I measured the PSB M4U 8s using a G.R.A.S. Model 43AG ear/cheek simulator and RA0402 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 amp. On the Model 43AG I used the original G.R.A.S. KB0065 simulated pinna for most measurements, as well as the new KB5000 pinna for other measurements, as noted. For tests in Bluetooth mode, I used a Sony HWS-BTA2W Bluetooth transmitter to send signals from the Clio 10 FW to the headphones. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed.

Frequency response

The M4U 8s’ frequency response is pretty much in accordance with contemporary ideas of which headphone FR is most pleasing to most listeners: fairly close to the so-called “Harman curve.” The bass is shelved up below 120Hz, and there’s a somewhat larger-than-usual peak at 3.5kHz; normally, this peak might be centered at more like 2.5kHz, and it might be a few dB lower in magnitude. This is almost certainly why I found the treble a bit bright overall. Note that the left channel did not exhibit this effect; while it’s possible that the two channels measure differently (a common occurrence in active headphones, because the acoustics can be different due to the batteries being on only one side, etc.), this headphone’s response is tuned with digital signal processing (DSP), so I suspect that this may be a measurement anomaly. However, none of my attempts to reposition the earpiece on the ear/cheek simulator were able to match the peak in the right channel.

Frequency response modes

This chart shows the right-channel frequency response of the M4U 8s measured in some of their operating modes: wired passive, wired active, wired active with NC on, and wired Bluetooth with NC off. You can see that only the wired passive mode deviates greatly from the others, which is no surprise -- it bypasses the active circuitry and the DSP used to tune the other modes. Switching on NC seems to boost the bass by a couple dB, and the top and bottom look slightly rolled off in Bluetooth mode -- although Bluetooth’s latency can introduce measurement anomalies, so I’m not 100% confident in this measurement.

Frequency response

This chart shows the M4U 8s’ measured right-channel frequency response in wired mode with NC on, measured with the old KB0065 pinna (which I’ve used for years) and G.R.A.S.’s new KB5000 pinna, which I’ll eventually be switching to because it more accurately reflects the structure and pliability of the human ear. (I include this mostly for future reference rather than as something you should draw conclusions from; I intend to show both measurements in every review until I completely switch to the new pinna later this year.)

Frequency response

This chart shows the M4U 8s’ measured right-channel frequency response compared with those of three other NC headphones: the PSB M4U 2s, the Sony WH-1000X Mk.2s, and the Bose QC25s. The M4U 8s deviate from the norm in only one clear way -- they have less midrange response between about 150Hz and 1.8kHz. In my experience, this doesn’t mean the midrange will necessarily sound recessed, but it does make the treble peak more subjectively prominent, even if it’s not really higher relative to the 500Hz normalization point used for this graph.

Waterfall

This spectral decay (waterfall) chart shows the results in passive mode; the results in active mode are similar, but the latency introduced by the DSP prevented me from getting a chart comparable to those I typically publish. There’s a bit of resonance in the bass, but it’s along the lines of what I’ve measured from many other over-ear, closed-back headphones. There’s also a series extremely high-Q (i.e., narrow bandwidth), low-magnitude resonances in the treble that correspond with the M4U 8s’ measured response peaks; I see this effect in most of the open-back planar-magnetic headphones I measure, but rarely in closed-back dynamic designs. While the response peak is certainly audible, these resonances are narrow, and low enough in magnitude, that I doubt they’d be audible.

THD

Because I had to measure the total harmonic distortion (THD) of the M4U 8s in passive mode, again because of latency problems, this measurement shows only the distortion of the driver. It’s very low, rising to just 1% at 20Hz even at extremely loud listening levels.

Isolation

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 85dB; this doesn’t change the way the isolation curves look, but an 85dB level allows me to get better measurements of NC headphones, which demand a lower noise floor.) The isolation of the M4U 8s isn’t exceptional, but it does get down into the ballpark of some of the better NC headphones; Sony’s WH-1000X Mk.2s beat it by a hair, and the Bose QC35 IIs by a few dB more, though the Boses (which have the best NC performance I’ve measured from over-ear headphones) are clearly much better -- at the cost of the “eardrum suck” noted in the review.

Impedance

The M4U 8s’ impedance magnitude in wired passive mode averages about 38 ohms, with a nearly flat phase response. The impedance in the wired active mode was above the Clio 10 FW’s 1500-ohm limit for impedance measurements.

The sensitivity of the M4U 8s, measured between 300Hz and 3kHz with a 1mW signal calculated for the specified 32 ohms impedance, is 108.2dB in wired passive mode, or 110.8dB in wired active mode with NC on. The M4U 8s should deliver ample volume from any source device.

. . . Brent Butterworth
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