I would agree with Mark on this. The outdoors are very difficult to properly get any acoustic impact from when so much sound simply disappates.
You aren't getting annoying reflections that aren't wanted, but you definitely don't get the reinforcing effects which a room can provide other than the ground.
I have a client who has about 40 speakers on the perimeter of his yard and despite that number, it's not always enough to drown out the sound of passing traffic from a nearby roadway and there is almost no bass at all. A good midrange, fine with the highs, but no bass, and 40 speakers. Typically two speakers to a channel of audio, so plenty of amplification. It's just necessary to really push a ton of power and a lot of speakers when the audio is going into the abyss of free air.