I measured the PSB M4U 4s 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 supplied Comply foam eartips because that’s what designer Paul Barton used when voicing this model. For comparison, I also include a measurement taken with one of the supplied silicone eartips. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.
Here is the frequency response of the M4U 4s using a supplied Comply foam eartip. It’s fairly similar to the responses we measure with most high-quality earphones, the only aspects of note being a little more energy than usual around 1.5kHz, and a little less than usual from 8 to 10kHz.
This chart shows the M4U 4s’ frequency response with the Comply foam eartip (red trace), and with the smallest of the supplied silicone tips (purple trace). The silicone tip causes a boost of 2-3dB in upper-midrange/lower-treble energy, a deep dip at 7kHz, and a 3-10dB increase in output between 8 and 12kHz. PSB’s silicone eartips will likely sound significantly brighter.
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 only a very slight effect on the M4U 4s, boosting the bass by 0.8dB at 20Hz (this will be inaudible), and reducing the output between 2 and 3kHz by 0.5-1dB (might be barely audible). Most balanced-armatures headphone show a much bigger and more significant frequency-response swing in this test.
You can see from this chart that the M4U 4s (blue trace) have more bass and treble output than do NAD’s Viso HP20 earphones (green trace), also designed by Paul Barton, or Sony’s XBA-H1s (red trace), a well-regarded hybrid design.
Other than the bass resonances seen in almost every spectral-decay measurement of headphones, the M4U 4s are essentially resonance-free.
The M4U 4s’ total harmonic distortion (THD) is very low, with just a couple of slight, narrow peaks at 1 and 4kHz, where the THD rises to 2.5-5% -- and that’s only at the extremely loud level of 100dBA, measured with pink noise.
In this chart, the sound-pressure level (SPL) of external noise is 75dB; the numbers below that indicate the degree of attenuation of external sounds. With either the Comply (green trace) or the silicone (purple trace) eartips, the M4U 4s’ isolation is outstanding for passive earphones. In fact, I had to expand this chart’s Y axis to accommodate the traces; normally, the lowest number along the Y axis is 30dB.
Here’s why, unlike with almost all other headphones using balanced-armature drivers, the M4U 4s’ response doesn’t really change when the user switches to a high-impedance source device. The impedances of most balanced-armature ’phones increase radically at high frequencies, but the M4U 4s’ impedance remains between 18 and 22 ohms throughout the audioband.
The M4U 4s’ sensitivity, measured from 300Hz to 3kHz with a 1mW signal calculated for the claimed 16-ohm impedance, is 99.7dB. Technically, that’s a little lower than average for earphones, but it’s plenty high enough to get a satisfying volume level from any portable device.
. . . Brent Butterworth