Re required port surface area for a given speaker: ports at large signals can and will suffer from harmonic distortion, noise, and compression. You'd have to define what goals you want to reach for each problem area - how low do you want distortion to be at full signal, how low noise, how low compression? Because they'll never be 0.
From the conclusion of "Maximizing performance from loudspeaker ports", Salvatti, Button - imo they say it best:
There's also all kinds of m/s values floating around. In the aforementioned paper, <10 m/s is thrown into the ring. Small afair stated an acceptable value of 15-20 m/s. My personal experience falls somewhere in the middle of that. But to each their own.
From the conclusion of "Maximizing performance from loudspeaker ports", Salvatti, Button - imo they say it best:
There's also all kinds of m/s values floating around. In the aforementioned paper, <10 m/s is thrown into the ring. Small afair stated an acceptable value of 15-20 m/s. My personal experience falls somewhere in the middle of that. But to each their own.
The interesting discovery in the respective papers and in the experiments shown in this thread (at least for me) was that airspeed is not that relevant by itself. It's the air displacement at the port terminations that matters.
Airspeed in the central section of the port can be huge and still pose no problem.
Yes. I actually think I will agree with this.
It's in fact almost worth putting it a bit more officially out there. 👍🏻
On the other hand, lower air velocity is always better in all cases.
But yes I am mostly curious to see how this will change certain things on a more practical level.
Because it could mean a smaller diameter and port length would be sufficient.
Meaning we're pushing that port resonance further up!
Definitely! Make the port (terminations) as big as possible.
That's the main benefit and one of the goals developed in this thread.
In fact the initial idea (and thread title) of resonance absorbers turned out to be less important once the port geometry was optimized!
Another benefit of a narrow center section is to attenuate enclosure resonances as much as possible.
Thanks, I appreciate your encouragement!
I still would like to confirm the results with a higher spl 12" driver ...
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Correct, the pioneers recommended Av = Sd for a baffle thickness vent, so when larger was needed a simple slant panel parabolic horn was added where St = Sd, so similar using round, square, rectangular tubes + the needed flare works great as Danley did in at least one of his consumer? designs.
Muffler adapter design I used as a guide: pg. 838, Fig.10 (Note the 'powerhouse' authors)
It will be clear from the chart heading that the Strouhal number applies to the port outlet.
(Hornresp assumes that the port tube is a cylindrical pipe).
Any recommended air velocity numbers cited anywhere would be for non-tapered ports, in which the air velocity would be relatively constant. I suppose you could calculate / simulate max. air speeds in flared ports with AKABAK, COMSOL or the like, but who, especially in the non-commercial space, has ever done that?
I must say though I was surprised how small the bottleneck of a flared port can be to still work well. Both via third party discoveries, like the aforementioned AES paper, as well as in my own tests. For example, as some here might know, I did the most extensive public review of the L'Acoustics KS28, including extracting the chassis and testing / comparing it in an enclosure with a 1/2 sd sized normal straight port with the same tuning. The L'Acoustics "L-Vent" is only 1/4 sd large at its bottleneck, meaning half the area, and yet performed similarly. It still had portnoise however
- not less than the straight port.Props for the thread. I only recently stumbled across it, on a research on port absorbers, within the development of a fullrange BR box with large port, and it helped me quite a bit.
It should probably be noted that this equation is appropriate for small velocities (= absence of turbulence), when we can assume that the system is linear, so we can describe the oscillations by a harmonic law. In the presence of turbulence, this ratio will give some error.
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All of these kinds of equations are only ever valid in linear systems and conditions.
So to me it sounds a bit obvious.
But yes it's a valid point nevertheless. 🙂
No, it's still not entirely obvious to me . I opened to read the Fluid Dynamics by Landau and Lifshitz (2nd. ed., p1) , and that's what it says there
That is, hydrodynamics equations operates with velocity distributions (v in the notation by Landau and Lifshitz) in space rather than velocities of particular particles(u in the notation by Landau and Lifshitz). Whereas the Strouhal number is defined through the mean flow velocity u rather than v. So, probably David's redifinition of Strouhal number through the mean particle displacement is quite correct in a wider range of u. Sorry for the perturbation of the thread, it's just important to clarify for myself the physics behind equations.
. I opened to read the Fluid Dynamics by Landau and Lifshitz (2nd. ed., p1) , and that's what it says thereThat is, hydrodynamics equations operates with velocity distributions (v in the notation by Landau and Lifshitz) in space rather than velocities of particular particles(u in the notation by Landau and Lifshitz). Whereas the Strouhal number is defined through the mean flow velocity u rather than v. So, probably David's redifinition of Strouhal number through the mean particle displacement is quite correct in a wider range of u. Sorry for the perturbation of the thread, it's just important to clarify for myself the physics behind equations
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I agree with you, but it's not too difficult for ports that are mostly designed by a simple radius such as the 'NFR' ports in the Harmon papers (IIRC). Most ports can be approximated similarly even if they have an additional radius at the very end.
Stil, my overall point was only that it would help less knowledgeable designers to note that parameters of flared ends are far more important than the central section, in contrast to the common reliance of max. air speed.
Yeah, my idea was mostly coming from a theoretical point of view.
I rarely buy of the shelf port to begin with and just use PVC tubing with a round-over on the baffle as well as inside the cabinet (entrance).
That round-over on itself is not very well defined, and mostly depends on thickness of the wood, diameter of the port etc etc.
So yes, it's a simple radius, but also one that is not really defined.
It probably doesn't matter that much with a small amount of end-correction, but it's good to investigate that as well as to keep it in mind
Probably, but to maximise the flares and the entrance/exit diameter without exceeding the flare rate around 30° (see #526) it makes sense to keep the central length as short as possible.
I'll try to show that in the next update.
I understand, I am just thinking a bit more practically.
It's just a bit easier to have a straight tube in the middle to be able to tune, or make different lengths.
In that case we only have to print out the port ends (flares) and not just an entire port every time,
Printing only the ends is also a much easier printing proces.
I think the impact in absolute performance won't be that big?