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Acoustic Ecologies: Half-Life 2’s City 17
by Michel McBride-Charpentier on 05/09/11 06:05:00 pm   Featured Blogs

The following blog post, unless otherwise noted, was written by a member of Gamasutra’s community.
The thoughts and opinions expressed are those of the writer and not Gamasutra or its parent company.


City 17 Soundscape (2.50 min) with annotations on Soundcloud (sorry, embedding doesn't work on Gamasutra).

I made this short recording of a walk through City 17 in order to more closely examine the impact of the soundscape on the game environment. Put on some headphones, close your eyes, and enjoy the atmosphere.

Just as the mis-en-scène of a game is everything in front of the camera, including the HUD and held weapon, a soundscape in a game is the total combination of everything the player hears, from footsteps to achievement notifications. It can be predicted to a degree, but will never really be known until one is immersed in it. This is because a soundscape is constructed out of, among other things, dynamic sounds and the mobile player's constantly changing distance to 3D sound sources. I like to call this soundscape possibility space the level's acoustic ecology. The acoustic ecology is a fundamental part of what transforms the abstract space composed of physical relationships between objects into a place that communicates context for narrative meaning to be derived from.

In the Source engine a “Soundscape” is the name given to a script that takes a bunch of .wav files and mixes them together on top of a looping base track, to be played when the player is inside a certain volume of space. Sound effects are divided into logical groups (distant trucks, alarms, aircraft, etc), and every few seconds (within varying random ranges) a sound in each group is played in a random location with a random volume. The resulting “soundscape” is a unique drone of city activity on the edge of hearing. It’s background noise in not just an acoustic sense, but a spatial one as well.

This background drone is to the acoustic ecology of the level as the 3D skybox is to level geometry (The Source engine’s 3D skybox is a 1/16th scale set of buildings that is “projected” outside of the actual level geometry). The sound effects in these scripts are randomly positioned within a volume of space the player occupies, but filtered and at low volume so that they seem to be originating from beyond the level’s façade, outside of the geometry. This background noise, like the skybox, is an illusion designed to make the player believe they are in a place with depth, and not an enclosed, constructed level. Layered on top of this are the local sounds, generated from entities inside the level and much more sensitive to the positioning and actions of the player.

R. Murray Schafer identified three elements of the soundscape: Keynote sounds, sound signals, and soundmarks. They’re a good place to start thinking about acoustic ecologies from a level design perspective, and I’ll walk through each of them in turn.

The keynote sound is so ubiquitous that it often doesn’t consciously register, and because of this ubiquity it more or less accurately represents the character of the environment and people who inhabit it. In modern cities traffic is the dominant keynote sound, as it is in the exterior portion of this City 17 recording. There is, however, an additional local keynote sound: the flying, shutterbug City Scanners. I was surprised to hear the warbling, beeping, clicking scanners almost constantly throughout the soundscape I recorded. I definitely saw a lot of scanners when I played through, but I never realized how often I could hear them.

This is a perfect example of an implementation of a keynote sound to support the themes of the game. It’s a common but sometimes only subconsciously perceptible sound that represents the character of the environment: a police state without privacy. Consider if there was only one scanner that took your photo when you got off the train, and some mounted cameras like the one you can hear snapping a series of photos halfway through the recording. Or even if the scanners moved silently. The same point would have been made visually, perhaps, but the player would have been a lot more comfortable moving around City 17. There would be areas of surveillance and areas of privacy the player could consciously recognize. But with the scanners providing a keynote sound, the unsettling sense of being watched etches itself in the subconscious and never really goes away.

Sound signals are the sounds that punctuate the soundscape. They’re listened to consciously, and therefore have a large impact on the acoustic ecology of a level. What players probably remember most about this part of City 17, other than the Citadel and Breen, is the Combine chatter and introduction to the stomping Strider. They’re visually interesting too, but the sounds are free from line of sight restrictions and contribute to that lasting impact. The Strider is heard before and after it’s seen, and you don’t need to be looking at a Combine patrol to hear the familiar radio barks. A sound signal, as the name implies, is used to communicate something directly to the player—they are meant to be listened to. In terms of level design, sound signals can be used to guide the player to points of interest, hint at the location of enemies, and so on. I think this aspect of sound design is fairly well understood already.

Soundmarks are Schafer’s term for sounds that are, like landmarks, unique to an area. Breen’s broadcast would be a soundmark in this recording. However, because it loops indefinitely, players probably try to ignore it after having heard enough. It fades into the background, becoming more or less a keynote sound. Making Breen’s words heard almost everywhere in this level, but maybe only subconsciously absorbed by the player (and presumably residents of City 17), is a kind of perfect, if unplanned, acoustic design that emphasizes the Big Brother theme.

It also appears to be good practice to tie the soundmark and landmark together, but this probably isn’t a hard rule. The landmark of this area is the Citadel. When the Citadel is first viewed, Breen’s display is right there, hanging on a nicely juxtaposed obelisk. The ultimate destination and ultimate enemy—the landmark and soundmark source—are elegantly connected to one another. A few minutes later we'll find out Breen is also inside the Citadel. There are many other cases where this type of connection would make sense: Church spire and ringing bells, castle wall and cannon fire, lighthouse and crashing waves, and so on.

