Reviewed on: SoundStage! Solo, November 2020

I measured the Philips Fidelio X3 headphones using laboratory-grade equipment: a G.R.A.S. Model 43AG ear/cheek simulator/RA0402 ear simulator with KB5000/KB5001 simulated pinnae, and a Clio 10 FW audio analyzer. For isolation measurements, I used a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. The headphones were amplified using a Musical Fidelity V-CAN and Schiit Magnius. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed. If you’d like to learn more about what our measurements mean, click here.

Frequency response

The first chart shows the frequency response. The relatively flat response up to about 1.2kHz is common in open-back audiophile headphones. What’s unusual is that the peak centered at about 3.3kHz is perhaps 1kHz higher in frequency than normal, and the peak at 7kHz is about 8dB higher than I’d expect to see, which suggests a little extra brightness and maybe some lack of energy around 1.5kHz to 3kHz.

Frequency response

This chart shows how the X3s’ tonal balance changes when they’re used with a high-impedance (75 ohms) source, such as a cheap laptop or some cheap professional headphone amps, or some exotic tube amps. There’s about a 1dB boost in the bass with the high-impedance source, which would be barely noticeable.

Frequency response

This chart shows the Fidelio X3s’ right-channel response compared with several audiophile headphones, and with the AKG K371s, which closely track the Harman curve. You can easily see how much more energy the Fidelio X3s have between 5 and 10kHz.


Like many large-diaphragm headphones, the Fidelio X3s show many very high-Q, low-magnitude resonances between 1 and 2kHz, which likely represent reflections between the diaphragm and the plate and pinna of the ear/cheek simulator.


Harmonic distortion in the Fidelio X3s is high in the bass frequencies. At the loud level of 90dBA, it’s probably not high enough to notice, but considering that the generally accepted level of distortion audibility in subwoofers is about 10% THD, you might notice some bass distortion at 100dBA -- although that’s a crazily loud listening level.


In this chart, the external noise level is 85dB SPL, and numbers below that indicate the degree of attenuation of outside sounds. The lower the lines, the better the isolation. The isolation of the Fidelio X3s is comparable to that of other large, open-back audiophile headphones. I added the Dan Clark Æon 2 Closed headphones so you can see how an audiophile closed-back model performs on this test.


The Fidelio X3s’ impedance magnitude is typical for a dynamic-driver headphone, running about 33 to 34 ohms through most of the audio range, with the expected low-frequency impedance bump at about 90Hz. Impedance phase angle is mostly flat, swinging mildly more inductive in the treble.

Sensitivity of the Fidelio X3s, measured between 300Hz and 3kHz, using a 1mW signal calculated for 30 ohms rated impedance, is 96.2dB, which is a little low, so a smartphone or tablet with a weak internal headphone amp might not be able to get them really cranking.

. . . Brent Butterworth