Thank you Gyro. It is good to know that i can replace the impedance load by a largely resistive load. And a series resistor is delightfully simple and easy to place into a connector housing.
I was able to talk a maintenance guy at the airport in letting me borrow a hedset tester which is just a fancy amplifier for the headsets own microphone as well as a 300 hz tone generator.
Using some cheap 18 ohm (measured dc resistance) smartphone earbuds i tested a simple series resistor and the 330 ohms i had on hand turned out to be a good value for the most quiet setting. Playing some music from my phone to the microphone of the aviation headset (using the 18 ohms earbuds to listen) i noticed no difference in the frequency response. This is looking promising
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magic: Thanks for making me aware of the multiple balanced armature topic. Checking some available options the single armature ones seem to be the cheapest which fits me just fine
. Right now i am leaning to a custom fit silicone IEM which would cost me about 250€. They are rated to provide 26dB of noise attenuation which is about as much as some fully closed silicone earplugs that i got. Sound quality of the armarture really should not be an issue as aircraft raidos always sound like you live in a tin can.
bson: Absolutely, now knoing that a series resistor will do just fine i will use a regular resistor to set a fixed value followed by a maybe 100 ohm pot to adjust the volume per ear. As the manufacturers of the IEMs never put the values of their volume controls into the datasheet id rather make it myself.
richard.cs: Looks like being too quiet will not be the problem so for now i will pass on using transformers. You raise a valid point with the significant attenuation:
Checking around on the internet it seems old school ham radio guys used to built something like a limiter for the audio signal in their headphones by using 2 diodes in reverse configuration to short the speaker if the voltage is to high. This seems easy enough to put in my adapter box too. Has anyone of you done something like that before? I dont quite understand how to adjust the point at which the diodes would cut the signal off (doubling the diodes for twice the forward voltage seems to be a much too coarse adjustment).
What kind of resitors should i use? Metal film, Carbon film? Or is that irrelevant?
Thank you all so far for your great help.
To get everybody on the same page i attatched a generalized schematic.