A Big Bottom and a Lively Top: The Philips Fidelio X1 Measurements
Raw frequency response plots show a headphone that's fairly immune from tonal changes in various placements. Compensated response plots show a very well behaved headphone. Frequency response falls off quickly below the primary driver resonance at 70Hz, but remains above the level of the 4kH region at 20Hz. Response between 70Hz and 6kHz has a significant warm tilt, but is the longest straight line on a headphone frequency response plot that I've ever seen. In listening these cans never seemed to be missing anything in that region. Quite extraordinary! The bump at 1kHz was not heard in listening. Response between 10 and 20kHz is a bit lower than I'd like to see.
30 Hz square wave response show the typical curved top due mainly to the hard knee in frequency response at 70Hz and attendant phase shift going through the primary driver resonance. This, coupled with the rise in distortion below 100Hz, is a clear indicator for the somewhat loose sounding low bass response.
300Hz square wave shows a fast rise and single overshoot followed by some ringing, which is also evident in the impulse response. If there's a downfall with these cans it's in the slightly grainy sound that these data might represent. None the less, the very clean initial edge of the X1 probably results in their good imaging qualities. I think DIYers might find this a great headphone to experiment with; if you could rid it of this ringing, I bet they'd be spectacular.
The impedance and phase plot show a nominally 37 Ohm headphone with primary driver resonance at 70Hz. Isolation plot shows an open headphone with little in the way of isolation from outside sounds. With 181mVrms needed to achieve 90dBspl at the ear the X1 will reach reasonable listening levels from portable devices, but they're not really designed for them.