 Hz region i will en up with the same response as the single hole with same resonance , no increment in SPL?
Is that correct ?
If it is, i cant possibly tune lower than maybe 26 Hz or the whole band will fall downwards .
Time to throw in some numbers.
Driver: Fs 22,5 Vas 280, Qts 0.145 Qms 3.(57), Qes 0.151, Vd 957cm3, Xmax 8,7.
Box: 210 Liter (after bracing, driver volume and 2 estimated tubes 110 mm dia and 46 cm length). 18 Hz. which was a goal i have no problem giving up.
But that length is probably too long anyway.
/Mike
If i tune those two holes with tube down to the 22 Hz region i will en up with the same response as the single hole with same resonance , no increment in SPL?
Is that correct ?
If it is, i cant possibly tune lower than maybe 26 Hz or the whole band will fall downwards .
Time to throw in some numbers.
Driver: Fs 22,5 Vas 280, Qts 0.145 Qms 3.(57), Qes 0.151, Vd 957cm3, Xmax 8,7.
Box: 210 Liter (after bracing, driver volume and 2 estimated tubes 110 mm dia and 46 cm length). 18 Hz. which was a goal i have no problem giving up.
But that length is probably too long anyway.
/Mike
BillFitzmaurice said:
There was a discussion on a Swedish forum recently that pretty much concluded that there actually is an advantage in using several narrower tubes than one big tube. The reason for this would be that the narrower tubes would start to go into turbulent flow at a higher SPL, than the single one. Also, a tube with a rectangular cross-section would be preferrable for the same reason. The higher "aspect ratio" on the rectangle, the better. However, neither the port diameters, nor the height of the rectangular port should be less than a few centimeters.
The people invoved in the discussion are very trustworthy in my opinion, so this is definitely something to consider.
The link is in swedish, sorry about that.
www.faktiskt.se
Svante said:
Oh, the conclusions were no guesses, believe me...What kind of errors do you mean?
A hypothesis does not make a fact. While laminar fow has less friction on a moody diagram, many times things that make sense theoretically do not pan out practically. I wouldn't believe it until I saw test results. Fluid dynamics is still very empirical, even with CFD programs.
AFAIK, the only errors in a high aspect ratio port would be in the end correction. I do know that 6:1 ports seem to have the same end correction as a circle, so it might need to be extreme...
Svante said:
Sounds to me like they got it wrong on both counts.
1. Smaller vents have a higher surface to cross sectional area ratio. This is likely to cause more turbulence, not less. If you replace a single vent with two vents of the same area, there would probably not be a significant distance.
However, if you follow this through to a silly extreme, you get lots of small whistling straws!
2. Rectangular vents are handy as they are easier to fit into the box, but they aren't as good as a circular vent of the same area regarding turbulence - more difficult to get decent flares and they tend to have sharper bends. Increasing the aspect ratio ie making one dimension longer - this will only increase the surface to area ratio and I can't see how this can be a good thing.
Perhaps if you could mention some of the points that were made to try to support these points of view. Unless there are some surprising discoveries, they sound like silly statements.
paulspencer said:
The point is that this should not be taken into the extreme. That is why I wrote that the smallest dimension should not be smaller than a few centimeters. As I have understood it, turbulence needs space to occur. This would mean that for a given velocity, turbulence is more likely to occur in a wide tube than in a narrow tube. Before this "other thread" (which I regret that you probably cannot read since it is in swedish), I was also convinced as you.
As I have understood it there are two effects that are important in the design of the port, the first being the laminar flow resistance that dominates at low flows, and the second being the minimum flow that triggers turbulence. An extremely narrow tube will give a high laminar flow resistance, and this is not desirable. On the other hand, an extremely wide tube will go into turbulent flow at a lower velocity, than several narrower tubes would. The point here is that the laminar flow resistance is very small as soon as the dimensions of the tube reaches a few centimeters. This, in turn, would lead to that high-Q resonators could still be built with the tube split in several smaller tubes. Such a resonator would have a higher max amplitude before going inte turbulence, than the resonator with a single circular tube.
paulspencer said:
I hope "Isidor" finds this thread, and writes here, he is not the kind of guy that makes silly statements... But you would not know, so I understand your sceptisism. I think, without understanding it fully, that the Reynolds number was an essential part of the reasoning. This number, as I understand it, is crucial when determining the flow at which turbulence starts to occur in a tube. This number can be calculated, at least approximately, for a few common cross-sectional shapes, such as circular or rectangular.
I came upon this old thread searching for possible advantages of slot ports.
