Open (with no insulation) or closed (passive noise insulation) or semi-open (intermediate version). Open headphones sound more airy and provide a wide stage, closed ones provide noise isolation and amplify low frequencies.
No higher than 32 Ohms for smartphone headphones. When using a good player or amplifier, it can be up to 100 Ohms. For stationary models you should coordinate the impedance with the amplifier power
Specified by -dB. The lower the value modulo (closer to zero), the easier it is for the microphone to record quiet sounds. It does not affect the quality
The number of drivers in each headphone. However, quantity does not equal quality. Driver coordination, their configuration, correct crossover, sound bores affects the sound
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Cable length, m
The length should allow you to lay the cable with a small margin. The longer the cable, the more impact it can have on the sound.