Reviewed on: SoundStage! Solo, December 2021

I measured the Apos Audio Caspian headphones using laboratory-grade equipment: a GRAS Model 43AG ear/cheek simulator/RA0402 ear simulator with KB5000/KB5001 simulated pinnae, and an Audiomatica Clio 12 QC audio analyzer. For isolation measurements, I used a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. For most measurements, the headphones were amplified using a Musical Fidelity V-CAN amplifier; I used a Schiit Magnius amplifier for distortion measurements. 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

This chart shows the Caspians’ frequency response. This isn’t far out of the ordinary; the only unusual thing I see is either the elevated upper bass/lower mids between 200 and 700Hz, or the dip in the mids at 1kHz—depending on how you look at it. There’s about a 2.5-octave-wide bump in the bass centered at about 50Hz; I’d expected to see this bump from my listening, but more around 100Hz, so I’m guessing I was thrown off by the elevated upper bass.

Frequency response

Here we can see the difference in the headphones’ response when a high-impedance (75 ohms) source is substituted for a typical low-impedance source (5 ohms). The high-impedance source bumps the bass up about 1dB, so with high-impedance sources (most tube amps, some cheap or older laptops, and some professional headphone amps), the Caspians will sound very slightly warmer.

Frequency response

This chart shows the Caspians’ right-channel response compared with a couple of other open-back models and the AKG K371 headphones, which are said to be very close to the Harman curve. Despite the designer’s disdain for the Harman curve, the Caspians aren’t far off it. The big difference is (again, depending on how you look at it) the elevated upper bass/lower mids or the dip at 1kHz. There’s also a couple dB less treble energy above 4kHz, relative to the 3kHz peak, in the Caspians versus the K371s.


The Caspians’ right-channel spectral-decay plot looks pretty good—there’s a bit of the super-high-Q “hash” we usually see with open-back models up in the 2kHz range, and perhaps a little more resonance around 200Hz than we normally see (and which corresponds with the elevated upper bass), but nothing really out of the ordinary.


Here’s the THD vs. frequency chart, measured at 90dBA and 100dBA (both levels set with pink noise). This looks clean—yes, at the extremely loud level of 100dBA, distortion hits about 6.5% in the bass, but at such low frequencies, it’d need to be about 10% to be audible.


In this chart, the external noise level is 85dB SPL (the red trace), and numbers below that indicate the degree of attenuation of outside sounds. The lower the lines, the better the isolation. You can see that the Caspians are somewhat less open to outside sounds than most other open-back models. I included the AKG K371 headphones so you can see how a closed-back design compares.


The impedance magnitude, rated at 33 ohms nominal, runs about 33 ohms through most of the audio range, except for a resonant peak at about 65Hz, all of which is common for a dynamic driver. Electrical phase shift is pretty mild.

Sensitivity of the Caspians, calculated for 33 ohms impedance and averaged from 300Hz to 3kHz, is 103.6dB—much lower than the rated 116 (Apos Audio doesn’t specify the frequency on that), but plenty enough to get satisfying volume from pretty much any source device.

Bottom line: Other than that excess energy in the upper bass and lower mids—which is easy to EQ out—no red flags here.

. . . Brent Butterworth