Port Length Question - 8 VS 10 Subs

You have a good question and it seems that it is a common perception of the value of tuning a box.  But when you tune a vented box to a particular frequency you are constructing a Heimholtz resonator that will add to the sound of the woofer at that frequency.  You're not changing the actual sound of the woofer....you're adding to it.  (...totally dependent on proper setup, of course, because you can also ruin the sound of a woofer with a box that is built wrong...)

A 12" woofer will play freqs to a lower range than will a 10", and we have to presume comparable quality of drivers in any comparison.  An 8" , being smaller in area still, will not be able to achieve the frequency range of a 10".  It's at the low end of the freq range where all three subs will differ.  But with most music material all three subs will perform, but with diminishing authority at the low end as you decrease the size of the sub.

Tuning the enclosure to a given frequency will add about 3 db SPL to that frequency because the enclosure sound is added to the driver's output.  You could tune boxes for all three size subs to 36 Hz and all three will be louder at that frequency, but the 12" sub will perform better at the lowest frequencies in the music source.  But in the purest sense of tuning a box it is generally done to add SPL at a frequency where the driver is lacking SPL......at about the point of driver resonant frequency where there is a substantially greater impedance imposed.  And this is done to smooth out the output curve and add to the low end.

You're welcome and I'll add this visual of what the port tuning does using a little project  I'm currently working on.  You'll see in this graph that the driver output is represented by a black line and at one point it dips low because of driver impedance.  The port SPL is represented by a turquoise line and at that same point it peaks.  The combination of everything together is shown in the final result by the heavy line, which is a smooth output and an extended low frequency for this driver.  The WinISD graph gives you a combined representation of output but doesn't break it down into separate responses on the same graph like this does, but you can use the WinISD program to see the same end result.  This graph was generated using Unibox.

VB_Response_Infinity_TC11MG-07-04.gif