It is certainly true that the Bose picks up interference, and it will do so just dangling on your head without being plugged into the aircraft, but it picks up only strong stuff that way: cellphone/satphone radiation, and radar.
Picking up wingtip strobes etc is a different problem and is probably caused by the various equipment being badly wired (wingtip strobe inverters are such a common problem that I can often hear them on others’ radio transmissions) and feeding into the intercom inputs, via radiation or conduction.
And once it gets into the intercom, you cannot get rid of it.
So the place to start is shielding the offending equipment.
The “problem” with the Bose headset is that is contains electronics. A PN junction will occure at active components. This will form an AM detector weather it is caused by a diode (which is normally used as AM detector) or a PN junction in another active component.
Once the signal is demodulated as audio (low frequency) their is NO way of filtering it out. The only way of filtering is to remove it when this signal is still on its RF carrier (high frequency)
Also, is there anyone else in the aircraft with headsets connected? Recently, someone I fly with upgraded to Bose X from another headset and suddenly all of the ‘odd’ electrical sounds that we had been hearing disappeared… Our assumption, without any detailed electronics knowledge was that their previous headsets (which were suspect to be on ‘their way out’) were picking something up and feeding it into the intercom. The only thing I really notice now, as Peter says is a distinctive noise when passing close through a radar wave. At EGHH you get this on every departure.
Yes, obviously 
but is there any evidence that interference sources are causing the problem specifically by passing heavy currents through the airframe, which then creates an issue on any signal which is single-ended (airframe-referenced)?
It would be much easier to re-wire the said signal with a twin cable with an overall shield, and not use the airframe for the return path.
The avionics I have been involved with (1980s stuff and later) all seem to have “hi” and “lo” terminals for everything.
How exactly would one use a transformer to solve this issue?
It can resolve ground loops.
How exactly would one use a transformer to solve this issue?
In the audio context, a transformer is normally used to convert between a balanced (two wire, differential) signal and an unbalanced (airframe referenced) signal.
An audio transformer is a relative big and heavy component. capacitor to short the signal are less expensive, smaller and weight less.
Would an audio line transformer be difficult to fit to my Bose X? I get quite a lot of interference no matter what aircraft I’m in from C150’s to PA28’s to SR20.
If you want to go any further you can buy some audio equipment and build a gate/threshold circuit you plug between the headset and the aircraft. The effect is if the audio signal drops below a certain level the line will be shut off completely eliminating all background noise.
This works only when the interference frequency is low. The RF signals suchs as a phone, transponder etc will get demodulated at the most logic devices. The RF signal must be stopped before it causes a problem. You actually hear the modulation, not the RF carrier
We had similar problems. Once the new Bose headsets went in I started to hear all kind of interference. Those are the lessons I learned.
What you hear depends a lot on the pilot. I hear all kind of interference while my copilot will not hear much at all.
What kind of headset does your copilot use?
Certain types of “interference” can be defeated with audio line transformers, these are on sale for car audio systems. Better to avoid the need, though, by wiring correctly. And if one decides to try the transformer way, some luck and some insight and a fair deal of trial and error will be required.
