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 or bright object, which attracts the Infected.

Another type of Crescendo Event is known as a Gauntlet Crescendo Event where rather than wait out a horde, the Survivors must keep moving in order to survive. These types of events are only found in Left 4 Dead 2. Examples of Gauntlet Events are the Crescendo Events of" The Streets, The Mall, The Fairgrounds, The Coaster, The Barns, The Park, and The Bridge.

No Mercy
After exiting The Subway rails in the second level, Survivors have to activate a loud electrical generator to open a metal hangar 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 noisy 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. The horde will keep coming until the elevator arrives. On occasion, the director sends forth a final horde when the elevator arrives. It is up to the players to decide whether to deal with this final horde or to simply rush in the elevator and skip it.

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.

Crash Course
In The Alleys, the Survivors encounter a bridge blocked by a military barricade, and must fire an artillery cannon to destroy it. The resulting noise attracts a massive horde of Infected.

In The Truck Depot Finale, the Survivors reach a vehicle lift, with an armor plated bus at the top. They must then lower the bus, via a rather unreliable generator, which sometimes requires to be started twice. Half way through, the power goes out, and the Survivors must fight in finale conditions in pitch black, until the power reactivates. Soon, the bus arrives, and the Survivors board, setting off for what is presumably the beginning of Death Toll.

Death Toll
While scouring The Drains, the Survivors reach a flood control room. They must trigger a flood gate to drop to proceed further. The noise of the gate dropping 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 when the load drops, the vibration sets off a car alarm, 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 wooden barricade with a "bomb" (made out of several gas cans tied together) propped in front of it. To start the event, a Survivor must shoot the gas bomb. Doing so causes the barricade to burn slowly at first, but suddenly a nearby generator explodes, summoning a horde. The Survivors cannot continue until the gas bomb is done burning, which takes about three times the time a regular can takes to burn.

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 safe house 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; however, it is unique because the Survivors do not activate or set off anything. It is also right before a finale.

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.

The Last Stand
In the Last Stand, activating a generator is what causes the horde to appear. It seems that the generator's sounds and the light at the top of the lighthouse is enough to attract a very large amount of Infected.

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.

Dead Center
On The Hotel, the Survivors have to ride an elevator to the ground floor, but their descent is halted when smoke from the hotel reaches the elevator. The doors must be noisily pried open to progress.

On The Streets, the path to the Liberty Mall is blocked by a gas tanker, and a man named Whitaker can blow it up, yet he won't unless you fetch him some cola from the convenience store. The store however, is armed with an alarm.

On The Mall, after traversing through the back rooms of Liberty Mall, the Survivors must break through the windows of a clothing store or opening an emergency exit door, depending on your path, tripping an alarm. You need to run to the security room to deactivate it.

Dark Carnival
On The Fair Grounds, the Survivors must activate a Merry-go-Round to progress. The noise attracts an endless wave of infected until they shut the ride down.

On The Coaster, Survivors have to activate the roller coaster to clear a blockade, attracting the horde until the coaster is shut down.

On The Barns, to enter the stadium, the survivors must open the entrance gates, alerting the horde, which will continue to spawn until the end of the level.

On The Concert Finale, the survivors must activate the lights and firework effects of a concert to attract a helicopter to save them. The ensuing music and fireworks attracts the horde.

Swamp Fever

 * In Plank Country, the Survivors have to call a small boat for they pass through the swamp which will make a large a loud noise, after which it stops once the Survivors pass.
 * In The Swamp, the Survivors encounter a wrecked jet, and are forced to open an emergency door to pass through, sounding an alarm.
 * In The Shantytown, a loud, rusty ramp must be moved to get to a shack above, creating a loud, Horde-attracting grind.
 * In the final chapter, The Plantation, the Survivors hold out in a mansion, in a way similar to the original Left 4 Dead Boathouse Finale.

Hard Rain

 * In The Sugar Mill, you and the other survivors need to reach the sugar mill below you, but a very loud elevator is the only way to get down.
 * The Town Escape finale takes place in the middle of a hurricane on top of a Burger Tank restaurant, where the Burger Tank sign must be activated to flag down the ferry from Swamp Fever to pick you up again.

