Last year, SoundCloud upgraded its AAC encoder for the first time in over a decade. The new one (Fraunhofer’s libfdk_aac) delivers higher perceptual audio quality across every metric we track. But if you look at a spectrogram, you may notice something unexpected. Here’s the scenario: you upload a lossless master. You download your track from SoundCloud and open it in a spectrum analyser. There’s a hard shelf at 17 kHz where there used to be energy all the way to 20 kHz. The old encoder kept…