Crescendo Events are actions that cause a massive onslaught of Common Infected, often called a Horde, to find and attack the Survivors. 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. An important factor during game play is that Special Infected (including Tanks) are invariably intermingled or interspersed with Common Infected during Crescendo Events.
Most Crescendo Events involve causing a loud noise or bright object to appear, which attracts the Infected.
Another type of Crescendo Event is known as a Gauntlet Crescendo Event where rather than wait out a horde, the horde is continuously spawning and the Survivors must keep moving in order to survive, usually to turn off/disable whatever is attracting the horde. 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, The Bridge, and The Underground
Crescendo Events by Chapter Edit
Left 4 Dead Edit
- 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 from the second level of the room, through the hole in the floor inside the small locker room, and from the entrance door.
- 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 causes its noisy descent, which in turn summons a horde from the surrounding walls and hallways, 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.
- In The Alleys, the Survivors encounter a bridge blocked by a military barricade, and must fire a Howitzer 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 truck at the top. They must then lower the truck, 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 truck lowers, and the Survivors board, setting off for what is presumably the beginning of 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. You must go over to the door and converse with him. When you ask the Church Guy to open the door, he refuses to do it, and instead rings the church bells, all the while telling the Survivors to "prove their humanity." The bells summon the horde to attack.
- 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. In the second game, the event is slightly different: instead of a car alarm, the forklift now has its own alarm that activates when it is used. This alarm summons a horde to attack the Survivors, and it operates as a Gauntlet Crescendo Event that lasts until they get to the safe room.
- Now at the Boathouse Finale, the Survivors discover a radio. When they call for rescue, a man named John Slater answers their calls by informing them that a boat is arriving to rescue them. They must survive an onslaught of Infected, then make a break for the docks just outside of the boathouse.
- 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 five 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 longer than a regular gas 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.
- During 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, Survivors encounter a freight train with a lever that 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. In Left 4 Dead 2, this becomes a gauntlet event.
- During the Farmhouse Finale, the Survivors have to cross through a tall corn field to reach the farm. Crows were living in this corn field, and the Survivors' movement through it startles them, causing the crows to make a lot of noise, attracting the horde's attention. Like the other events, this is unavoidable; however, it is unusual because the Survivors do not activate or set off anything. This event's unusual placement of just before the finale, as well as the relative lack of warning related to this event, is likely to compensate for The Train Station (the fourth chapter of Blood Harvest) not having a Crescendo Event. No Mercy, Death Toll and Dead Air's fourth chapters all featured a Crescendo Event. In Left 4 Dead 2, this event is now a Gauntlet Crescendo Event that goes on until the Survivors reach the farmhouse.
- After reaching their holdout in the Farmhouse Finale, a radio on the table in the abandoned house is used to summon a military rescue via a military armored personnel carrier. In doing so, the team also calls a horde to their doorstep. They must hold off the horde until the vehicle arrives, which they enter from an open hatch in the back.
- At The Docks, the Survivors must open the door to a train car with a Tank in it. The Tank's loud roars will then cause a horde to attack.
- Like in Farmhouse Finale, the Survivors accidentally startle a flock of crows while climbing to the top of a gravel mound in The Barge. The birds' loud crowing attracts a horde. This is avoidable in L4D2 version of this campaign as seen in this video (skip to 3:23).
- In the Port Finale, starting any of the three generators causes a horde to attack, and later, a Tank.
In The Last Stand, activating a generator causes the horde to attack. It seems that the generator's sounds and the light at the top of the lighthouse are enough to attract a very large amount of Infected. While not technically a Crescendo, Gauntlet Crescendo or Panic Event, it is the act in-game that begins a round of Survival on The Last Stand.
In Left 4 Dead 2, the strategies for dealing with Crescendo Events are different, as they are more difficult to deal with. A common tactic in Left 4 Dead's Crescendo Events was to sit in a corner and wait it out, making them fairly easy. However, in the sequel, after the Survivors start a Crescendo Event, there will often be a second event to turn off the trigger of the horde and stop the waves of Infected from coming. Sometimes, it is necessary to flip the second switch to proceed, while other times, it is not actually necessary to stop the horde, but definitely preferable. There are also Panic Events that occur much more frequently than Crescendo Events; an extreme example of this is the Impound Lot from The Parish's third chapter, The Cemetery. 
