Sound reproduction and headphones is all about Impedance.
Impedance is a combination of Ohmic Resistance and Inductive reactance to alternating current.
Mosy metal detectors are designed to operate into a headphone impedance of between 8 and 32 ohms,
(Usually marked on the HPs) which is why those cheapies from the garage sales and trash and treasure markets work so well, they are usually old stereo types that are almost always 8 ohms imp.
If you plug in a set of high impedance HPs (64 ohms and up)into a detector 8 ohm audio output the responce will be flat, possibly distorted and volume will be down.
If you connect low impedance HPs into a high impedance audio output you could possibly damage the audio output amplifyer stages.
The frequency response is also important.
metal detectors have a narrow low frequency response curve (they only generate sound in a range of between say 40hz to around 2000hz, therefore fancy wide frequency response High fidelity HPs are not required. If you connect a set of wide frequency response HPs to a detector the sound may be somewhat sharp and screechy because they are reproducing a lot of the high frequency harmonics that the cheapies will not respond to.
Most Underwater detectors use Piezioelectric HPs and are therfore High Impedance and so low impedance HPs will be no good.........Although for some reason my Infinium works great with 8 ohm headphones
