Hello,
I have a little question about port calculations.
I wanted to design a subwoofer box for two 12’’ speaker and wanted to make it look like the ks28 from L-acoustics but they use what they call “L-vents” (first picture) that are curved and I don’t know which measure to use for the vent surface in software like winISD.
Do I have to use 340mm or 140mm or something else (second picture) ?
Thanks for your help 🙂
I have a little question about port calculations.
I wanted to design a subwoofer box for two 12’’ speaker and wanted to make it look like the ks28 from L-acoustics but they use what they call “L-vents” (first picture) that are curved and I don’t know which measure to use for the vent surface in software like winISD.
Do I have to use 340mm or 140mm or something else (second picture) ?
Thanks for your help 🙂
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At the frequencies involved a “curved” vent will not be seen, but in the sketch you have shown one has a fairly radical vent with flared ends… and no straight section.
dave
dave
Start with the smallest surface area (140 mm wide vent), and then adjust if necessary.
There are many ways to shape a port. Flares at both ends should prevent port noise or at least push it to a higher driver excursion. This is helpfull if the port would get too long using a sufficient diameter.
Sometimes you run into a nice looking design, that only pleases the eye, but has no real acoustical advantages. It is only designed to look different and the unknowing customer will interpret this as an innovation. If it is not to radical, the disadvantages from the designe feature will not make it a bad product.
I suppose in this case some product designer had an idea for a "new" feature and some engineer had to match driver and cabinet to make it sound half way acceptable.
Really, after more than 100 years of speaker building, there are hardly any revolutionary designs to invent any more. Real speaker building is to match and tweak within known physics and choose the best driver for a cause.
In this design any horn function is erased, as the slow wave entering the port gets faster in the middle and then slows down again. So all you can expect is losses during the transformation slow-fast-slow. A real horn transforms the wave to match it to the room / audience. So no horn function here.
Whether this special design is a real disadvantage will depent on the driver and the power feed into it. If you have these parameters, you can calculate the air speed at the most narrow point. To fast= high losses. If speed stays inside the usual limits, it is just a reflex port shaped differently and sacrifying some precious box volume. So you could get the same result with identical drivers from a smaller cabinet with a conventionaly shaped port.
The scetch shows no bracing inside, the large panels will resonate like hell = not good for precise bass.
Sometimes you run into a nice looking design, that only pleases the eye, but has no real acoustical advantages. It is only designed to look different and the unknowing customer will interpret this as an innovation. If it is not to radical, the disadvantages from the designe feature will not make it a bad product.
I suppose in this case some product designer had an idea for a "new" feature and some engineer had to match driver and cabinet to make it sound half way acceptable.
Really, after more than 100 years of speaker building, there are hardly any revolutionary designs to invent any more. Real speaker building is to match and tweak within known physics and choose the best driver for a cause.
In this design any horn function is erased, as the slow wave entering the port gets faster in the middle and then slows down again. So all you can expect is losses during the transformation slow-fast-slow. A real horn transforms the wave to match it to the room / audience. So no horn function here.
Whether this special design is a real disadvantage will depent on the driver and the power feed into it. If you have these parameters, you can calculate the air speed at the most narrow point. To fast= high losses. If speed stays inside the usual limits, it is just a reflex port shaped differently and sacrifying some precious box volume. So you could get the same result with identical drivers from a smaller cabinet with a conventionaly shaped port.
The scetch shows no bracing inside, the large panels will resonate like hell = not good for precise bass.
'Something else' IME, so FWIW/YMMV; from back in days when an averaged approximation of a crude parabolic horn vent was all I had available:
(Mav) cm^2 = (54.1*34) =1839.4 cm^2
(Tav) cm^2 = (54.1*14) =757.4 cm^2
Av^2 mean = (Mav*Tav)^0.5 = ~1180.323 cm^2
Dv mean = (1180.323*4/pi)^0.5 = ~38.77 cm/15.26" dia.
(Mav) cm^2 = (54.1*34) =1839.4 cm^2
(Tav) cm^2 = (54.1*14) =757.4 cm^2
Av^2 mean = (Mav*Tav)^0.5 = ~1180.323 cm^2
Dv mean = (1180.323*4/pi)^0.5 = ~38.77 cm/15.26" dia.
Whatever shape you choose, ensure that the area is proportionate (I start with port area = Sd and work backwards until it fits in the box and has an aspect ratio no greater than 2:1) and that airflow symmetry is as good as you can make it to avoid port rectification. Gains in output and reduction in THD result from good ports.
What kind of drivers are you planning to use? If comparable to something like a 12HP1060 etc (this is a PA drive though) the curved vents could be useful. If going for lower xmax drivers, a rectangular or some round ports are the easier way to go.
Check out the JBL paper on curved ports, it contains useful information to get you started. Still, you would need to build and verify.
You would need also some high power measurements to see if the ports stays within tolerance regarding tuning.
Check out the JBL paper on curved ports, it contains useful information to get you started. Still, you would need to build and verify.
You would need also some high power measurements to see if the ports stays within tolerance regarding tuning.
wtf is all this ?? I don’t know (but whenever I understand it and apply it ) when previously you said it , it AaLWAYS works!!
back to two stroke exhaust and expansion chambers ..
Or straight steps of pieces of ‘pipe’ in rectangle shapevery single time when they are at that 1/3 , 1/3, 1/3 (0.3490658 as driver entry/excite offsets ) thing Nobody seems to understand
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I never understand the numbers at first (i suck at math ) but whenever I figure them out and see what you mean it, always works.
I still don’t understand
0.217, (0349), 0.424,0.6,
0.712,0.848
(except for 0.349)This one is blatantly obvious once found.
i dont understand these others … but playing around with them sometimes they land in places that show you where they are in a qw pipe(sorta)
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Ah! 😎
Apparently someone's ~ (corrected) perfect odd harmonics of a 1/4 WL resonator, i.e. they shift down a bit due to its end correction that makes it acoustically a bit longer.
Mine confirmed using MJK's math FWIW/YMMV: ~ 0 [top], 0.217, 0.349, 0.424, 0.561 0.651, 0.714, 0.848, 0 [bottom]
Apparently someone's ~ (corrected) perfect odd harmonics of a 1/4 WL resonator, i.e. they shift down a bit due to its end correction that makes it acoustically a bit longer.
Mine confirmed using MJK's math FWIW/YMMV: ~ 0 [top], 0.217, 0.349, 0.424, 0.561 0.651, 0.714, 0.848, 0 [bottom]
