Sound propagation and the 'Stealth' Genre

'Sound propagation' is the term used to explain the behavior of sound in an environment. When creating a game, we want to accurately reproduce the acoustics of the virtual environment as it would sound in reality. This is no only so that the player can have an understanding as they relate it to the real world, but also to help increase immersion into the game. Accurate reproduction of sound propagation in games consists of multiple aspects, such as Reflection (Reverberation), Refraction (Re-direction of wave) and Attenuation/spatialisation (Where the sound is situated and its drop-off properties).

Reflection and Refraction
See the link below and select 'sound from air to water' to get a basic idea of how it propagates:

www.phys.hawaii.edu/~teb/java/ntnujava/propagation/propagation.html

Essentially, this behavior can be reproduced through using the correct reverb. In UDK, it would require too much CPU to run an accurate propagation model within a map. So to prevent this problem, it provides pre calculated reverb effects in which you can choose from. You can apply one of these presets which seems appropriate using the [Reverb Volume].

Attenuation and spatialisation

Spatiailisation is basically the realisation of the sound in a 3D environment, it allows a sound to emanate from 3D space. A lot of the things we hear in games are off screen due to our field of vision and spatialisation of sounds helps inform us of our surroundings. Attenuation and sometimes filtering when designing games can give a sense of distance from the object to the player. There are many types of distance algorithms to consider when designing attenuation and in games, these are:

• Linear - Unrealistic in the real world, but can be used for crossfading between looped audio and room tones.
• Logarithmic - For sounds more suited to be heard at a closer distance.
• Inverse - Good for loud and distant objects. Quiet at max radius but can be heard from far away.
• LogReverse - Loud at the max radius.
• Natural Sound - Realistic attenuation curve.

In UDK, these algorithms can be selected using the 'Attenuation' node.

Sound Propagation in the stealth genre

Accurate reconstruction of sound propagation is crucial in more genres of gaming than others. One example of this is the stealth genre, in which the player relies on sound for the most part to receive information on NPC whereabouts and the geometry of the environment. 'Dishonored', a recent release from Bethesda Softworks looks into this idea carefully:

http://vimeo.com/45113835#at=0

The sound changes when the player is in another room and the NPC is behind a door, also when the NPC walks in to the same room, the sound informs the player of this through adding clarity to the audio.

Another game which springs to mind is 'Thief', a similar game to Dishonored which used the same idea of accurate and carefully considered sound propagation. It seems essential in stealth games to accurately reproduce the behavior of sound dependent on the geometry of the game.

A final example taken from a fun 2D game called 'Mark of the Ninja' demonstrates the importance of sound within the stealth genre:

Sound is so important to the flow of the game,that it is emphasized through the ability to see it.

Aspects taken from:
The Game Audio Tutorial - (Book by Richard Stevens and Dave Raybould)
The Propagation of Sound - http://www.jhu.edu/virtlab/ray/acoustic.htm
UDK Documentation -http://udn.epicgames.com/Three/SoundCueReference.html