Hi everyone, I recently acquired a pair of Red Line 45 speakers. The bass speaker drivers surprisingly look like new but the surround for the ABRs had of course completely disintegrated. I have seen that some have replaced the surround with some other material. But considering that the original surround had the same shape as a regular speaker foam surround it would seem impossible to replicate it's behavior with something flat. A flat surround could possibly be made to have the same behavior at low amplitudes but would be much more non linear and quickly get stiffer with a bigger excursion, hence a bass reflex port seems to be the only resonable fix. I have also seen that others have taken this route, but unfortunately I haven't seen any exact dimensions mentioned anywhere. I'm creating a printable model for the bass reflex tube, so I tried to do some calculations and measurements to hopefully get it right the first time. But unfortunately it produced new questions. Login to view image Here's what I've done so far: The ABR is 30 cm x 12 cm with 3 cm radius rounded corners Area metal = 30 * 12 - 4 * 3 * 3 + pi * 3 * 3 ~= 352 cm2 Additional area of 1/3 surround ~= 0.2 * 2 * 13 * 12 = 10 cm2 Equivalent diameter with same area = 2 * sqrt(362/pi) ~= 21.5 cm Weight of metal = 300 g Guesstimate of foam weight (1 mm thick, 150kg/m3 density) = 0.0362 * 0.001 * 100 kg ~= 6g Then using the method with 16 l box until it fits gives fb: 31 Hz I have measured the driver and the values seem to vary quite a bit with amplitude, suggesting it might not be all that linear. This is roughly the mean of the measurements: Fs = 80 Hz Re = 7.3 Ohm Le = 0.4 mH Qes = 0.75 Qts = 0.6 Mms = 8 g Vas = 14 l SD = 133 cm2 Then plug this in Login to view image Plot thickens, did I screw up my measurements, my calculations or is this how it was designed? Or will this be a bit more regular if you put the RL 60 drivers there instead? Is the RL 60 and RL 45 ABR the same weight? On the other hand, considering close to wall or corner bass gain, maybe this makes sense. This slope would probably work well with different corner or wall placement but sound thin further from walls. Is this how it behaves in practice when it works? I won't be able to test myself until I have fixed it. But let's also check if the bass reflex conversion kit matches this fb then. Taking some measurements from a picture of it gives that the radius of the tube is about 2 cm and the length is about 10 cm. It's a bit hard to estimate the area of the tube but I'm calculating it as if it was circular. It definitely looks bigger than a semicircle. I think the different end correction factor when the port lays flat on the panel also adds about half the diameter to the length making it about 12 cm with correction. Plugging in the values Login to view image Has anyone else measured this driver? Or taken actual measurements on the conversion kit, if that's even meant to be compatible with RL 45?