Hi,
pipe with two open ends has modes so that velocity nodes are at the ends of the pipe. Thus, lowest mode in the pipe itself has pressure node in the middle of the pipe. Holes middle of the pipe would prevent pressure buildup, and reduce the first pipe mode. I remember this trick worked quite nicely just with holes to the enclosure, but of course could add more noise like schuffing or reduce output of the main Helmholz resonance, and so on, never tested any further. Kwesi measurements show very good performance in this regard, as there seems to be very little other "noise" than the pipe mode, which is then reduced quite nicely.
Midrange noise ~15db below main Helmholz resonance peak is already quite good, rarely see any better on data posted on the forum, while less than 10db kind of calls for improvement. Worst case, the mid leak could be louder than the Helmholz resonance making the port ruin the whole speaker.
Besides pipe modes, any noise inside the box wants to leak out if pressure node is at the inlet. Especially bigger enclosures easily have box modes within pass band and leak all kinds of midrange poo out of the port, in which case positioning of the "inlet" makes a difference. Also positioning of the transducer(s) now affect port performance, since they affect amplitude of box modes, and so on.
ps. people talk that 3-way speaker is hard project compared to a 2-way speaker, likely due to more complicated crossover, but things like the port and box noise is more problematic with 2-way speakers as port is about mandatory and woofer has wide bandwidth to cover, while 3-way speaker could work without one while also having less of a bandwidth to mind about. Crossover are basically trivial to come up with in simulator, while misplaced port requires building the whole box again unless port is made to be tweaked for best performance, so, my opinion is that 2-way speaker is lot more work than a 3-way speaker. I found port debugging a lot of fun and spend quite some time with, but result wasn't as good as hoped so just got rid of ports and have been concentrating other things for now. It is fun and informative thread, a lot of work, so I hope you'll arrive to very good performance!
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I think this only happens in two cases.
1 - when the surface area of the port is relatively large.
If we forget the standing waves of the port itself for a second. The port itself has a low pass effect.
This effect will be less when we make the surface area bigger, until we reach an almost open enclosure (aka dipole)
2 - above the first mode
Also, this midrange backwave should give a bit of a dipole /cardioid response, since its phase is the opposite.
A PR is also not 100% leak free (plus has a whole bunch of other problems)
In practice, the issue is just 2-way systems where we can't pick a crossover lower than the first port resonance.
Often it's possible to get this around 1-1.5kHz or so, so it's mostly about the first mode.
For anything big, I would always go for a MLTL. Which should have a lot less issues. (a MLTL is far more BR than TL)
Also with internal standing waves of the cabinet.
Unfortunately it's not a good option when something has to be small and compact.
It's roughly 15-20% bigger or so compared to a regular BR.
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That's all for passive systems.
For a more active system, I would pick a 2-way (either closed, or passive/active cardioid) and filter around 100 (or higher in stereo) with some subwoofers.
These subs can be in the same cabinet.
Bookshelf or floorstander.
Subs can be closed or BR.
that's exactly what I am after. but instead of many holes with low absorption efficiency and high tendency of turbulent air flow around the holes I am trying to solve the issue with one very efficient opening, respectively one very light membrane.
also note that we are speaking of very different things here:
- the port resonance (high frequency) has a pressure maximum at port center.
- the helmholtz resonance uses the air in the port as weight. the whole port air volume should move in and out (idealized case). there is no pressure maximum at the port center for the low bass reflex tuning frequency. the pressure maximum is inside the box (and it can be huge).
I realized the latter, when seeing (and hearing) the very strong correlation of H3 harmonics and tiny air gaps or leaks. it's also very audible at high volume.
that could actually be a very good testing method: whenever around the tuning frequency of a bass reflex system there is more H3 than H2 the chances are high that there is an air leak (or you push the system above linear behaviour).
that's my point:
why opening the absorber holes to the high pressurized box interior? that does not seem to make any sense to me.
not only it reduces the bass reflex efficiency but it also (unnecessarily) introduces a huge chuffing problem with air being pressed through damping material and small holes.
I suppose you have seen clearly that @Kwesi used a second sewage tube to isolate the absorber airtighly from the box interior! that makes all the difference!
edit: here is an excerpt of the philips/roozen paper that mentions port holes as possible solution to reduce resonances.
note that the port is not INSIDE the box! the paper does not mention this explicitly but it is highly relevant.
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Hi, yes of course, that's exactly why I chose the wording like I did, that the holes are for the pipe mode and do work even without the sleeve for the pipe mode, but that isn't usually the only issue although very visual in the graphs. Should not have posted at all, seems to add just noise to the thread as you seem to be on top of it. Harmonics are something augerpro also measures in his thread, perhaps a good way to evaluate audibility of issues?
not at all, please keep the discussion up! I appreciate your contributions to diyaudio (and my thread) very much!
you mean: best practically achievable result?
or: goal to be met?
if the (port) resonance is outside the driver passband I would not bother much.
for 2-way midwoofers the resonance should be kept as low (and directed away from listening spot) as possible.
generally small ports will have much less resonant behaviour (length/diameter ratio is better) and further reduction is possible with resonance absorbers.
have a look here. (edit: just saw, that was a response to your post, so never mind)
also note that my bandpass box is quite bad regarding bass output/resonance peak ratio as the chamber is small but nontheless has a rather big port (except for the small port examples).
this means:
- less absorption of high frequencies due to low volume and surface.
