A HS125 flight crew responded to smoke in the cockpit.

Narrative

At approximately 6000 FT; on our arrival; we experienced abrupt smoke coming from behind the avionics console below the windscreen. I instructed my First Officer to declare an emergency with ATC and tell them we would be landing at a nearby airport and to clear airspace ahead. Also requested him to call ahead to tower and have emergency equipment standing by. After lowering; the nose and applying full throttle; I turned and announced to out passengers (which they were already fully aware) that we had smoke and would land shortly. Approximately 2 to 3 minutes later; we were on the ground surrounded by emergency vehicles. I immediately shutdown the airplane as my First Officer evacuated the passengers. Total smoke duration in flight lasted about 30 seconds. Heavy at first; then tapering slowly towards the 30 second or so point. Odor was present; however; no smoke was emitting during final phases of flight or after shutdown. Passengers were fully debriefed and then escorted by our local service representative into the FBO.