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A very important and so-obvious-it's-easy-to-overlook way to get the most out of sound on a mobile or web platform is to be creative.
Early sound designers were making an unbelievable array of sound effects and musical instruments with nothing but square waves and noise generators.
While we have refined consumers' tastes and expectations over the years and have more tools and capabilities now, we must still use our brains to figure out ways to make sounds interesting, evolving, and efficient.
When dealing with looping sounds on mobile and web platforms, we often need to err on the side of smaller file size. Modulation is a surefire way to get the most out of small looping files so that they evolve and change over time without getting stale or annoying.
There are a couple different methods used to modulate sounds depending on the engine you are using. Some engines give you means to modulate sounds via their toolset. For example, in Unreal you can build a SoundCue with an oscillator module to change the pitch and/or volume over time (Figure 1).
In Unity, you can use an animation component to modulate volume, pitch, pan, low pass filter, or pretty much any other parameter over time (Figure 2).
While vastly powerful, the drawback of these methods is that the modulation is uniform; therefore it will always be the same every playback. If your engine does not provide a tool to modulate sound or you want more control over modulation, you can create modulation in script.
I have written scripts in C# for Unity to provide for modulation of volume, pitch, and a low pass filter. Doing so gives me far greater control over the modulation control and also allows me to randomize the modulation each cycle or each playback to keep the sound continually evolving.
For the sake of brevity, I have not discussed what other engines offer in terms of modulation and effects, although most can support modulation in some form or another via scripting. In fact, one enterprising developer has written some filters and effects in ActionScript for Flash and posted the source code online.
One thing that is crucial to remember when adding modulation to your sounds is to be subtle in the amount of modulation you add. A little goes a long way and adding too much will draw the player's attention to the changes in sound, which is what you are trying to avoid when creating dynamic evolving sounds from small loops.
Here is an example of a Reactor Core ambience from Free Range Games' forthcoming browser game Freefall, played back in realtime in Unity. The first clip in the movie is just two versions of the sound playing at different pitches in the game world. The second version is the exact same layout using modulation curves on both volume and pitch. As you can hear, the difference is drastic. Furthermore, while you can hear the repetition in the first example, the second example continues to evolve for several minutes before looping. You can watch this movie by clicking here.
A final means which can improve your design while keeping file size down is through stylistic design choices. Based on the type of game your team is developing, the art style or the intended user experience, the sound designer can choose to remove or augment certain video game tropes for the sake of the game's aesthetic and for optimization of the sound footprint. It is essential in all sound design to use your skills to shape the aural atmosphere in a way that makes sense, is unobtrusive and immersive.
However, when trying to create consol-quality audio on a smaller platform it becomes even more important to use your sound design creatively to augment the mood and tone of the game while culling the fat of unnecessary sounds.
Are footsteps really necessary in your dungeon crawler? Will anyone hear that background ambient loop, or is it a better idea to use point source triggered one-shots? If your on-rails space shooter is really all about fun ways to blow up enemies, maybe you should focus on those sounds rather than the player's ship's engines? The way we playback sound and the elements of the game we choose to sonify in the end will inform the final quality of the game's audio.
With a great designer, great sounds, and a solid toolset, amazing sounding games can exist on any platform. Tools available for mobile and web games are exploding with Unity, Flash's forthcoming Molehill 3D API, and even Unreal on iOS and Android.
This is a boon for smaller game developers looking to create console quality experiences in the browser or phone. Through the use of scripting, ingenuity, and aggressively balancing compression quality and sound needs, we can approach parity where one day soon the audio you hear coming out of your browser window or tablet or phone speakers will blow you away (and ideally keep people from pressing the mute button).