- On The Hotel, the Survivors have to ride an elevator to the ground floor, but the ride is abruptly halted when smoke from the hotel reaches the elevator. The doors must be noisily pried open to progress further, which, naturally, the horde does not take too kindly to.
- On The Streets, the Survivors pass through a gun shop owned by a man named Whitaker. Whitaker has barricaded himself on his roof, but now he needs some cola. There is a gas truck that is blocking the team from reaching Liberty Mall, but Whitaker has the firepower to blow it up for you. Realizing the situation, he makes a deal with you: fetch him some cola from the nearby Save 4 Less convenience store, and he'll blow up the truck and clear the way. The store, however, is armed with an alarm, and the resulting event lasts until you get back to Whitaker's gun shop and give him his cola through a slot in the door he is hiding behind.
- On The Mall, after traversing through the back rooms of Liberty Mall, the Survivors must either break through the glass windows of a toy store or open an emergency exit door (whichever path the AI Director fancies for the players) to progress. This event goes until you go all the way up to the third floor's security room and deactivate the alarm. This event does not have to be stopped to proceed, although it is definitely preferred to do so before continuing.
- On the Atrium Finale, the Survivors take an elevator down to the atrium area. While they are inside of it, they plan to take Jimmy Gibbs Junior's race car, fuel it up (as Ellis points out that cars aren't fueled for car shows), and escape the mall at full speed. Upon the elevator opening, the finale begins.
- On The Riverbank, the Survivors come across a wedding, where there is a Witch who was apparently to be the bride just before turning infected. The Witch is avoidable or unavoidable, depending on how you approach her. Walking to the nearby speaker and turning on the music will play the Midnight Riders song "Save Me Some Sugar (This Won't Take Long)", which will then startle her and lead her to attack whoever turned it on. If the Witch is startled in any way, including the speaker, this causes a mini Crescendo Event, leading to all of the Infected wedding guests to attack you. Exercise extreme caution if turning on the music on Expert. You can just go one-by-one past the Witch if you prefer not to fight the wedding guests, but caution is recommended if trying to pass the Witch on Expert, Realism or Versus.
- On The Underground, the Survivors have to open a gate, which starts a Gauntlet Crescendo. From here, they must run to the safe room through a dark sewer, constantly slowed down by the water.
- On The Port, the Survivors have to gas up a generator so that they can lower the bridge, get back to Jimmy Gibbs Junior's car, and drive off to continue their journey to New Orleans. This finale is unique, as the team is helped by the Left 4 Dead team (minus Bill) who can cover them with gunfire and throw items down to the Left 4 Dead 2 Survivors while they're busy fighting to survive.
- On The Fair Grounds, the Survivors must activate a Merry-go-Round to progress. The ride's music loops endlessly, attracting a wave of infected that will spawn in kind until the team shuts 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 turnstile gates, which angers the horde and causes an endless horde that will continue to spawn until reaching the end of the level. While it is one of the shortest areas to go through for a Gauntlet Crescendo Event, the area that players fight through is extremely narrow, so they need to be careful, particularly watching out for Special Infected, as getting trapped here will likely end in at least one death.
- On The Concert Finale, the Survivors must activate the lights and firework effects of the Midnight Riders' abandoned concert stage to signal to a helicopter to evacuate them from Whispering Oaks. The ensuing music and pyrotechnics show attracts the horde.
- In Plank Country, the Survivors must call a small ferry for them to pass through the swamp; However, it will produce a loud noise which will attract a large horde towards them. Sometimes, this event can be skipped completely by using the nearby Jukebox to play Jonathan Coulton's "Re: Your Brains," a song that attracts the horde, then heading to the Gator Farm to activate the ferry.
