What kind of effect is this and what is it caused by? In the screenshot below I have two audio signals. The upper one is the audio signal that is created  with python and then played, whilst the lower one is the &quot;same&quot; signal but recorded via microphone on an android device. Tried my best at aligning them in audacity but I am unsure where the &quot;fade-in&quot; comes from. The same thing happens on the other end of the signal, but in reverse of course.

Also, there is no other noise in the recording, as I am recording from the audio output relayed to the input and not directly from the speakers (so no background noise).
Don't know which terms to search for. Any help would be much appreciated.
Looks like pre-ringing. That happens whenever you apply a linear phase filter to a signal.  In your case that could be the sigma-delta converters or any type of sample rate conversion or other unknown processing that the operating system or the drivers doo.