I measured the Reference X20i’s using a G.R.A.S. Model RA0045 ear simulator, a Clio 10 FW audio analyzer, a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface, and a Musical Fidelity V-Can headphone amplifier. I used one of the medium-sized eartips (not the Super-Slim tips) because it’s what best fit the simulator. (The simulator has a round canal, not an oval canal like a real human ear.) This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.

Frequency response

This chart shows the frequency response of the X20i’s -- a very typical, by-the-book response curve for earphones, suggesting that the X20i’s will have a fairly neutral sound.

Frequency response

Adding 70 ohms of output impedance to the V-Can’s 5 ohms, to simulate the effects of using a typical low-quality headphone amp, has a huge effect on the Klipsches’ sound. The greater the output impedance of your source device, the more the X20i’s will tilt toward a trebly sound. This characteristic is typical of headphones using balanced armatures, but this is one of the more extreme examples I’ve seen.

Frequency response

This chart confirms what I stated above: the X20i’s have a very standard response. Here they’re compared with the PSB M4U 4s, the NuForce Primo 8s, and the Shure SE846s: all high-end earphones having balanced armatures or a combination of balanced armatures and dynamic drivers. It’s important to note that all of these headphones have a fairly neutral sound, which is reflected in the relatively even amounts of bass and treble shown in the chart.


Resonance in the X20i’s is generally mild; you can see the usual bass resonances, and also, at 10kHz, an unusual resonance of very low amplitude (approximately -40dB) and very narrow bandwidth that is nonetheless poorly damped. It’s hard to imagine, given the frequency, amplitude, and bandwidth of the resonance, that anyone could hear this other than an expert listener using test tones.


The total harmonic distortion (THD) of the X20i’s is pretty low, at 90dBA (measured with pink noise) -- a very loud listening level. At 100dBA -- an extremely loud level I include not because it applies to real-life listening but because it’s a hurdle some headphones can clear and some can’t -- the X20i’s exhibit significant distortion: 5-10% between 2 and 4kHz. You can probably hear that, but if you listen for long at 100dBA you won’t be hearing much for long.


In this chart, the external noise level is 75dB SPL (red line); the numbers below that indicate the attenuation of outside sounds. The X20i’s’ isolation (orange trace) is typical, at least when measured by the simulator and compared with the RBH EP3s (green) and PSB M4U 4s (purple). If the Super-Slim eartips are as good as Klipsch says, they should improve the isolation in an actual human ear canal.


The X20i’s exhibit a huge impedance swing, going from a low of 24 ohms at 20Hz to a high of 300 ohms at 8.7kHz. This, along with the accompanying large shift in impedance phase, is why the sound changes so much with higher-impedance sources.

The sensitivity of the X20i’s, measured between 300Hz and 3kHz with a 1mW signal calculated for the rated 50 ohms impedance, is 111.9dB, which is extremely high -- you can get very loud levels from the X20i’s with any source device.

. . . Brent Butterworth