Hi I’m looking for opinions around port calculations, measurement. For a 30” mid-bass from
18 Sound - 15MB 700 with Helmholtz Resonator – Port values below.
All calculators seem to give a different result. Is 1+1 not 2 ? 😊 So all opinions or guidance more than welcome. I read the similar threads here but no obvious answer is found.
Vb 90 L / Fb 45 Hz / Sd 850 cm2
Rule of thumb - Minimum port diameter (1/3) 283 cm2 = 19 cm diameter (283.53 cm2)
18 Sound - 15MB 700 with Helmholtz Resonator – Port values below.
All calculators seem to give a different result. Is 1+1 not 2 ? 😊 So all opinions or guidance more than welcome. I read the similar threads here but no obvious answer is found.
Vb 90 L / Fb 45 Hz / Sd 850 cm2
Rule of thumb - Minimum port diameter (1/3) 283 cm2 = 19 cm diameter (283.53 cm2)
• SpeakerBox Lite Length 34 cm
• MH Port calculator Length 30.52 cm
• Basta Length 30.3 cm
• Mobile information labs Length 32.94
• Omnicalculator Length 45 cm
• The 12 volt Length 32.48
• Boomspeaker Length 32.8
• Lautsprechershop.de Length 31.5
• …
Help 🙂• MH Port calculator Length 30.52 cm
• Basta Length 30.3 cm
• Mobile information labs Length 32.94
• Omnicalculator Length 45 cm
• The 12 volt Length 32.48
• Boomspeaker Length 32.8
• Lautsprechershop.de Length 31.5
• …
Hopefully, the speaker designers will now chip in.
Could it be that the different calculators apply different 'end corrections' to the length of the port?
Could it be that the different calculators apply different 'end corrections' to the length of the port?
Apart from the Omnicalculator result, the difference in port lengths calculated is not significant, ~3 Hz difference from short to long (30-34cm) which might be less than the variation in driver Fs. It may come from rounding errors in the underlying formula and assumptions about of the cabinet (e.g. amount of damping material and/or internal surface porosity).
Edit: ...or port end corrections, thanks Galu.
Edit: ...or port end corrections, thanks Galu.
imho a port length + - 10 % difference s virtually a non event in port calculation.
it depends on the software.........losses , method and so on.
so your 30.....34 cm should work (45 cm is a little bit far away).
it depends on the software.........losses , method and so on.
so your 30.....34 cm should work (45 cm is a little bit far away).
As to why you've used so many calculators - that's a long conversation involving several sessions with a good therapist.
To answer your question; port specification is derived from cabinet volume. Different programs assume different variables. A 1 metre cube is a 1000 litres. Construct it using 19mm timber and the internal volume is 890 litres. A cabinet that size will need serious bracing, reducing the internal volume by another 5 litres. Then there's the driver, the chassis and magnet may occupy another 5 litres reducing the internal volume to 880 litres. Then there's the port itself, taking 10 litres of internal volume.
The variables account for 13% of volume - this covers the spread of results ignoring the outlier.
To answer your question; port specification is derived from cabinet volume. Different programs assume different variables. A 1 metre cube is a 1000 litres. Construct it using 19mm timber and the internal volume is 890 litres. A cabinet that size will need serious bracing, reducing the internal volume by another 5 litres. Then there's the driver, the chassis and magnet may occupy another 5 litres reducing the internal volume to 880 litres. Then there's the port itself, taking 10 litres of internal volume.
The variables account for 13% of volume - this covers the spread of results ignoring the outlier.
Seem when I start to experiment with the k factor the numers come closer together. Also I did noticed as suggest with playing with 1 Hz fb difference to ports size changes much more.
I'll stick to the MH port calculator. Woofer Port Calculations
Thank you for addressing my concern !
I'll stick to the MH port calculator. Woofer Port Calculations
Thank you for addressing my concern !
