Suggestion for box volume

sbronf

Member
2017-11-20 8:07 pm
Italia
ok Thanks, could you explain A thing to me?
if the tuning frequency can be lowered by Increasing the length of the tube, what is the need to change the volume of the Box?
that is, with 16L if I increase the tube to get tuning at 30hz, is it the same thing as having a box of 50L with a shorter tube tuned to 30hz?
 
ok Thanks, could you explain A thing to me?
if the tuning frequency can be lowered by Increasing the length of the tube, what is the need to change the volume of the Box?
that is, with 16L if I increase the tube to get tuning at 30hz, is it the same thing as having a box of 50L with a shorter tube tuned to 30hz?

No, above the port tuning the box behaves mostly like a sealed enclosure, so if your port is tuned low, you need to make sure that the frequencies above also are present, so you need a bigger box to be flat to the port tuning to simplify the theory. Download winisd or hornresp (the former is simpler to use, the latter is more complex) to simulate yourself. You will see if you make the box too small for a tuned port, the frequency plot will not be flat.

If you want a small box, ported or sealed, you will have to tune it higher. If you want it lower, the box will be bigger. If you tune the port too low for the box volume, you will have a peaky uneven response wich translate in a one note sub.
 

jReave

Member
2012-10-30 4:34 pm
To the OP, it may help to understand that you are conceptualizing only in terms of 3 variables - Vb (the volume of the box), Fb ( the tuning frequency of the box) and port length whereas you actually need to add at least 1 more variable, port SPL or loudness, to fully understand the relationships at work here in a bass reflex enclosure. (I'm ignoring port diameter and box stuffing for simplicity's sake.)

So for example in your case if you keep Vb the same but increase port length, Fb will indeed be lowered but so too will port SPL. Now if you increase port length and increase Vb, Fb will again go down but port SPL will now increase as well. Or in other words, if you want the bass to be both lower and louder, you have to increase both the size of the box and the length of the port. Keeping the box size the same and increasing the port length will lower the tuning but it will also lower the port SPL as well.

As mentioned above, there is no substitute for playing around with a box simulation program to really get a feel for how all the variables interact.

Now if you don't want to build a new box at present, try a port between about 12.5 to 15.5cm long with a 5cm diameter - Fb is around 50Hz, F3 around 55Hz and the FR is fairly flat with small peaking around about 104Hz when the box is heavily stuffed.
 
Or in other words, if you want the bass to be both lower and louder, you have to increase both the size of the box and the length of the port. Keeping the box size the same and increasing the port length will lower the tuning but it will also lower the port SPL as well.

ok but always relative to the volume?
with 42L for a 38hz tuning frequency I need a 6.4cm x 10cm tube
while in 16L to have it of 60hz I need a longer tube (14cm). quite right?
 
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jReave

Member
2012-10-30 4:34 pm
Yes, relative. All the variables work relative to the others.

I hesitated to write that 1st sentence (above in your quote) because it could possibly be misleading. The larger box has a lower tuning but also a shorter port? But that is correct in your example as long as the port diameters are the same. In my example, I was using 5cm. If you use a 10cm diameter, the equivalent port length would need to be about 62cm, which besides being overkill is just plain ridiculous for a 16L box. Don't forget that you should also be subtracting the volume the port (as well as other interior objects) from the volume of the box.

** Or was that a 6.4cm diameter port? Which would need about a 25cm port in 16L for the same tuning in my example. Except again, I'm not accounting for the volume the port now takes up.......
 
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Another opinion..

Played with this driver a bit... Works well in a 26-28 liter enclosure (gross volume) Tuned per Thiele's alignment. 2" dia. X 5" port tunes to 41 Hz. Or, 2.5" dia. X 8" port tunes to 40 Hz. No use tuning much lower as the volume of displacement of this driver will not support enough output below 40 Hz and if you do tune much lower your group delay is a real mess. Sounds nice, balanced, tight and smooth set up per above specs. All of the above assumes your drivers are close to spec. Have you measured them? Good luck!https://files.diyaudio.com/forums/images/smilies/biggrin.gif
 
Yes, relative. All the variables work relative to the others.

I hesitated to write that 1st sentence (above in your quote) because it could possibly be misleading. The larger box has a lower tuning but also a shorter port? But that is correct in your example as long as the port diameters are the same. In my example, I was using 5cm. If you use a 10cm diameter, the equivalent port length would need to be about 62cm, which besides being overkill is just plain ridiculous for a 16L box. Don't forget that you should also be subtracting the volume the port (as well as other interior objects) from the volume of the box.

** Or was that a 6.4cm diameter port? Which would need about a 25cm port in 16L for the same tuning in my example. Except again, I'm not accounting for the volume the port now takes up.......

Now i use in 16L a tube of 5cm diameter x 13,5cm lenght: tuning frequency sound be bear 60Hz.

For 42L box softwares give me a tube of 6cm diameter x 11cm lenght: now the tuning frequency should be 38Hz
 
Played with this driver a bit... Works well in a 26-28 liter enclosure (gross volume) Tuned per Thiele's alignment. 2" dia. X 5" port tunes to 41 Hz. Or, 2.5" dia. X 8" port tunes to 40 Hz. No use tuning much lower as the volume of displacement of this driver will not support enough output below 40 Hz and if you do tune much lower your group delay is a real mess. Sounds nice, balanced, tight and smooth set up per above specs. All of the above assumes your drivers are close to spec. Have you measured them? Good luck!https://files.diyaudio.com/forums/images/smilies/biggrin.gif

42L or 28L :confused:
Should try...