Yes, and this is the best way to do it, just build above an ocean or light up the ground below. Video below of a very good mob trap, stay below the height limit a bit in case of glitches. I built mine around 210, it is great.
http://www.youtube.com/watch?v=HXrxZ98k7kQ --- o---|==============> MC, Steam ID, most other IGN's: KITTYONFYRE
If you mean a mob spawner as opposed to a dark room, I know there was a problem with trying to get squids above sea level. --- Proud to be EPic. Those whose memories fade seek to carve them in their hearts...
In 1.2.5, a bug prevents mobs from spawning in the top 16 layers (layers 240 to 255). This bug is also present in snapshot 12w26a, so I suspect it's probably going to be in 1.3 too, but I haven't checked the pre-release. The same bug is what requires you to build "mob antennas" to get mobs to spawn in layers 128-239; at least one block in the 16x16x16 mini-chunk above the spawning floor must not be air.
However, it's also worth noting that when you build above layer 143, you reduce the odds of mobs spawning everywhere in that chunk. This is because normally the game picks a random spot to attempt a spawn from y=0 to y=127, but when you build higher it extends the range of the random pack spawn point without increasing the frequency of attempts to spawn. I think the effect of this is very small since the mob limit tends to be quickly reached, but I haven't done any tests to see. It's possible that a mob trap would be more effective if it were built entirely below layer 144, but it would probably have to be an extremely efficient mob trap for that effect to be noticeable. But maybe it is significant, I'm not sure. --- What senses do we lack that we cannot see and cannot hear another world all around us? -Dune
From: Mr Hangman | Posted: 8/1/2012 3:04:53 AM | #004
I'm not too familiar with the spawning mechanics, but wouldn't a high altitude mob trap be effective because there would be less total places for mobs to spawn? If I'm not mistaken, mobs can only spawn within a large sphere around the player, so if you are far enough off the ground very few mobs would spawn down there. --- Death is the solution to all problems. No man - no problem. ~Joseph Stalin
Yes, that's correct. Mobs outside a 128m radius sphere will automatically despawn. The obvious advantage of a high-altitude mob trap is that you can prevent all mob spawning on the surface and underground without having to go to the effort of lighting it up. I'm just pointing out that there is also a non-obvious disadvantage. Building at a high altitude is definitely an improvement over an old, pre-1.2 mob trap, but if you really want to go for efficiency there is a way to take advantage of both aspects: make your afk point on a tower at ~y=172 or so, but still build the trap under y=127. The downside is that you'll need an item elevator or to transport the items outside the loaded chunk area to prevent despawning.
But for less hardcore trap builders, then yes, build above 172 (over the ocean), but below 240. --- What senses do we lack that we cannot see and cannot hear another world all around us? -Dune