I measured the Monolith M300s using a G.R.A.S. Model 43AG ear/cheek 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 amp. With the G.R.A.S. I used the original KB0065 simulated pinna for most measurements, as well as the new KB5000 pinna for certain measurements, as noted. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed.

Frequency response

The M300s’ frequency response is quite unusual, even for such an unusual set of earphones. Basically, if you moved the whole curve 1kHz higher, it would look reasonably normal. Most earphones and headphones have a big response peak between 2 and 3kHz, but the M300s’ peak is centered at 1.2kHz. The deep dip at 1kHz represents a cancellation, I would guess due to the internal acoustics of the eartube, but considering that it’s a dip, and a very narrow one, I doubt it would be readily audible.

Frequency response

This chart shows the M300s’ right-channel frequency response measured with the old KB0065 pinna (which I’ve used for years) and G.R.A.S.’s new KB5000 pinna, which more accurately reflects the structure and pliability of the human ear. I plan to show both measurements in every review for at least the next year before I begin to use only the new pinna, and include this measurement here mostly for future reference rather than as something you should draw conclusions from. Because of the KB5000’s more realistic construction, I’d consider the measurements taken with it more representative of the Monolith M300s’ performance, but unfortunately, at the moment I have only the right-ear model of the new pinna.

Frequency response

This chart shows the results of adding 70 ohms output impedance to the V-CAN’s 5-ohm output impedance to simulate the effects of using a typical low-quality headphone amp. The difference is zero; those little squiggles in the bass are due to noise caused by the much lower recorded level of the signal with the higher-impedance output. (I scaled up the 75-ohm result by 10.3dB in the chart for comparison purposes.)

Frequency response

This chart shows the M300s’ right-channel frequency response compared with that of the very similar Audeze iSine10s, the well-regarded PSB M4U 4 hybrid dynamic/balanced-armature earphones, and the Focal Sphears (rather “normal” single-driver dynamic earphones). The PSBs and Focals have the broad bass hump and strong upper-mid/lower-treble peaks typical of most earphones; the Audezes’ response looks much like that of a typical over-ear planar-magnetic model; but the M300s occupy a world of their own.


The spectral-decay (waterfall) chart looks unusual: While the resonances are well damped and die out quickly, they’re higher in magnitude than the norm, with an especially strong resonance centered at 1kHz.


The M300s’ total harmonic distortion (THD) is effectively nonexistent, even at loud listening levels, with just a narrow blip of distortion centered at 1kHz -- the same frequency as the dip in the frequency response, and the strongest resonance in the spectral-decay measurement. In a rather extraordinary display, the measured distortion at 100dBA is no higher than at 90dBA -- there’s actually an orange trace representing the distortion at 100dBA, but it’s completely obscured by the green trace at 90dBA.


In this chart, the external noise level is 85dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. (Note that I recently switched from measuring at a level of 75dB to a level of 85dB. This doesn’t change the way the isolation curves look, but 85dB lets me get better measurements of noise-canceling headphones, which demand a lower noise floor.) Like the Audeze iSine10s, the M300s offer very little isolation from outside sounds; voices and other sounds will leak right in. However, the M300s do offer a little more isolation than typical over-ear, open-back, planar-magnetic headphones, such as the Monoprice M1060s included in this chart.


The M300s’ impedance magnitude and phase are about as flat as they could be, respectively at 26 ohms and a maximum phase shift of about +6° at 20kHz.

The sensitivity of the Monolith M300s, measured between 300Hz and 3kHz with a 1mW signal at the M300s’ specified impedance of 22 ohms, is 109.5dB. That’s high -- any conceivable source device should be able to drive the M300s to very loud levels.

. . . Brent Butterworth