Originally Posted by
rwanttaja
Since the helmet Tony posted is an aviation unit, it is probably compatible with aircraft radios. Many hand-helds, though, have low-impedance output jacks for the earphones. However, just about all the companies sell headset adaptors. That's what I used on the built-in ICOM handheld in my airplane.
The issue with compatibility is that aviation has maintained backward compatibility. When aircraft radios first came out, they used carbon microphones and plugs/jacks that were used in manual telephone switchboards (hence the weird size for the mike plug). As aircraft microphones improved, they had to include the appropriate interfaces so that they behaved like carbon microphones.
Civilian equipment is much different: Since something like a CB radio had a hard-wired mike, they could use a modern device in its most efficient guise. Hence, you can't count on being able to plug a modern mike into an aircraft radio. It might work, it might not.
I'm not sure, but I expect the headphone side was similar (except they used the standard 1/4 plugs that were ALSO used on the phone switchboards). The headset impedance was about 300 ohms, but over the years, consumer-grade equipment standardized on eight ohms. Fortunately, impedance matching is a simple process for earphones.
Ron Wanttaja