The Parish

 * In The Park, the Survivors encounter a trailer with an alarm. Exiting the trailer and breaking the threshold of the street activates an alarm, which summons an endless wave of infected; endless, that is, until the Survivors reach a tower with the alarm shutoff.
 * In the 3rd chapter, The Cemetery the Survivors emerge from a manhole into an impound lot filled with alarmed cars. This appears to be a potential "slippery slope", in that setting off one alarm will inevitably cause many more to be set off by stray gunshots from the ensuing battle. (Considered an "optional crescendo event" instead of a series of panic events.)
 * Near the end of The Quarter, the Survivors reach a hotel complex. The Survivors must get across a gap between the roofs of a pool house, so they use a parade float to bridge the gap. Starting up the float makes it play a a variant of "The Saints Come Marching In". This tune attracts hordes of zombies until the float crashes. The tune also replaces the horde music in this event.
 * In the final chapter, The Bridge, the survivors must lower a two story bridge to reach a military helicopter. After the bridge is lowered, the heavy metal gate locks up for a second, before slamming harshly into the ground, the loud noise alerts the horde.



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 Infected 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), however there is also a certain range where shooting at an alarmed car will not activate it. 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 directly (obviously for gameplay reasons). However, it is possible for a Smoker to set up with the alarmed car in between himself and a survivor, then pull that survivor into touching the car and setting it off. Remember that, unless you're going after kill-related achievements, or if you just feel like killing a lot of Infected, you should walk away or avoid them, like a Witch.

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. Pulling Survivors through metal detectors is a favorite past-time of Smokers in Versus.

Boomers
Boomer bile causes a small panic event for each Survivor that gets covered by it. The horde that is summoned will only attack the Survivor(s) that it was summoned for. Once the bile is gone, any remaining Infected will attack the nearest Survivor.

Time
After certain periods of time (controlled by the Director), a horde will naturally attack you. Staying in one place too long or backtracking to a location you have already been will also trigger a horde attack. The frequency of these Events is based on the current difficulty level, and, occasionally, the chapter being played (Dead Center chapter 3 is notable for having many of these attacks.).

Triggered Events
Some events can be triggered by survivors activating certain items unnecessary to completing the level. In The Parish and Swamp Fever, turning on the Jukebox and playing the Jonathan Coulton song "Re: Your Brains" will cause a panic event at the first refrain ("All we wanna do is eat your brains"). Winning carnival games in Dark Carnival may also trigger a panic event.

Trivia

 * Occasionally, before a Crescendo Event, if one player goes idle, a seemingly random horde may spawn. This should not be possible, as no enemies should spawn until after the trigger for the horde is set off.
 * Sometimes hordes will appear out of strange areas such as bathroom and closets which usually acts as comical relief due to the sight of entire hordes simultaneously flooding out of small cramped locations. This happens because zombies may only spawn in areas out of the survivors line of sight. Since its rare for the survivors to actually go into closets or bathrooms unless low on supplies, they will often be chosen for spawning
 * All of the cars with alarms in Left 4 Dead use the same model. There is one car in Death Toll, however, which is alarmed and uses a different model, but only goes off once the forklift's load is lowered. It can be seen by triggering the Crescendo Event, then turning around.
 * In the sound files, the Panic Event is called MiniFinale.
 * It is not uncommon for a Tank to spawn right before the Crescendo Event in the Tunnel. If this happens, attempt to shoot him through the wall before activating the event. He won't be able to break down the door, or come in through the vent above you. Eventually he'll kill himself.
 * Valve recycled a sound (the_horror1.wav and the_horror2.wav to be exact) from Half-Life 1 and used it for the screams that are heard after a Crescendo Event starts.
 * Crescendo Events are never mentioned by survivors - however, they do say how to get past one, and ask each other if they're ready (however, they always do this).'
 * It appears that there was an electric fence crescendo event somewhere around No Mercy in Left 4 Dead 1. It's campaign can be judged by the way the Survivors call each others in the cut lines, so it's logical how it could be in No Mercy, the first campaign of the game.