- high transmission and excitement of resonances due to big port surface
as b_force also mentions:
you can furthermore try to reduce the level of resonance by dampening the box interior (dapening material thickness should allow absorbtion of resonance peak frequency range, bass cannot be absorbed anyway, if we stay within reasonable box sizes). works even better with MLTL, as the port is far away from the driver, possibly even behind a TL fold.
combining all these measures should allow to reduce the port resonance peak way below 20 dB of bass output.
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Haha tell me about it!!
Having holes in the port open to the speaker enclosure does effectively reduce resonances. I will go look for some data in a moment. I put this down to allowing a pressure release from the port node, but I have not considered the phase relationship between the inside of the port at tube resonance and the inside of the cabinet. Anyone know what that is doing?? It might be a different relationship than at fundamental helmholtz resonance.
the "elephant in the box" is the air leak from enclosure to port at the bass reflex tuning frequency. you absoultely don't want that air leak! it reduces the bass output and in addition creates chuffing noise.
maybe i should do a measurement to prove it...
Thanks, don't worry not quitting
I should have chosen words wiser thats all, on both of my posts. Bad wording adds noise and confusion, while better reduces, and in this case perhaps it was unnecessary to post at all as everything was already written I think. I should never write in a hurry, not the first time, and probably not the last time either You are right, it reduces efficiency, so the trick is finding a balance where the unwanted resonances are reduced more than the fundamental. This is the balance I was going to use, until I noticed how bad the chuffing was. 53x 0.5mm holes, larger holes has a higher loss at the fundamental. Even the yellow trace is not actually a bad result IMO, that was a slot port made with 3mm MDF.
I think with small you refer to its length?
Or better, the ratio between surface area vs length.
Because the resonance with ports with a small diameter is far more obvious.
Until it's so small that the boundary effect becomes so significant that it influences the output even at very small levels.
My simulations (Martin's old MathCAD sheets) and most typically posted here showed very much worse port contribution. I did find it possible to, at least in software , tune it mostly out by sliding the driver down and port up on the front baffle but the alignment was titchy to the extent of requiring readjustment depending on stuffing density.
BTW, -20dB is equivalent to 10% distortion. My current project's driver runs under 0.2% right in the range the port would be dropping it's delayed contribution. Wasn't worth the trade off for me with a full range.
Yeah, philosophically port is bad compromise, it trades-off some midrange fidelity for more bass. Midrange fidelity is now lost forever, it can be restored only by blocking the port and adding more bass with some other way, like increasing volume displacement by using bigger/better/multiple drivers.
Quadrupling volume displacement of average 6" driver is relatively easy if there is some more space and some more currency units so getting rid of ports in home hifi is not too difficult. Quadrupling 32x double 18" on an EDM concert might make the size and cost too much and ports make a lot of sense. And as already said ports aren't much of an issue with subs, so just something to think about if one starts thinking about philosophically Nice bass is quite big part of good sound, so perhaps losing some fidelity for more bass is fine if space and money is premium, and there is no possibility for separate subwoofer system.
Quadrupling volume displacement of average 6" driver is relatively easy if there is some more space and some more currency units so getting rid of ports in home hifi is not too difficult. Quadrupling 32x double 18" on an EDM concert might make the size and cost too much and ports make a lot of sense. And as already said ports aren't much of an issue with subs, so just something to think about if one starts thinking about philosophically
Nice bass is quite big part of good sound, so perhaps losing some fidelity for more bass is fine if space and money is premium, and there is no possibility for separate subwoofer system.
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We might not talk about the same thing?
A MLTL is basically just an additional board to break the internal reflections.
The downside is that because of the triangle type shape, its volume works a little less effective, so we often need a longer line.
All other things are the same.
But since it's folded for any practical purposes like @stv already mentioned, the midrange will have a much harder time reaching the port.
It does have downsides in another way.
Because dampening the reflection directly behind the driver can be challenging without chocking the air flow.
A closed box design is a bad compromise, very little bass output withe a lot of cone excursion.
An open baffle (dipole or cardioid) is even worse in that regard.
With a quarter wave TL or horn system these resonances are even bigger.
A 3-way system will be more complicated and more expensive.
Conclusion: they are all bad, just a different kind of bad 😉😂
Mass loaded transmission line. Not to pick on Tom but this link is very typical of the results I've seen using this alignment:
https://neurochrome.com/pages/designing-mltl-speakers
The partition's function isn't related to breaking up internal reflections. It permits folding the cabinet into a more compact form factor but MLTLs can also be straight.