three 8 ohm loads would be
(8 x 8) / ( 8 + 8) = 64/16 = 4

then take that load and parallel it to the other 8 ohm

(4 x 8) / (4 + 8) =32/12 = 2.66666667 is the load for 3 8 ohm loads

So, how do you do that? To keep it simple say you want to hook up 3 4 ohm subs to a mono channel amp. How do you calculate the ohm load?

Whoops you beat me to it! Thank you. It drives me crazy when I can't figure this kind of stuff out!

just because that worked this time I would always use the formula because, that worked out right that time and some times it will work out easy but usuallly it is wrong if you just divide.

Right. Thanks for the help.

So in my example 3 4 ohm speakers on a mono amp would be:

(4x4)/(4+4) = 16/8 = 2

(2x4)/(2+4) = 8/6 = 1.33 ohms total?

I guess going to school for degrees in mechanical engineering, electrical engineering and acoustical engineering finally paid off.

It sure did! So are my above formulas right? 1.33 ohms for 3 4 ohm speakers wired in parallel?

Also, is there a way to figure out power(wattage) that an amp poduces between the 2 and 4 ohms? In other words, if an amp is rated at 720 watts RMS at 4 ohms and 960 watts RMS at 2 ohms how do you figure out what power it would have at 2.67 ohms?

Ya but what is the formula for shear

depends on what kind of shear you are talking about, shear strength, shear force, horizontal shear vertical shear, they are all different actual forumlas.