I have a pretty ugly problem. Basically i made a XP mob trap, and have two questions to it.
1st Question: I actually found a zombie spawner and a caver spider spawner pretty close together. I connected them with water streams (http://www.abload.de/img/2011-11-27_13.04.0815ugz.png) and then they swim upwards to a drop. So the first question is: Why are the cave spiders able to see me? Look at the screenshot above, there are 2 zombies chilling in the back (cant see me), but the cave spiders want to attack me. Is there anything i can do about that?
2nd Question: I basically built the trap after tutorial. I have a 23 block deep hole (if i jump in it i only have half a heart left), but still about 70-80 % of the Zombies die from the fall. As shown in the video, i do not have water at the last block before the drop. I think i already know what causes this. I think that the spawner spawns most of the mobs higher than 3 blocks off the ground, which will make them loose health. So i thought of filling the whole area up, leaving only 2 something like a 5x5x2 area for the mobs to spawn. So my question is: Will this decrease the spawning rate or will the mob spawner keep spawning?
I apologize for my bad english, i am austrian (;. Thanks in advance, i would really appreciate an answer.
Have a nice day.
// edit: I forgot to mention that, even though there is a 23 block deep fall, all of the SURVIVING zombies actually take 2 hits to kill. I got no idea why that is.
Zombies have a small amount of armor. They can't die from a vanilla fist in one hit. I would recommend splash healing potions, but since your trap is a dual trap it would heal the spiders. Your best bet is probably making quite a few wooden or stone swords.
Spiders seeing through blocks
This is pretty much unavoidable, spiders see in a 16 block radius from themselves. You would have to design an ingenious trap in order to get around this as well as keep both spawners spawning.
Zombies dying from fall
It's likely that they are taking a small amount of fall damage like you suggested. I'd try raising the spawning pad up by one Y level. If that doesn't work, you may wish to consider raising the impact pad.
Reduced spawn rates from fixing zombie's fall damage
It shouldn't reduce the spawn rates significantly if at all. In my experience spawners will try pretty methodically to spawn until they do so. I design my XP farms with only a one block spawning pad and they work just fine.