Headphones

In its data sheet for the M4U 8, PSB offers no specification for its sensitivity in passive mode but that's not indicative of obfuscation. On the contrary, the sensitivity we measured – 119.2dB for 1V input at 1kHz, averaged for the two capsules – puts the M4U 8 in the upper echelon of modern headphones, almost matching the 120.1dB we recorded from the M4U 2 [HFN Dec '13]. So if you have to use the M4U 8 in passive mode you won't want for sufficient level. But you won't choose to use it this way if you wish to experience the tonal balance PSB intends, which is delivered in active mode.

As expected, the uncorrected frequency responses for this mode [Graph 1, above] show a mild turn-up in output at low frequencies – but it is only mild, and less than we saw with the M4U 2. Plus there is greater output at around 3kHz than before, and less at higher treble frequencies. All this suggests a more even perceived tonal balance – a hope confirmed by the corrected responses [Graph 2]. Although there is still some lower-midrange/upper-bass shelving up in the diffuse-field corrected response [green trace, Graph 2] it is less than with the M4U 2, and there is also less of a 'hole' in presence band than with its predecessor. Note that the Harman-corrected response [red trace, Graph 2] actually suggests a lack of bass output.

So while the M4U 8's tonal balance is mildly tilted towards warmth, it evinces nothing like the overly bass-heavy/treble-light character of many modern headphones. Not in active mode, at least. In passive mode there is a slow ramp up in the diffuse-field corrected response below 1kHz reaching 11dB just above 100Hz [not shown here], and in ANC mode there is much more output below 120Hz. In our earlier review I criticised the M4U 2 for being phase inverting in active mode. That's no longer the case in the M4U 8 – but the fix has been achieved by making it phase inverting in passive mode instead! KH