- In The Swamp, the Survivors encounter a wrecked airliner, and are forced to open an emergency door to pass through, sounding an alarm. A glitch exists where you can climb around the plane and skip the event completely, as seen in some gameplay videos. However, this has been patched for both the PC and the Mac versions.
- 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 plantation house, in a way similar to the original Left 4 Dead Boathouse Finale.
- In The Sugar Mill, the Survivors need to reach the sugar mill below from the third story of a building, but a very loud elevator is the only way they're going to actually be able to go safely. Naturally, the horde is not pleased.
- In Mill Escape onwards, weather effects are now active. Rain is falling at this point in the campaign. Every time there is a flash of lightning, you have been signaled that a massive downpour is about to occur, which will then attract a horde.
- The Town Escape carries the weather effects from the previous chapter. The 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 Virgil and his boat.
- In The Park, the Survivors find a CEDA trailer. As evidenced by a sign inside of the trailer, opening up the door, exiting the trailer and breaking the threshold of the street will activate a very loud alarm, which will summon an endless wave of Infected, until the Survivors reach a tower with the alarm switch.
- 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 likely cause many more of them 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 have reached a hotel complex. The Survivors must get across a gap between the roofs of a pool house, so they use a conveniently-placed parade float to bridge the gap. Starting up this float makes it play a jazzy tune called "The Saints Will Never Come", Left 4 Dead's twist on "The Saints Come Marching In". The horde apparently isn't too fond of jazz, as you and the Survivors will be attacked until the float crashes. The tune also replaces the horde music in this event.
- In the finale of The Bridge, the Survivors must lower a two story bridge to cross the bridge and escape New Orleans on a military helicopter that is waiting on a helipad near the end of the bridge. After the bridge is lowered, the heavy metal gate locks up for a second, slamming harshly into the ground. The sounds of an entire two-story bridge lowering and a heavy metal gate crashing onto the ground end up being the metaphorical "starting guns" for the race between the Survivors and the Infected.
Panic Event Edit
Unlike Crescendo Events, Panic Events are things that can be avoided as the Survivors proceed through a level, if they are careful. There are five different things that can trigger a panic event. The number of summoned Infected appears to have been tweaked by recent updates.
Car Alarms Edit
- "You have GOT to be kidding me!"
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 touches them or shoots them, provided that the shooter is within a certain range. They can be identified by a flashing red light inside the car that is refracted by the windows. Whenever a loud sound - usually a gunshot - is produced, the alarmed car will make a brief beeping sound that will encourage trigger-happy Survivors to remain cautious. Almost all cars rigged with alarms will share the exact same model, as seen above, and will be pink-red in coloration. Fortunately, the Infected can't set these off directly (obviously for gameplay reasons). However, it is possible for a Smoker to set off the alarm by pulling a Survivor against the car. A Boomer exploding next to the car will also trigger the alarm. A Tank punching a Survivor into the car will also trigger the alarm. In Left 4 Dead 2, a Charger or Jockey can also force a Survivor against the car, triggering the alarm much like the Smoker. In the Impound lot, if you are unlucky enough to encounter a Tank, any cars it punches will set off. However, if the car self is punched by Tank or being hit by a Tank's rock it will deactivate the alarm (possibly because Tank's punch is so strong that it destroys the car's alarm). Remember that unless you want many Common Infected kills, you should avoid proximity to a car alarm if at all possible.
Metal Detectors Edit
In The Terminal level of Dead Air, the Survivors approach the security area. As with any airport's security area, Metro International Airport has metal detectors. Walking through one of them will trigger the noisy alarm (and the Panic Event), as the Survivors are clearly carrying quite a lot of weaponry and ammunition made of metal, However, they can simply walk around the two functional detectors and avoid the event completely. Pulling the Survivors through metal detectors is a popular tactic for players that are using the Smoker in Versus.
However, in Left 4 Dead 2, the metal detector causes a Gauntlet Crescendo Event that lasts until the team makes it into the safe room. The areas around the metal detector are now blocked off, forcing players to go through the metal detector. This was probably done to make The Terminal more difficult for the players to complete.
