A couple port questions

[angus_m]

Thanks. I see he put the trap at 1/4 of the port length, while I put mine at 1/2. Maybe someone could shed some light on which is better and why.

Hi -

I'm new around here, but attached are some visualizations of acoustic pressure for the port modes.

Perforation at half the length of the port will work best on the half wavelength resonance port
Perforation at either 1/4 or 3/4 will act best on the full wavelength resonance in the port

I wonder if you could treat both with a perf section at ~1/3?

See attachments showing acoustic pressure for the main enclosure tuning, as well as full and half wavelength resonances in the port. These are cutaway images through the port.

Thanks everyone - this thread is a goldmine!

Angus

[4thtry]

I recommend staying with round (+flared if you like) ports vs. slot if you want accuracy. Slot ports have an additional problem where the internal end of the port inherits more "length" due to using one or more walls of the cabinet, in other words the airflow "sees" a longer port because the exit is not as "final" as a round port which is generally away from side walls. Not a problem if your 'slot' port is not using any cabinet side walls, i.e. a square port that you would position in the same place as a round port even though this isn't a slot port, IMO. This results in the requirement for more calculations to get correct tuning. How much one cares about this is up to oneself.

Thanks, Mainframe. On my latest build, my rectangular port calculated to 17 inches but measured like a 21 inch long port. This explains why. I did not account for the walls!

[4thtry]

Thanks. I see he put the trap at 1/4 of the port length, while I put mine at 1/2. Maybe someone could shed some light on which is better and why.

When building my Plumber Delight speakers, I experimented with putting the trap 1/4, 1/2, and 3/4 down the length of the port. As I recall, it did not make much difference. The critical thing was trimming the trap length and adjusting the trap stuffing density to get it just right. Your port is much shorter than mine was, so your resonances are much higher in frequency and lower in magnitude.

[Mainframe]

I recommend staying with round (+flared if you like) ports vs. slot if you want accuracy. Slot ports have an additional problem where the internal end of the port inherits more "length" due to using one or more walls of the cabinet, in other words the airflow "sees" a longer port because the exit is not as "final" as a round port which is generally away from side walls. Not a problem if your 'slot' port is not using any cabinet side walls, i.e. a square port that you would position in the same place as a round port even though this isn't a slot port, IMO. This results in the requirement for more calculations to get correct tuning. How much one cares about this is up to oneself.

[augerpro]

Among others, maybe you will find #6 [Port perforations] in the attachment interesting.

Now THAT is a great paper! Harman's must be based on it. I actually like this paper's method better than Harman's for exactly the reasons that I diverged from Harman: normalizing based on tuning, not length, and later on length but with varying diameter so tuning was still the same. Curiously Roozen's worst performer (D) superficially looks like mine, BUT Harman did several variants where the changed ratio of length vs wall curvature radius. The one I selected was called N=0.5, which was sort of in the middle. At the high end of N there was much more curvature, and at the low end it looked pretty similar to Roozen's E. So I'm guessing his D was more like the N=1 port from Harman's paper. Still he specifies the optimal contour as like Harman (and his own D) but where curvature from port center to exit is not faster than 6 degrees. I'm going to try that, and tapping the port at 1/2 and 1/4 length.

[keith_correa]

Among others, maybe you will find #6 [Port perforations] in the attachment interesting.

[augerpro]

Thanks. I see he put the trap at 1/4 of the port length, while I put mine at 1/2. Maybe someone could shed some light on which is better and why.

[Chris Roemer]

Found it.

http://techtalk.parts-express.com/fo...rs?view=thread

[augerpro]

I'll post full results this weekend, but just wanted to share some early results of my tapped port for dampening the port resonances. The port is based of my Precision port clone. I added square taps around the centerline. First plot compares results with taps open, then wrapped with wool batting, then duct tape over the wool to create a more or less sealed cavity. Second plot compares the final taped version vs the standard Precision port. The resonance at @1.3khz is the predicted first resonance. The broader resonance above that to about 3.5khz is a little unaccounted for.

[fpitas]

It's a trap!!!

Someone had to say it.

[augerpro]

Interesting. Is there documentation on this project?

[Chris Roemer]

Have you ever seen 4thtry's project where he successfully trapped the port resonance? This was from 4 yrs ago at Iowa DIY.

[augerpro]

Watch this thread over the next couple days: https://www.diyaudio.com/forums/mult...thread-26.html. I'm measuring some new ports and the results are promising. I've abandoned the Kef method of flexible center sections and went with tapped center sections feeding a cavity damped with wool. The initial results are very encouraging. Tomorrow I do harmonic distortion tests then I'll post results.

[augerpro]

I don't have the Kef study on hand (search for Kef LS50 white paper) but attached the Harman paper. One version of the Kef port did use pipe insulation foam, but I'm going to investigate tapping the center of the pipe and damping that energy instead of trying to find the perfect flexible material of the right diameter. Might be easier to read about the project here (round 3 had the ports): https://www.somasonus.net/box-construction-methods

Harman Port Study.pdf

[tvrgeek]

Spurious thoughts:

Not finding reference to the Harman port or the Kef study. Can you point to them?

The above plots are interesting. I am more concerned with performance at way lower mach numbers.
In manifold design. it was determined that a 260 degree round over was optimum but need not be larger than 1/2 inch radius to prevent the artificial narrowing of the port though a 90 degree elliptical onto a flat plate was almost as good. Too bad I have not found an elliptical radius router bit.

It would be logical, that to reduce resonances, a variable diameter would be adventurous, but much harder to tune.

A material for an absorbent center might be semi-rigid pipe insulation. It would need to be lined of course. I can think of gluing many materials inside a cardboard tube.