Bungie’s Audio Lead Jay Weinland has been opening the same door for the last hour. He wants it to creak like a door that hasn’t opened for hundreds of years, yet the creaking sound can’t overwhelm the sound of metal grinding on metal either. “I’m just about finished with the first pass on this level,” he says, opening the door again. In later audio passes, placeholder sounds evaporate and real ambient effects emerge – birds chirping in the background, the soft murmur of streams, the wind stirring through trees and foliage.

C Paul Johnson, our Audio Design Engineer is having a bad day – his computer is failing him and he’s buried in making noises (C Paul spends a lot of his time making the sounds that tickle your ears in Halo). Weinland told Marty O’Donnell that C Paul was “grumpy” that morning, only to hear C Paul bark “I’m am not!” from his office. If C Paul is grumpy, and we're not saying he is, he probably has good reason to be. Audio is one of the last pieces of to get put into a video game. Why? Because these guys could spend their time working on the sounds for a particular feature, only to hear the stale thud of that feature ending up on the cutting room floor. The final months of development on a project are the busiest for Bungie’s audio team. “Basically, we work on whatever is shipping next,” C Paul says.

Right now, Halo 3’s campaign is filled with placeholder music and Marty O’Donnell’s office door has a sign taped on it – it’s a skull with “Email or Die” written underneath it. He's busy.

Marty asked if I’d seen the Ivory Tower yet. I thought I had, probably for some 500 games of Matchmaking. But that’s not the Ivory Tower Marty is talking about – his office, buried in the back of Bungie’s computer-festooned gymnasium, is stuffed with chairs that look more comfortable than they actually are – is called the Ivory Tower.

The soundproofed room has three computers, an army of speakers and a keyboard inside.

The music in Halo 3 operates similarly to how it did in its predecessors, Marty explains. For example there are some pieces of music designated as “glue.” The glue consists of smaller pieces called “ins,” “loops,” and “outs” – these aren’t complete songs, not like those you’d hear on an official soundtrack. Instead, think of them as branches of a tree, interconnected but still individual.

Working side by side with the mission designers, Marty plans triggers that will begin and end these pieces of music. The trigger could be something as simple as when a player steps out into a sun-baked vista - a theme fades in. However, once the music starts it doesn’t just play a single song and cut out. An intro segment piece triggers in the mission and then a separate trigger cues the piece to move to an outro or an “alt” (an alternate version of the piece to act as more “glue”). What Marty played for me, the first piece from Halo 3 to tickle the studio’s ears, was a serene chorus of voices, still cooing, oscillating and shifting, music that could be the soundtrack to an ethereal foreign landscape. Twenty-seven seconds later the piece faded to silence.

But, the music doesn’t have to simply “fade out” – that’s the point of the glue. Instead of disappearing quietly, the song has multiple variations that have been arranged to fill earspace regardless of how long it takes a player to move from point A, to point B. Once at point B, an outro kicks in and gently exits the player from the scene – or could even begin a completely different piece of music.

“A lot of stuff is trial and error,” Weinland says. The creaking in the door gets louder and softer. Some of that trial and error went into setting 85 sound files for the Battle Rifle. Some of the sounds are obvious: the click of the reload animation, a round ejecting from the chamber and rolling down a hillside (there’s different sounds depending on what surface the round hits), some of them aren’t: the hollow click when the gun is fired on empty, the sound of picking up ammo, the sound of a dropped rifle hitting a surface, whether it’s the steel walkways on High Ground, the stream on Valhalla or something otherworldly on some place you haven’t been, yet.

Why does Halo 3 sound so much different than Halo 2? “The level of detail stuff,” Weinland says. “That’s what people are really latching on to.” C Paul explains some of the level of detail stuff: “Each weapon/vehicle or whatever else we want to do it for, has two separate sounds. One for close up and one for distant. We crossfade between the two based on the distance from the player. So a lot of the time you’re actually hearing a mixture of the two. In the cases of the battle rifle and sniper rifle we actually have 3 levels of detail." But why does Halo 3 sound so much different?

"One way developers create the sound of weapons firing in the distance is by simply turning down the volume on a sound as players move further away from it,” Weinland says. “Because we crossfade to a different sound, instead of a quieter Battle Rifle, it sounds like a newsreel from a warzone.”

It’s not a warzone and it’s not a Battle Rifle, but here’s the sound of a door opening somewhere in Halo 3.