Crescendo Event

Crescendo Events are things that causes a horde or massive onslaught of the Infected. The cause is often a part of the environment that The Survivors must activate in order to progress through a level, generally acting as mini-finales.

Most Crescendo Events are related in that starting one causes a loud noise, which infuriates the Infected and causes them to seek out its source.

No Mercy
After exiting The Subway rails in the second level, Survivors have to activate an electrical generator to open a door, which will alert the infected. They will start coming through the big windows on the second level of the room, and also through the hole inside the small locker room.

In The Sewer level, Survivors must activate a noisy lift to cross a roof and enter a warehouse. Infected pour over the metal roof and across the street below, climbing eighteen-wheelers parked below and the ladder up the lift to reach them.

Later, in The Hospital, Survivors must use an elevator. Pressing the button to cause its descent summons a horde from the surrounding walls and the hall from which they just came from, some even pouring in from the vents above their heads.

Reaching the Rooftop Finale, Survivors hear a voice from over a radio, which they use to call for a helicopter. Summoning rescue also triggers a horde event. They must survive several waves of Infected as the pilot makes his way to the landing pad nearby.

Death Toll
While scouring The Drains, the Survivors reach a flood control room. They must trigger a flood gate to drop to proceed further. Doing so summons a horde from the sewers below.

When Survivors reach the safe house at the end of The Church level, a mysterious man known only as "Church Guy" (so called from the name of his sound file) is already inside. When asked to open the door, he will instead ring the church bells, calling forth a horde.

After leaving an office building, Survivors are confronted with a forklift holding up a cement walkway in The Town. Triggering the forklift to drop its load allows them to proceed, but not before summoning a horde.

Having arrived at the Boathouse Finale, Survivors discover a radio. They are informed by the male voice heard over it that a boat is arriving to rescue them. They must survive an onslaught of Infected, then make a break for the docks just outside.

Dead Air
Upon exiting an apartment complex, the Survivors encounter The Crane from which a dumpster is suspended. They must lower the dumpster to proceed to the next rooftop. The loud noise the dumpster makes during its descent attracts a horde.

Next, clearing out The Construction Site, Survivors discover a barricaded alleyway with several propane tanks propped in front. Ascending the metal framework of the unfinished building usually yields molotov cocktails and pipe bombs, both useful in the horde onslaught brought on when the barricade is inevitably destroyed.

In order to proceed through the baggage handling area of The Terminal, Survivors must rev up a van crashed into the wall nearby, which will barrel through a locked gate. This difficult horde event will yield infected from the offices behind, over an adjacent pile of rubble blocking another hall, and the escalators below.

Exiting from the waiting area to the Runway Finale, Survivors discover a military plane and a voice from inside communicating over a radio. The pilot informs them that the aircraft needs fuel, and instructs them to activate a fuel pump. Once filled, the plane can carry them to safety. However, the noise the pump makes attracts several waves of the Infected, which the Survivors have no choice but to endure if they are to escape.



Blood Harvest
In The Tunnel, Survivors must leave a train shed through an emergency exit, which, once triggered, sets off an alarm, calling forth a horde.

During The Bridge level, Survivors encounter a freight train with a lever they must pull to remove the brake, causing it to roll backwards and knock down a bridge. Doing so allows access to the safehouse at the end of the level, but the collision summons waves of Infected.

On the way to the Farmhouse Finale the Survivors have to cross through a tall corn field. There are crows in this field that will become startled and fly away making a racket. A horde is attracted to the noise. Like the other events this is unavoidable but will catch players off guard.

The Survivors eventually reach a derelict farm during the Farmhouse Finale. A radio on the bottom floor of the abandoned house can be used to summon military rescue. In doing so, the team also calls the infected to their doorstep. They must stave off the horde until the vehicle arrives, which they enter from an open hatch in the back.

Panic Event
Unlike Crescendo events, Panic Events are things that can be avoided as the Survivors proceed through a level, if they are careful. There are several different things that can trigger a Panic Event. The number of summoned zombies appears to have been tweaked by recent updates.



Car alarms
Cars are strewn about the map in virtually every level. While most are harmless, you'll find the occasional alarmed one. These alarms are set off if anyone shoots them or touches them (especially if doing something like jumping on them). They usually can be recognized by a flashing red light from inside the car that constantly changes the tint of the windows. Fortunately, the Infected can't set these off, no matter how much they attack it (obviously for gameplay reasons), there is also a certain range where shooting at an alarmed car will not activate it.

Metal detectors
In the Terminal level, Survivors are advised to stay clear of metal detectors. Walking through one will trigger the alarm (and the Panic Event), as the Survivors are obviously carrying quite a lot of metal.

Boomers
Boomer bile causes one panic event for each Survivor that gets covered by it. The Horde that is summoned will only attack the Survivor that it was summoned for. Once the bile is gone, any remaining zombies will attack the nearest Survivor.

Time
Staying in one place too long or backtracking to a location you have already been will also trigger a Panic Event. The frequency of these Events are based on the current Difficulty level.

Left 4 Dead 2
In Left 4 Dead 2, crescendo events are made considerably harder. A common tactic in Left 4 Dead ' s crescendo events is to sit in corners and wait out the storm, making them fairly easy. However, in the sequel, after the Survivors start a crescendo event, there will usually be a second event to turn off the trigger of the horde and stop the waves of Infected from coming. There are also panic events with significantly increased odds of happening; imagine a parking lot full of cars with alarms.

Trivia

 * Occasionally, if there is more than one player before a Crescendo Event, if one player goes idle, a seemingly random horde may spawn — one that shouldn't, as no enemies should spawn until after the trigger for the horde is set off.
 * Tanks never appear during a Crescendo Event except in the intro movie when the Horde appears and in the final level finales
 * Sometimes Hordes will appear out of strange areas such as bathroom and closets.