These three .wav files replace the standard ones that come with the XR series of vessels (DG-XR1, XR2 and XR5). I find the original sounds quite loud and also make it seem like the APU is right under your seat when it could perhaps be located at the back of the spacecraft. This sound is more of a distant hum and is less annoying as a result.
Listen here:Â http://www.youtube.com/watch?v=gxahRNJmMNI
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Mixed using sounds of a real APU and a generator.
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Installation instructions in the Readme.Â
These sounds are free to re-use, distribute, modify etc.