I have this system property on Asus Zenfone 3 phone(max output only 0.3V)
[use.audio.eu.parameters]: [true]
A call to Asus confirmed that it is due to EU regulations.
Again, this is most likely due to Asus taking French law and applying it to the larger scope of the EU to avoid having to deal with subtle differences between EU countries. I highly doubt the people answering the phone at the Asus hotline are experts in EU regulations.
EU law is public on the Internet, so, if there is such a regulation, it should be easy to find it, but I was unable to. This leads me to suspect there is no such thing. (In contrast, it was quite easy to get to the French decree.)
While doing that research I dug a bit more into the
French decree. That decree references the 60065 standard. I have obtained a copy of that standard and the relevant section is "Z1 Protection against excessive sound pressure from personal music players", which states:
[In] a personal music player provided with an analogue electrical output socket for a listening device, the electrical output shall be ≤ 150 mV measured as described in EN 50332-2, while playing the fixed “programme simulation noise” described in EN 50332-1.
EN 50332-2 states:
The player input signal shall be as specified in EN 50332-1:2013, Clause 5 recorded on the relevant medium with the specified level
All hardware and software processing systems, gain and tone control settings shall be set such as to reach the maximum sound level output.
The analogue audio output shall be loaded with a resistive load of 32 Ω per channel.
The maximum output voltage V m shall be defined as unweighted r.m.s. voltage at the load
EN 50332-1 states (emphasis mine):
The test signal used to determine the maximum sound pressure level of headphones shall be programme simulation noise, as defined in HD 483.1 S2. [...] This signal is a weighted stationary Gaussian noise and can be obtained from pink noise with a suitable filter network.
For digital listening devices, the test signal shall be applied to the listening device with an r.m.s. amplitude of - 10 dBFS, where 0 dBFS is defined as being the maximum RMS amplitude of a sinusoidal signal corresponding to the full scale of the digital interface.
Okay, so that means my
other source was actually misleading, it's not 150 mV max output, it's 150 mVrms
at -10 dBFS. So the actual maximum output that French law allows, at 0 dBFS, is actually… 474 mVrms. Which is more reasonable, and matches the max output of the A2155 model quite well. So yeah, the 0.5V max output for models sold around the EU might be ultimately be linked to this regulation, as wiggum suspected.
That said I still suspect that Apple is being overparanoid here, because the French decree clearly states that it only applies to "devices mainly consisting of a listening apparatus that can be worn inside, over, or around the ears" which seems to exclude devices that don't come bundled with headphones, such as this Apple adapter.