wondering what is a good roll off for my 7". considering pointing them down at your feet under the dash.
my subs play at 30hz down

 

WHOA! 30hz down is crazy! They will hardly ever be playing at that crossover point. Move the crossover point up to 40 or 50 hz to actually use them.

As for the question about sound being directional, you won't have any problems with direction at those crossover points.

Question:
Do you have enough room to mount 7" under your dash with enough room for an enclosure big enough to let the subs play down to 30 hz?

 

Sound is supposed to be non-directional under about 100hz. Up to 250-300hz pathlengths are still considered more important than direction.

 

a lil off topic..but what is a path length?

 

path length is the distance from a speaker to your ears .

 

l have 4 10 inchers x overed at 30hz.
using the 7 inchers to play 30hz and up. they will go with a 3 way up front.
as they wont be directional pointing down. the enclosure needs to be 7 litres sealed. l will make it fit under the dash somehow and also a set in the doors, that will play alittle higher and be more directional

so around 250 hz sound becomes directional. thinking of making my midrange cross at the 250 and tweets at 4500. alittle high but the 4 inch can play up to 20khz lm guessing less fatigue playing a tweet to low.

dealing with a 4 way 10" 7" 4" and 1/2" tweet

 

sound is said to become directional when the wavelength of the sound being produced approaches the size of the radiator.... this however really has more to do with off axis frequency response, and the line between near and far field behaviours...

It follows that there will probably not be a difference in how your speakers sound up to frequencies around 2khz... but that is not to say you would not be able to tell what direction the sound is comming from...

So, from the standpoint of tonality Id say DO IT and cross them over at no more then 500Hz @ 12db/octave... but that may still affect your stage and would probably not be considered ideal for pure SQ..

as far as true directionality it will have more to do with the dimensions of your vehicle versus wavelength... ie you can probably tell frequencies over 113hz are comming from the back of your vehicle if the interior is over 10feet long....

 

that is the same system design that i run in my civic. dynaudio 7 in door and 10 in rear. xover for sub is 50hz at 24db/oct and 7 inch is 60 to 250 at 24db/oct.

 

lm working with an S 10 chev ext cab.
l think lve got 6-7 feet of wavelength
subs are less than 3 feet to your ears
l read that the first harmonic is around 250 hz

 

30Hz and down is a little low. IMO I would run 70-80Hz down set a sub-sonic just above the capability of the lowest Fq of the subs by around 5Hz and then over lap the Fq range with your 7" around 60-70Hz and cut it where your next size driver is starting its range. This way no Fq is missed.