Oppo PM-1 - Manual (Page 6)

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Unique to the OPPO PM-1 is the use of a double-sided diaphragm,
which allows us to place twice as many conductors within the
magnetic field and eliminate any passive return zones where the
conductors do not work. This results in the use of 100% of our
conductor length, which in turn results in greater efficiency. In
addition, our flat conductor pattern eliminates inductance-related
intermodulation distortion, common with dynamic headphones,
and the OPPO PM-1's purely resistive impedance means that
sound quality is unaffected by a headphone amplifier's output
impedance.
The OPPO PM-1 combines high sensitivity with low weight,
allowing it to be used freely with portable devices without
requiring additional amplification. Its circumaural, open back
design provides the best in sound quality and comfort, and the
overall rugged construction ensures that the headphones can
withstand mechanical and environmental stress.
These aspects of the OPPO PM-1's design combine to provide a
transparent, highly dynamic sound that exhibits well-balanced
tonal qualities with very little distortion, resulting in a listening
experience that is both free of fatigue and capable of
communicating every nuance of your favourite music.
We are proud of the work we have done with these headphones,
and we hope they bring you years of enjoyment.