As i recently discovered on an Onken thread, k is just a way of specifying an alignment. It is, to my mind, much more informative to look at the curves generated when you change the vent size.
Here you see what happens when you take a fixed volume with 40/50/60/70 Hz tunings. Each would have a different k factor.
What thw ks are i forgot already, GM mentions 2 in the recent Onken thread , more commn today a max flat, which would be somewhere between the 2 middle curves. The other number he mentions, for a tradiitonal Onken tuning gives a curve something like this. Do note that to get this alignment i had to increase the box volume.
You will see this kind of alignment in many Tannoy’s for instance, and is a way of trading off bass extention for box volume, really needs good match to room gain, and IMO some quality & finesse.
It does bring up that there is a large space of possible reflex designs. Volume and tuning are the variables (speaker data being fixed) and as one moves thru that space there ar eliterally an infinite number of potential results. It is very easy to get into the weeds.
And don’t make the enclosure a tower. Because it is now an ML-TL.
dave
Here you see what happens when you take a fixed volume with 40/50/60/70 Hz tunings. Each would have a different k factor.
What thw ks are i forgot already, GM mentions 2 in the recent Onken thread , more commn today a max flat, which would be somewhere between the 2 middle curves. The other number he mentions, for a tradiitonal Onken tuning gives a curve something like this. Do note that to get this alignment i had to increase the box volume.
You will see this kind of alignment in many Tannoy’s for instance, and is a way of trading off bass extention for box volume, really needs good match to room gain, and IMO some quality & finesse.
It does bring up that there is a large space of possible reflex designs. Volume and tuning are the variables (speaker data being fixed) and as one moves thru that space there ar eliterally an infinite number of potential results. It is very easy to get into the weeds.
And don’t make the enclosure a tower. Because it is now an ML-TL.
dave
Attachments
Hmm, the Onken 'k' is VERY different from the pipe end correction 'k', which adds another level of confusion re the Onken's.
FWIW, the late bjorno's worksheet is TTBOMK the most accurate: https://www.diyaudio.com/forums/att...-dayton-port-length-effective-port-length-jpg
FWIW, the late bjorno's worksheet is TTBOMK the most accurate: https://www.diyaudio.com/forums/att...-dayton-port-length-effective-port-length-jpg
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Huh? The net internal volume is all that's important, and the simulator designer cannot have any idea whether you are heavily braced or zero, or whether the driver is of tiny frame/magnet or huge, so it could never be factored in to give an accurate result.
End factor correction K
and number of ports Nv
and various opinions on what formula / constants used predicts more accurate results.
so various software likely has used different formulas from different sources.
but all very similar.
slot ports usually share walls, so in real life the port seems longer.
Also the efficiency of non round ports like, rectangle, square, triangle etc etc
are not as efficient. So will also appear to be longer.
if a slot port shares multiple walls usually a shorter length will be slightly more accurate.
also a good point, in real life. Driver Fs can vary up to 10%
and yes it is also up to you the designer to compensate
for displacement of bracing, ports and driver.
Net/ Gross volume
Also models can compensate for Leakage Ql, absorption Qa, and port efficiency Qp and every calculator can have different default values.
And not all of them compensate port length for said values, but might show a accurate impedance curve.
So model might say 45 Hz for a certain port length , but if you check the modeled impedance curve it may be
different.
The onken calculator, I would assume would change K since it shares walls, also depends what constant he used for Nv and what formula
was decided to be more accurate for multiple ports. Since it uses multiple slot ports.
if well designed could likely be more accurate for multiple slot ports which also share cabinet walls
in a nutshell, round ports might need to be longer in real life, and slot ports might need to be shorter in real life.
round port having best efficiency, and rectangle/ slot port having very poor efficiency. Specially if poor height to width ratio
and number of ports Nv
and various opinions on what formula / constants used predicts more accurate results.
so various software likely has used different formulas from different sources.
but all very similar.
slot ports usually share walls, so in real life the port seems longer.
