Investigating port resonance absorbers and port geometries

step can't provide parametric adjustability - which is key to have a useful model that can be adjusted following the excel sheet!

But I could provide a collection of different printable models in stl, 3mf or step format.

I will also try to model a parametric port in freecad.
FreeCAD I have as well as Ondsel
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I'll let @augerpro decide whether he wants to upload a step file of his port model!

FreeCAD NFR=0,5 port example including STL and STEP​


In the meantime here are my models for a NFR=0,5 port.

The freecad model is parametric and can be adjusted as required.
Please note that mounting holes are not (yet) connected to the port exterior diameter/radius, so the respective circle radius in the sketch eventually needs to be adjusted, in case of port diameter adjustment.

The step and stl models (and the as-is-freecad model) have the following parameters:

actual length= 120 mm
effective exterior diameter = 80 mm (defined as radius = 40 mm)
flange width = 25 mm
thus, total exterior flange diameter = (40+25)*2 = 130 mm
flange thickness = 4 mm
port wall thickness = 3 mm

one important hint: the interior port surface should be as smooth as possible (mainly in the central are).
so if the port is to narrow to be smoothed by hand you might eventually want to print two halves for better access to the interior surfaces.
I'll post some pictures of my workflow.

Have fun!


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simulated/built tuning frequency deviation graphs​

Here are two graphs indicating the tuning frequency deviations of my excel sheet, based on the salvatti/devantier/button formula compared to the actually built examples of @augerpro and mine;
interestingly the "port length"/"min.diameter" ratio behaves nearly exactly as the max.surface/min.surface ratio. I guess there is a very logic geometric explanation, but I haven't thought about that yet.

In other words the "port length"/"min diameter" ratio defines how long and narrow (high number) or wide and short (low number) a port is.

The examples built are not many, but the "geometry ratio"/"deviation" pairs on both sides of the atlantic seem to match quite well, except for my two low ratio examples (red most left data points).
I included a first guess of a possible deviation pattern in light yellow.


As mentioned previously I will buld a bigger test/subwoofer enclosure using a 12" driver and will run further port geometry tests!

thanks once more to @augerpro for the valuable contributions!
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I am still working on whether the strouhal number limit of 1 is really useful for high quality music reproduction or if it should stay even higher.
But the number one limit can be a great guideline that needs to be respected with some safety distance.

In the next update the Hornresp default Strouhal number limit value will be 1, but the the user will be able to change it to 1.5 or 2 if they wish.
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Any as built/measured vs. calculated tuning frequency comparison using a nfr=0,5 port would be very useful for getting enough data for a correction factor to be added to the excel sheet. So any input is welcome!
Let me get my head around designing a port and I'll get it printed by a service. I don't have a printer yet. You guys need greater air volume measurements I think unless I missed something in the thread.
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how I make my 3d printed ports​

this is how I make my ports used for testing and hopefully finally also for my loudspeaker projects!
there are probably several different ways, but I found my method works well to get a smooth interior surface.
this is essential, mainly for the central high airspeed section.
see my tests in post #230!

still no mounting holes, because the ports are temporarily mounted with play dough.

01: printing upright in two halves, flange at the bottom. I use "brim" surround for better adhesion to the printer bed. this is essential with long ports that may otherwise come loose because of the "lever" length.

02: print complete!

03: inaccuracies and printing layers (perpendicular to air flow) must be sanded, filled, sanded

04: applying wood putty after first rough sanding of 3d prints. I use 80 grit sandpaper.
for big ports it may be possible to print one complete piece and sand, fill and smoothen the interior surface. But it's still much easier to do that with two open surfaces.
use 120 grit (and finer) sandpaper to smoothen the filled parts, once the putty is dry.

05: smoothed port halves. I like to apply a coat of grey primer to better see any remainig grates or holes.

06: guing the halves. I use pva wood glue but epoxy or superglue are probably much stronger.

07: filling the joint gap (3d printers are not good at printing sharp edges)

08: filled gaps

09.JPG 09a.JPG
09: sanding of joint. for my very small port I use small files.

10: sanded joint

11: using 320 grit pad to further smoothen surface

12: applying primer to fill small scratches. I use a matte primer that dries very fast and can easily be polished.

13: primed surface, some particles and brush strokes need to be smoothed out ...

14: smoothen surface again with 320 grit pad

15: smooth surface of finished port (still not optically perfect, but perfectly usable in acoustic terms).
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because the graphs above are mostly interpolated between the measurements at 0,5 - 1 - 2 - 4 - 8 V, I will provide a new measurement with smaller increments and additional subjective impressions.

detailed measurement of straight and flared port​

I used the straight tube port with small roundovers (0.6 mm) and the "BIG port" described earlier with a NFR=0.5 and took measurements with small one dB steps for the relevant range.
It was quite easy to determine the point where the turbulent/chuffing noise was clearly audible, using the 50 Hz sine test tone.

It was also very interesting that the straight tube port tone is getting ugly quite early (strouhal number around 2), but this is only related to the sharp chuffing resonance noise peak, which I suppose is not strongly related to strouhal number.
I suspect that even for the flared port the low frequency noise is not very audible, because it is masked by the fundamental 50 Hz tone. The noise gets audible once there is a slight resonance hump, at strouhal number of about 1,4.
Once more I noticed that compression and THD/H3 distorsion are not audible per se. It's the chuffing noise that is annoying.

As I noticed earlier with very high outputs (strouhal number far below 1) the straight tube port has less turbulent low frequency noise.
But as I pointed out earlier, the chuffing/turbulence noises for these levels is far beyond everything that is bearable for serious music listening.

Here are the chuffing spectrum screen shots and the comparison graphs for compression, H3 and THD:




conclusion for these port examples:​

keep strouhal number above 2 for straight ports
keep strouhal number above 1.5 for flared ports with NFR=0.5
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Do you any idea what the velocity levels would have been?
here is an overview of strouhal numbers, port exit air velocities and my subjective descriptions, referring to the test posted above.

the speaker has a sensitivity of roughly 84 dB @ 2,83 V / 1 m and was about 1 meter distant (except if mentioned in the description) positioned on a stand 1 m high (near 4 pi radiation).
the strouhal numbers and velocities are slightly different between the two ports because the straight tube port has an effective exterior radius of 2,77 cm and the flared port 2,88 cm. see plan in post 529.
furthermore the velocities/air particle displacements/strouhal numbers correspond to the real output. they reflect the real, compressed output (see post 565 for graphs!).


by the way: the air velocities in the central smallest part of the port is 123% of port exit velocity for the straight port and 369% for the flared port.
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