This is my first post - after reading many interesting posts and answers.
First of all, I think this is a great forum where I see lots of DIYers not only buidling, but also helping each others. A great place to be.
I'm kind of novice to this hobby. I plan my first project, using WinIsd. It works fine, but one thing is not clear for me. When I get the result for a ported box, should I add the volume of the reflex tube to the calculated enclosure volume, or it is already taken into consideration by the program?
Thank you in advance.
Zoltan
First of all, I think this is a great forum where I see lots of DIYers not only buidling, but also helping each others. A great place to be.
I'm kind of novice to this hobby. I plan my first project, using WinIsd. It works fine, but one thing is not clear for me. When I get the result for a ported box, should I add the volume of the reflex tube to the calculated enclosure volume, or it is already taken into consideration by the program?
Thank you in advance.
Zoltan
No. You have to subtract it from the volume. The program can't know if the port is inside or outside of the box or how much volume the port takes up, it depends on the wall size too (made out of MDF etc.). With a simple r5x10cm plastic tube it usually doesn't matter at all if you are subtracting it at all, but for extreme cases (very small enc.or very deep tuning at high mass/very compact subs i.e.) the port and its walls can make a huge difference.
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Zoltan,
The internal volume taken up by the ports, speaker, and bracing must be reduced from the enclosure volume.
As will become clear, the large volume ports required to reduce "chuffing" and "port compression" also reduce the cabinet volume- when tuning low a compromise must be reached between port and cabinet volume.
The internal volume taken up by the ports, speaker, and bracing must be reduced from the enclosure volume.
As will become clear, the large volume ports required to reduce "chuffing" and "port compression" also reduce the cabinet volume- when tuning low a compromise must be reached between port and cabinet volume.
Absolutely correct. A too small port also translates into high losses and compression at high volumes. That depends on the air velocity and can be simulated too. The geometry of the port can also lead to port air noises. A narrow slit should be avoided, though rounding the edges can reduce it and so on.
A too large port - especally on the front - lets out a lot mid-garbage though. And with very long ports you get length resonances as well.
A too large port - especally on the front - lets out a lot mid-garbage though. And with very long ports you get length resonances as well.
For modelling programs like WinISD, the modeled volume for the enclosure is a "net volume", i.e. excludes any internal bracing, driver volume and ports. It also excludes the volume occupied by the crossover PCB and components.
So, when deciding enclosure dimensions, such element volumes must be considered "overhead" and added to the modeled volume to reach the "gross volume" which will be used to reach dimensions for the enclosure.
So, when deciding enclosure dimensions, such element volumes must be considered "overhead" and added to the modeled volume to reach the "gross volume" which will be used to reach dimensions for the enclosure.
Thank you all for the answer. I will consider these volumes. Though to calculate the volume taken by the speaker is not that easy. do you have any good method?
Yes. An approximation is close enough, so reduce it to the very simplest possible geometical shapes, don't calculate every single corner and bit. All in all an error margin of +/-10% of the volume is negligible without audible differences.
As a rule of thumb an 8" woofer takes up ~2l, a 12" ~5l.
As a rule of thumb an 8" woofer takes up ~2l, a 12" ~5l.
IME, calculation of volume for drivers, tube and bracing is only necessary with very small enclosures, because a bit of stuffing has the effect of "enlarge" the enclosure.
For all the vented box I've built, the calculated vent always gave a lower tuning than desired. So an impedance measure is the only method to find and correct the tuning.
Ralf
For all the vented box I've built, the calculated vent always gave a lower tuning than desired. So an impedance measure is the only method to find and correct the tuning.
Ralf
Here's a link with a suggestion for measuring volume displaced by a speaker.
The Subwoofer DIY Page - Box-Building FAQ
That is not an error in the simulation, that's typically a problem of the port ending near a wall of the enclosure or even a corner, which virtually increases the length of the port. Also - and that is not voodoo - the speed of sound is lowered because of the stuffing, that's what 'makes the box bigger' with damping material. Bigger volume translates into lower tuning, mystery solved. You can either adjust the speed of sound in your simulation program or adjust the volume by a certain % to compensate for that.
That's simply not true. There is no 'THE' correct tuning, you can vary it by a margin, there's usually even very much room for adjustments, it doesn't have to be the exact calculated frequency. Very extreme, very small speakers/subs can be tricky, that's true but in a lot of cases you get a much better result if you adjust it to your taste and the room. For PA or high-spl use (dB drag etc.) you should verify the maximum excursion and maximum spl/power simulation though.
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