Also the efficiency of non round ports like, rectangle, square, triangle etc etc
are not as efficient. So will also appear to be longer.
if a slot port shares multiple walls usually a shorter length will be slightly more accurate.
also a good point, in real life. Driver Fs can vary up to 10%
and yes it is also up to you the designer to compensate
for displacement of bracing, ports and driver.
Net/ Gross volume
Also models can compensate for Leakage Ql, absorption Qa, and port efficiency Qp and every calculator can have different default values.
And not all of them compensate port length for said values, but might show a accurate impedance curve.
So model might say 45 Hz for a certain port length , but if you check the modeled impedance curve it may be
different.
The onken calculator, I would assume would change K since it shares walls, also depends what constant he used for Nv and what formula
was decided to be more accurate for multiple ports. Since it uses multiple slot ports.
if well designed could likely be more accurate for multiple slot ports which also share cabinet walls
in a nutshell, round ports might need to be longer in real life, and slot ports might need to be shorter in real life.
round port having best efficiency, and rectangle/ slot port having very poor efficiency. Specially if poor height to width ratio
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I find it strange that all simulators end up with really long ports, like 30cm. I have never seen a consumer nor pro manufacturer using any more than 10-15 cm. Lots of PA even just have a button opening in the baffle corners and no real port at all. Who’s wrong?
^^ I couldn't care less what Boxnotes does. I know how it's calculated and I want to work out my own bracing, port volumes and driver displacement, so I need to know the actual net volume.
There are far better programs out there.
There are far better programs out there.
Because good programs calculate the correct port diameter to get a low enough port air speed, usually 0.05 mach. This means that for typically undersized consumer and pro boxes, the port is too long and the port volume too large to make the final product as small as they want it, so they make it samller diameter and therefore shorter length and hope that at high SPL people won't notice the compression and chuffing. DIYers generally do not have the restrictions commercial manufacturers have, so they can do it to get best performance.
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That makes sense. Is it possible to chose that SPL/size- or Mach factor yourself? Assuming I don't plan to play very loud (in most cases, and when I do I dont care about noise) I can have smaller ports? Isn't it just a formula, while the simulator takes into consideration the actual driver as well?
All the speaker simulators I know (WINISD, Hornresp, VITUIXCAD etc) definitely do allow the user to choose their own preferred port diameter (or width & height) and will then show the resulting length required and airspeed that will happen at your modelled SPL.
It's up to you to make the (hopefully well informed) choice of what compromise to accept between lower airspeed vs smaller port.
It's up to you to make the (hopefully well informed) choice of what compromise to accept between lower airspeed vs smaller port.
That's because the OP wants to use a 19 cm diameter port, using the manufacturer suggested box and tuning (90L 45Hz). The choice of the diameter of the port depends heavily on the SPL the OP wants, something that is limited by the x-max of the driver.
I did a simulation with Unibox, and the box/alignment chosen results in the driver hitting the xmax at:
with 32W, 26Hz with max SPL 113.6 dB
with 64W, 31Hz with max SPL 116.6 dB
with 128W, 35Hz with max SPL 119.6 dB
with 256W, 70Hz with max SPL 122.6 dB
With 32W a single 10 cm tube is perfectly fine, for 64 and 128W dual 10 cm tubes are needed, even for the last case dual 10 cm tubes can be used, but I'd avoid such high power as it will destroy the driver at low frequencies despite what the mfg is saying (not enough x-max).
With 128W Unibox calculates a 8 mm cone excursion. The max cone excursion is not stated in the specs, but can be roughly this value. I'd build the speaker with dual 10 cm tubes. The calculated length is 18 cm, I find that the real length is always lower than calculated, just measure the tuning.
As a final note, forget about calculators and use a simulator.
Ralf
Note that Unibox calculate with the given T/S parameters a sensitivity of just 100.5db/2.83V/m .
Did you add in a HPF below tune (at least 2nd order) like you should always do with a ported enclosure to prevent overexcursion?