One of the major issues that was raised from our survey is when a device gets overloaded when presented with excessive sound levels.  A common issue is recording the audio at a rock concert where the device is simply unable to cope with the sound pressure levels it is exposed to.  In order to understand how devices respond when placed in this situation an experiment was designed to attempt to capture the kind of non-linear behaviours that may occur.

The full report is accessible here:

http://usir.salford.ac.uk/29371/1/Distortion_of_Portable_Audio_Devices.pdf

The performance of a series of common devices was quantified including the; Cannon 550D, Edirol r44, Neumann U87ai via Focusrite 2i4, Shure SM57 via Focusrite 2i4, Zoom H2, Zoom H4, Google Nexus 4, Apple Iphone and a Sony camcorder (vx2000).

Most devices have some form of dynamic gain control to prevent signal clipping, but the implementations clearly differ considerably.  Some devices have many  settings for different situations indicting that there is no one particular method suitable for all cases.  The attack and release times of the measured systems range from 5 to 17 ms and 30 and 400 ms respectively.  Some devices may also demonstrate a nonlinear gain curve with no attack or release but which try to limit audible distortion by using a compression ratio of between 1.4 and 10.  While other systems have no protection and when presented with excessive sound levels will exhibit hard clipping.