Ok first lets be very careful on how we used the word Full-Range as this really means 20hz - 20khz to us by our standards of hearing and no small driver is going to do that in a vehicle point blank. Wide-Band is a better term to use for Mobile Audio use. Now with that siad let me address the way most companies rate their Wide-Band drivers (or full-range for the sake of this thread). The power rating are usually taken one or two ways; testing it completely full-range (20hz - 20khz) or high passed at its Fs, and with these methods there is no way a 4.5" and smaller driver is coming to able to handle any type of even moderate power.
Now what dose the above mean? What it means is if you use that driver in a safe operating range with a high-pass crossover; let’s just say it’s a 4.5 wide-band driver so I would say 250hz is ok starting point. The Manufacture may rate that driver at 20 watts rms and it has an Fs of 70hz and a sensitivity or 84db. First thing you think is 20 watts and 84 db, this thing is not going to be able to play with any kind of volume and you would be right, but now remember that rating is taken by 1 of methods I stated above; 20hz -20khz or Fs – 20khz. Now high-pass the driver at 250hz, you just remove all the low end; which it shouldn’t be trying to reproduce in the first place in a vehicle. You also cut down on all the massive excursion requirements on the small driver. Guess what; your power handling just went up way up. More power handling means more volume. The bottom line is; the less of the bottom octaves the small driver has to reproduce the cleaner, more accurate and precise it will sound.
Now to say a 4” is too large to be a wide-band/full-range is a little miss leading, as there are 10” full-range/wind-band drivers on the market. But, IHMO would have to agree that is getting a little large for A-Pillar mounting unless you have one of the new cars with the little window on the dash/a-pillars like the newer Honda Civic and Mazda CX-7, but that’s not to say it can’t be done or will not work, as it can and has been done many times over with amazing results. It’s just a matter of vehicle and/or your willingness to have you’re a-pillars on the large side.
Now if you can get the driver say at least or no more then 15 degrees off-axis you will not have to worry about the beaming issue of most 6.5” and smaller Wide-Band/Full-Range drivers. Now as you move father off-axis this does start to become more of a concern and you want to start looking for a driver with less Sd (cone area).
Now the above is not a given/seat in stone; As some drivers have better extension/handling into the upper frequencies then others and with this higher extension it’s a phantom sort of a affect that leads us to think it has better off-axis response when it’s really that it reproduces the upper range a lot better. Here are few things that have quite bit to do with this:
1. Cone material
2. Cone size
3. Cone shape
4. Motor design
5. Phase plug and/or dust cap design and/or material
6. Excursion, yes xmax of said driver
With that said; if you can get the driver mounted on-axis the deciding factors will now be:
1. Does the driver play the range I desire to mate with my mid-bass seamlessly? Which should be at least 800hz; which any good mid/bass should be able to play that high without any issues.
2. Next can it handle the available or plan wattage you have, but power can be controlled so this shouldn’t be an issue. Also at higher frequencies no where near the rate wattage is being used.
3. Does the driver really have extension high enough for your tasted to be used with out a tweeter? Key word “YOUR”
4. If it does meet the above, now does it have the sonic signature you desire. Key word “YOU”.
Now with all that said. If you want a small driver to play fullrange or reproduce the woofer - sub frequencies take it in your home built yourself a transmission line enclosure and tune it as low as that small driver will allow which is usually around its Fs or a little lower.
Hope this helps and good hunting for loud and clean sonic bliss. O ya, sorry so long.
