Yes: if AudioClip is in stereo (or another number of channels), downmixes all channels down into mono.

No: does not modify channel count.

Yes: readjusts the gain of the AudioClip so that the now-mono sound is the same volume as the original stereo file.

No: does not readjust the gain.

Check this box if the clip has ambisonic-encoded audio. Ambisonic audio is useful for VR, AR, and 360 video etc, but it’s not really germane to this guide.

These tabs let you specify a default setting and platform-specific settings for the settings below. Some platforms have compression formats that aren’t available on other platforms, and some might just have different hardware that requires you to optimise differently. See Notes below for more on platform-specific compression formats.

Make sure to check the platform-specific settings, even if you want everything to use the general settings – Unity may set some platform-specific settings for you automatically. For example, iOS builds may default to “specify sample rate: 22kHz”, which can cause aliasing (a sound glitch introduced by downsampling incorrectly).

Decompress on Load: audio is stored on disk in the Compression Format specified, but is decompressed and loaded raw into RAM as PCM. This takes up a lot of RAM and increases load time a bit, but is very cheap in terms of CPU and very fast to access.

Compressed in Memory: audio is stored both on disk and in RAM in the Compression Format specified. This takes less RAM and less load time, but costs CPU when the sound is played, as it needs to be decompressed on the fly each time it is played.

Streaming: Audio is streamed directly off the disk without going into RAM at all. This uses some disk throughput and CPU, but on PC/console, it does not have a huge impact on performance so long as there are no more than two sounds streaming simultaneously. On mobile (particularly lower-end and old devices) streaming more than one stereo audio file simultaneously starts taking a heavy toll on CPU use (see Warnings below).

PCM: straight-up raw audio data, fully uncompressed and takes up loads of disk space and RAM, but playing it is basically free because it does not need to be decompressed.

ADPCM: very old compression format with a compression ratio of 3.5:1. Pretty cheap to compress/decompress compared to Vorbis or other compression formats, but introduces digital noise artifacts into the sound, so should only be used for noisy sounds in which this would not be noticed. If you are unsure whether ADPCM is right for a particular sound, preview the sound in both PCM and ADPCM formats – if you hear a difference, I recommend going for PCM instead.

Vorbis: compressed format which is compatible with most major platforms. Can handle quite high compression ratios whilst maintaining decent sound quality, but is somewhat expensive to compress and decompress on the fly.

70-100%: Practically indistinguishable from full quality PCM to all but audiophiles with expensive audio equipment

1-69%: Varying degrees of quality, the lower values introducing a lot of gross noise artifacts, removing dynamic, and making the audio sound flat and lifeless. You can use the preview button in the inspector pane to see how noticeable the drop in quality is for that particular sound.

Preserve: Just uses the sample rate at which the sound was made.

Optimise: Unity analyses the audio for the highest frequency that it contains, then uses the Nyquist theorem to determine the lowest sample rate that can be used without losing any of those frequencies. E.g. if the highest frequency that the sound contains is 10kHz, the sample rate can be lowered to 20kHz without any loss of sound content. This setting can only be used with PCM/ADPCM.

Override: If you want, you can manually apply a new sample rate to the AudioClip. I generally would not recommend this unless you know what you're doing.

Streaming multiple audio files at once is relatively light on CPU use for PC and consoles, but can present a big problem on mobile - especially low-end or old devices. Below is a chart of measurements I took using Unity's profiler, comparing the effect of streaming multiple audio files on various Samsung Galaxy phones and my own PC. The first chart shows 1-12 simultaneous streaming audio sources, the second is a zoom-in on sources 1-3.

If a sound is set to Decompress on Load, using Vorbis compression will only make it 1/10th of the size on the disk, not the RAM, which still stores the raw PCM data. If you set the sound to Compressed in Memory, you do get the RAM saving, but at the cost of CPU to decompress it on the fly.

Every load type except for Streaming will by default load audio data into RAM and leave it there until the scene is unloaded. If you are running your whole game in one big scene, this could fill up all of your available RAM. To make matters worse, manually removing AudioClips from RAM is hugely inefficient and can cause a frame hitch, as can leaving it up to garbage collection. If you have a lot of audio data, you may want to optimise for lower RAM use by sacrificing in other areas, utilising Unity asset bundles. The Preload Audio Data setting has no bearing on this either, as it only determines when the data is loaded into RAM, not what happens to it afterwards..

If your target platform supports the MP3 format, be aware that it does not automatically loop seamlessly, so I do not recommend using MP3 compression for atmospheric or music loops. Because of the way MP3’s are encoded, silent padding is often added to the end of the file to make the overall number of samples perfectly divisible by “frames” of 1,152 samples. There are ways to create seamless loops with MP3’s - but that’s for another guide.

Disabling Preload Audio Data and enabling Load in Background will make a large file play late, but will not cause a CPU spike. This is because it takes time to be loaded but is also not stalling the main thread.

Disabling Preload Audio Data and disabling Load in Background will cause a large file to stall the main thread when the sound is called for the first time. However, this is not an issue if you are using FMOD, which runs decoding on a separate thread.

Always make sure that the files you are about to import into Unity are in an uncompressed format like WAVE (.wav) or AIFF (.aiff). Many compressed formats are lossy, which means that information is lost when they are encoded. If you import a compressed file such as an MP3 or Vorbis into Unity, Unity will first decode it into an uncompressed format, then re-encode it into whichever format you select – even if it's the same format that you started with. This can introduce more compression artifacts, which are generally undesirable.

There is a great write-up on various audio compression formats, their pros and cons, and which platforms support them in the documentation for AudioKinetic’s Wwise middleware.

If you're using FMOD, you have access to its FADPCM format, which is a vast quality improvement upon the old ADPCM format. However, this is not built into standalone Unity.

You may want to use MP3 instead of Vorbis on iPhone, for example, as it has a hardware MP3 decoder which saves your CPU from having to deal with decompressing MP3 files stored in RAM or on the disk. But beware that this may not be appropriate for looping sounds (see Warnings above), and that it can only decode one MP3 at a time. If multiple MP3's need decoding simultaneously, these will be done in software just like Vorbis. This shouldn't present any major issues, but it is worth noting that MP3 decoding is slightly more CPU-intensive than Vorbis decoding.

If your target platform is Playstation 4, the ATRAC9 format offers a fairly high compression ratio with less CPU overhead than Vorbis or MP3.

Likewise, for Xbox One, Microsoft’s XMA format will serve well instead of Vorbis or MP3. Microsoft recommend a compression ratio of between 8:1 and 15:1 for the best performance and quality.

In the Suggested Settings tables, I've listed some sounds with PCM as their compression type (actually uncompressed) and Compressed in Memory as their load type. Using Decompress on Load would also be fine, I'm just pointing out not to set these sounds to Streaming.

Some middleware solutions such as FMOD and Wwise have their own way of handling audio importing, making these Unity settings redundant (unless you also want to run some sounds without using the middleware, for some reason).

The sound types in the Suggested Settings tables include the category “Foley” which may be cause for argument, since it’s a film term with a very specific meaning, and by rights probably shouldn’t be used in games at all. However, I think it’s the most fitting term for the various sounds associated with the physical interactions between characters and objects in a game.

Although various target platforms have native Sample Rates, Unity plays around with mixing and matching sample rates all the time. What this means is that whenever a sound is played at a speed other than 100%, or if a sound is imported using Optimise Sample Rate, then the console/device has to do its own sample rate interpolation on the fly, to make everything come out at its native sample rate. This is usually a negligibly cheap operation, even on mobile, but could potentially pose a problem for some platforms, depending on how they handle it.