This friend of mine that was into pro audio told me many many years ago that once airflow through a vent becomes turbulent in nature, the bass reflex principle stops working, since turbulence leads to a fall in air pressure that blocks airflow. He claimed that by holding a cigarette lighter to the vent, he could demonstrate that as loudness goes up, there is a certain SPL, when the port can't cope anymore, where the flame would stop swaying back and forth and just lean away from the vent, as if the airflow has become DC in nature. I remember being intrigued by this at the time but have never tried it for myself.
He went on to claim that four 2" vents could deliver 6dB more SPL than one 4" vent before saturating, although their combined area is the same, due to some relationship I don't fully understand that relates to a greater Reynolds value allowing twice the m/s before the onset of turbulence). Back to Paul's remark about these conclusions eventually leading up to the use of small straws in lieu of a single vent (a familiar 'audiophile tweak' nonetheless), no it won't because you can only take this so far apparently, down to a limit defined by the so-called hydraulic diameter (again, beyond my grasp).
Building high-SPL bass reflex enclosures for sound reinforcement (and sometimes SPL car audio competitions), he was a big proponent of slot ports. He said that as long as they have a high width-to-height ratio (with the ideal height being between 1/4 and 1/2 inch), slot ports can produce higher SPL than round ones of the same area before choking. E.g. a 1/4" high slot port can take an air speed of roughly 3x that of a circular port before saturating, which means about 10dB in SPL capacity! Again, alleged reason being greater Reynolds - due to higher laminar flow - allowing for faster airflow....
I have no idea if what he said is true and I'm not knowledgable enough to challenge it. Regardless, a lot of air has flowed through bass reflex ports since he told me about these concepts and our understanding of flow dynamics, as it relates to speakers, ought to have advanced since. What are the latest theories on this interesting topic?
This friend of mine that was into pro audio told me many many years ago that once airflow through a vent becomes turbulent in nature, the bass reflex principle stops working, since turbulence leads to a fall in air pressure that blocks airflow. He claimed that by holding a cigarette lighter to the vent, he could demonstrate that as loudness goes up, there is a certain SPL, when the port can't cope anymore, where the flame would stop swaying back and forth and just lean away from the vent, as if the airflow has become DC in nature. I remember being intrigued by this at the time but have never tried it for myself.
He went on to claim that four 2" vents could deliver 6dB more SPL than one 4" vent before saturating, although their combined area is the same, due to some relationship I don't fully understand that relates to a greater Reynolds value allowing twice the m/s before the onset of turbulence). Back to Paul's remark about these conclusions eventually leading up to the use of small straws in lieu of a single vent (a familiar 'audiophile tweak' nonetheless), no it won't because you can only take this so far apparently, down to a limit defined by the so-called hydraulic diameter (again, beyond my grasp).
Building high-SPL bass reflex enclosures for sound reinforcement (and sometimes SPL car audio competitions), he was a big proponent of slot ports. He said that as long as they have a high width-to-height ratio (with the ideal height being between 1/4 and 1/2 inch), slot ports can produce higher SPL than round ones of the same area before choking. E.g. a 1/4" high slot port can take an air speed of roughly 3x that of a circular port before saturating, which means about 10dB in SPL capacity! Again, alleged reason being greater Reynolds - due to higher laminar flow - allowing for faster airflow....
I have no idea if what he said is true and I'm not knowledgable enough to challenge it. Regardless, a lot of air has flowed through bass reflex ports since he told me about these concepts and our understanding of flow dynamics, as it relates to speakers, ought to have advanced since. What are the latest theories on this interesting topic?
Last edited:
Turbulence happens fairly early on at low velocities. It's not until much higher velocities that chuffing becomes an audible problem and higher still velocities where the port ceases to function as a port. And even when it does become resistive enough to be mostly blocked, it will never become DC with a constant stream of output. That doesn't even make sense.
All of this is wrong and the science has not changed. There's a whole industry built around Reynolds numbers, moody charts, turbulence, and properly sized ducts. It's the HVAC industry. House venting is a large cost and so it makes sense that they have all kinds of knowledge and tools to make the vents perform optimally. Have you ever seen bifurcated house vents, or vents split into 4? They do not perform better than a single vent of the same cross sectional area. If there were an advantage the proof would be in your house ducts. And a large aspect ratio is not an advantage, it's a problem. The air in the middle of the vent can move freely but at the edges there is drag friction. A high aspect ratio means there is more surface area in the vent interior, this means a higher area of drag friction. A circular vent will vastly outperform a very high aspect ratio rectangular vent at high velocity. You won't see very high aspect ratio house venting anywhere unless it is necessary to fit into the walls.
It sounds like your friend learned a few cool words but got the concepts completely backwards. Either that or you are misquoting him.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.