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.
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 Edit
Some events can be triggered by Survivors activating certain items unnecessary to completing the level. In campaigns that feature a jukebox, 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 certain carnival games in Dark Carnival or turning on the radio (or by some other way startling the bride Witch) at the wedding in The Passing will also trigger a panic event.
- 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. It may, however, be due to the whims of the Director, or possibly as a type of punishment directed at that player because they are not actively helping their team.
- For events that need to or can be shot at to activate, sometimes the bot players will trigger them by accident (when shooting at nearby infected), causing the players to potentially face a horde unprepared.
- This can be especially annoying during The Mall, as that is a gauntlet crescendo, and triggering it too early will likely cause you to get swarmed. In addition, in Single Player and Campaign, Special Infected love to spawn just outside the windows (they can't actually break in) and will cause the bots to shoot them.
- In Left 4 Dead, Bots pay no heed to alarmed cars and will shoot it or jump on top of it like normal cars; players may have to try a different path to avoid unnecessary fights.
- This is however, fixed in Left 4 Dead 2. Bots will generally refrain from firing at a cars when infected is nearby. Furthermore, even if they do shoot a car by means of accident. It will not set off.
- The Infected may only spawn in areas out of the Survivors' line of sight. A horde of Infected may emerge from a small, enclosed space, such as a closet or a bathroom. Survivors rarely enter bathrooms or closets unless low on supplies, so they will often be used for spawning.
- If a bathroom or closet is nearby, and the Horde sound is heard, going into the bathroom may cause the entire horde to spawn inside the player. The Infected cannot move, and cannot attack, though they will turn and yell. A few shotgun blasts will kill them.
- 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 it only goes off once the forklift's load is lowered. It can be seen by triggering the Crescendo Event, then turning around.
- Only Survivors' attack, melee shove, or jumping onto alarmed cars will trigger the alarm. Touching the side does not set the alarms off. So dragging, carrying, or riding a Survivor to an alarmed car will not trigger it, unless a Survivor shoots it while attempting rescue.
- In the sound files, the Panic Event is called MiniFinale.
- Valve recycled a sound (the_horror1.wav and the_horror2.wav to be exact) from Half-Life and used it for the screams that are heard after a Crescendo Event starts.
- In Left 4 Dead, Crescendo Events are never directly mentioned by Survivors - however, they do say how to get past one, and ask each other if they're ready.
- It appears that there was an electric fence Crescendo Event somewhere around No Mercy in Left 4 Dead. Its 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.
- It is unknown why the finales attracts hordes, as none of them makes any noise (with the exception of both the Truck Depot Finale and the Runway Finale from Left 4 Dead and The Bridge and The Concert from Left 4 Dead 2). The "real world" explanation is that this is intentional to gameplay. A possible in-game theory is that when the Survivors linger around the finale area, the Infected pick up their scent easily and attack; this may also explain why hordes can also spawn when players stay in the same area or backtrack. In some cases, like the Town Escape in Left 4 Dead 2, or The Lighthouse in Left 4 Dead, the horde is attracted to light. A cut line from the helicopter pilot states that he believes the Infected have the ability to sense hope, and when they do, they "have to destroy it.", although this may simply be hyperbole. Another option is that all the hordes of infected that the players evaded by entering the safe rooms found a way to bypass them and finally caught up with the player all at once.
- Death Toll (not including the cresendo event tied to a car alarm) and Swamp Fever are the only campaigns in the Left 4 Dead universe that do not contain car alarms.
- If a Tank hits an alarmed car with a concrete slab, the car will turn white or purple.
- When the Survivors trigger a Crescendo Event but run away, the Horde will find them wherever they are, even when they are far away from the source of the loud noise that startled the Horde.
- Fire does not trigger any car alarms. Using a Molotov or Gas Can against the Infected near an alarmed car can be useful.
- There is also a glitch with the car alarms if they are shot with Incendiary Ammunition, which does not trigger the alarm. This makes keeping some Incendiary Ammunition near car alarms very useful.