I’ll end with a note on playground_memory.wav. It’s what first intrigued me aboutHalf-Life 2’s soundscape and inspired me to delve deeper. You can hear it near the end of the recording. While walking through a dilapidated playground the sounds of children playing can be heard fairly clearly for a moment. This is non-diegetic in that it does not exist in the fiction of the world. Yet it clearly exists in the fiction of the game. The filename hints at its nature: Gordon Freeman’s memory of a lively playground, in happier times. There’s a lot going on in this recorded soundscape. Give it another listen and try to picture the possibility space of sounds that it was created from.

Further Reading:

Stay a while and listen | Ben Abraham writes about "off-screen" wind chimes inHalf-Life 2, and the musicality of sound effects. | The inspiration for this digital soundwalk. Lots of great links to other sound studies resources.

The Soundscape: Our Sonic Environment and the Tuning of the World | R. Murray Schafer's seminal work.

Soundscape - Valve Developer Wiki | Technical description of the soundscape scripts used by the Source engine.

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Level Designer


Darius Kazemi
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"The Source engine’s 3D skybox is a 1/16th scale set of buildings that is “projected” outside of the actual level geometry" -- could you explain that in more detail? I don't understand and would like to!

Eric Schwarz
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If you look at most Source games, the skybox is actually played far-off in some corner of the level, because unlike traditional skyboxes or skydomes, they aren't objects actually placed "around" the game space. I have no idea the coding that is involved in this process, but it's as he says. Whether it's a superior approach or not, I'll leave up to Valve, but I imagine it lets them create unique level backdrops, change them quickly and easily by replacing the base model, and then manipulate the projection parameters, rather than having to rebuild the entire background object-by-object.

Steven An
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I don't know the details of how they do it, but play around with Forge and make some Source levels. This will help you understand how their sky boxes work.

Andrew Durdin
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Darius: Each level consists of two spaces: the first is the full-scale area that the player moves around in and interacts with. This area contains a high level of detail, and calculations are done every frame as to which parts of it should be visible from the player's position, in order to determine whether to render it or not (lookup BSP on wikipedia for more info on that).

The second space is the 3D skybox, which is always rendered. It is used for low-detail, background stuff that the player will never get near to: the surrounding hills, the ocean, distant apartment buildings, and the citadel (in most levels). It is usually created at 1/16th scale in the editor, like a scale model of the surrounding landscape, but is scaled up when rendering so it appears at full size. The level designer places a "sky_camera" entity within this landscape at a position that corresponds precisely to the origin of the full-scale area; this serves as a guide so that the engine can precisely line up the full-scale area and the 3D skybox—so that the full-scale area and the 3D skybox can blend seamlessly into each other.

Michel McBride-Charpentier
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An entitity called sky_camera "is used to mark the position of the map's origin inside the 3D Skybox. As your position changes relative to the maps origin it will be scaled and applied relative to the sky_camera." It's a real-time projection from a camera inside the 3D skybox on to the "sky" of the actual level.

Basic Characteristics of the 3D Skybox:

The gist of it is they are low-detail, non-interactive, can have their own props and lights, and don't use occlusion culling.

Nick Rudzicz
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I believe it's Menace II Society where there's a constant drone of helicopters watching from the sky.

Eric Schwarz
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Wonderful article. I've always thought Half-Life 2 had some of the best audio in just about any game I'd played, with so much attention to care, detail, depth, etc. that makes most titles sound flat in comparison (Crysis might be the only shooter I've seen that comes close). Working with other game engines, I imagine this largely has to do with the fact that few really offer up good tools for manipulating the audio space. Short of placing ambient vs. directional sounds, very few let you precisely control the exact volume, pitch, placement, intensity, EQ etc. within the engine itself, meaning you're largely reliant on the base sound effect itself.

A note on using sound to evoke fiction that does not exist visually: I think this is one of the best ways to make the world feel genuine and real. I spent quite a bit of time a while back building a city slums environment for my ongoing mod project, and as I ran around it without having yet placed the audio, I didn't "feel" as if the area was a slum. I actually spent a long while tweaking it visually just because of this, but I eventually discovered it had to do with the audio. Once I added things like the sounds of distant arguments and violence, dogs barking in the distance, tricking and dripping water, and the right reverb effects, it really came alive. Well-done audio really makes the world feel more spacious than it is.

Brad Borne
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I mostly agree, but HL2 seems to suffer from the same problem as a lot of first person shooters, especially on PC. The gun sound effects just sound weak, more sputtery than punchy. Maybe that's done to accentuate the environment? I don't know. Strangely, the only shooter I've seen recently with an awesome gunshot sound is that latest Wolfenstein game...

Zak Belica
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Well formulated article, and some good points are made. One workflow reality that should be mentioned is that you as a sound designer should establish great relationships with the level designers. You'll inspire each other to create a more impressive world, and audio is such an 'easy' way to sell it.

On a more inspirational note, as a sound designer, you are a dramatist and storyteller as well. Valve is fantastic at using visuals and audio to convey a story about a place- a story that is shown/heard, not 'told' via dialog or cinematics. It creates a powerful sense of player ownership of the game world.