Mucking about in Winisd (freeware speaker designing software) I can achieve a Qtc of 0.494 with bipole 5 inchers in a 24 Litre enclosure. Probably not much bass but it all depends on room gain really because the bipole is baffle step compensated by it's very nature.
In a small almost square room like my living room I will have to really careful with the bass loading otherwise it will be boomy. Oversizing the box will lower the Qtc and the Fsc (frequency at which the bass has rolled off to minus 3dB ) but there is a balancing act to be done here regarding my in room response versus box size (don't want massive cubes) and very low bass output which with a mid bass involves more excursion and inter modulation distortion. Have to find a happy medium here.
But it was only the other day that I realised there is an input for added cone mass in winisd
and voice coil temperature and other things. As doping adds mass then I had a play. I knew adding mass lowered resonant frequency and F3, and increased Qts and Vas (box size required) but I never knew by how much. I know this is just a simulator
I wish I knew how much dope I had added in weight but that would mean weighing before and after with accurate scales and it is too late now. Anyway playing around with the added mass was a useful exercise to see just how much the other parameters are influenced by different weights.
With two 5 inchers in 24 L the qtc is 0.494 and the Fsc is 67.79 Hz. Adding mass of 1 gram changes Qtc to 0.552 and Fsc to 60.1Hz. Add another gram and Qtc is 0.603 and Fsc is 54.53 Hz. By the time you have added 4.3 grams the Qtc is 0.707 and Fsc is 46.86 Hz. So with adding 4.3g we have gone from a sub Bessel alignment to bang on a Butterworth with the same 24 Litre enclosure throughout.
So what ? Well adding the 4.5g makes the speaker play lower in frequency (be that good or bad). And the output at 20Hz has increased from -22 dB to -14 dB. Won't be good for the midrange in a 2 way. Need to get a happy subjective balance here between everything.
Amount of dope added is really set by how much you need to attenuate the higher frequencies that are produced from the mid bass that is run fully open. We can't just say lets add 2g to suit bass output box size and room gain if the mid range is too hot and the cone's break up is still audible.
25 Litres is as high as I can go but you have to take off that things like the volume of the driver itself that protrudes into the enclosure and any bracing. But saying that I am currently running the two 5 inchers in about 16 Litres after deductions and the bass performance is very good. With no added weight in 16 Litres that sims at Qtc of 0.53 and Fsc of 73.1 Hz. With 3.25g of added weight that has changed to Qtc 0.707 (Butterworth) and Fsc 53.13 Hz.
So the 24L box will definitely be too big for two 5 inchers. But maybe big enough for two 6 inchers ? Need to do the sims for that. If going with the bipole 5 inchers then the box can be braced to within an inch of it's life to get the volume down for best subjective bass response with my room and range of music which ranges from stuff with hardly and bass to electronic music which can have loads. need tone controls