I measured the Bowers & Wilkins PXes using a G.R.A.S. Model 43AG ear/cheek simulator/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 KB0065 simulated pinna for most measurements as well as the new KB5000 pinna for certain measurements, as noted. Because I was unable to get the PXes to connect to my Sony HWS-BTA2W Bluetooth transmitter to send signals from the Clio 10 FW to the headphones, I had to use a wired connection; I couldn’t test with Bluetooth signals. However, this shouldn’t matter -- these headphones work only when powered on, so their internal amplifier (and, presumably, digital signal processing) are in the signal chain just as they would be when the headphones are fed a Bluetooth signal. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed.
The PXes’ frequency response looks pretty standard, with a mild boost in the bass and a strong peak centered at about 2.4kHz. This curve aligns with what is generally considered to be the proper curve for making headphones sound more like real speakers in a real room. Note that this is the best match I was able to get between the left and right channels in about a dozen attempts; note also that the narrow earpads made positioning the earpieces on the ear/cheek simulator far fussier -- the pads leave less margin for error in positioning. Not shown here is the result with a 75-ohm source impedance, which mimics the electrical effect of using the cheap headphone amp built into typical PC laptops and inexpensive MP3 players; because of these headphones’ high input impedance, using a higher-impedance source has almost no effect on frequency response.
This chart shows the right-channel frequency response of the PXes measured with noise canceling (NC) off, and with NC in the default settings of the three NC modes: Office, City, and Flight. (Adjusting the NC to the full Amplified setting did not affect frequency response.) The frequency response in Office mode is practically the same as with NC off, but clearly, the City and Flight modes will make these headphones sound substantially different.
This chart shows the PXes’ measured right-channel frequency response (NC off), measured with the old KB0065 pinna (which I’ve used for years) and with G.R.A.S.’s new KB5000 pinna, which I’ll 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 later this year, when I start using only the new pinna.) The difference here is far greater than I usually see, because it was so difficult to get the PXes consistently well seated on the ear/cheek simulator. This was the closest match I could get in more than a dozen tries.
This chart shows the PXes’ measured right-channel frequency response compared with three other NC headphone models: the Bose QC35 II, the PSB M4U 2 (generally considered to rank among the best-sounding NC headphones), and the Sennheiser HD 4.50 BTNC. You can see that the PXes’ response differs considerably from the others; it’s more of what’s often called a smiley-face curve, with pronounced bass and treble and a dip in the midrange.
The spectral-decay (waterfall) chart has an artifact I’ve never seen -- an apparent “echo” at about 6ms at frequencies below 1kHz. I don’t know what to make of it, but it showed up in repeated measurement attempts. The result shown is with NC off; activating the Flight NC mode somewhat reduced the artifact. Otherwise, this waterfall plot looks reasonably clean. The only resonance above 1kHz that lingers past 10ms is the one at 2kHz, which corresponds with the distortion peak seen below; still, it’s down about 40dB and so should be only negligibly audible.
The PXes’ total harmonic distortion (THD) is generally low except for the peak at 2kHz, which corresponds with the resonance shown in the waterfall plot. Note that switching NC on increases distortion, although it’s measurable only at what would be an extremely loud listening level.
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 to measuring at a level of 85dB instead of 75dB; this doesn’t change the way the isolation curves look, but an 85dB level lets me get better measurements of NC headphones, which demand a lower noise floor.) The PXes’ isolation, shown here in Flight mode at the default setting, is perhaps a little above average for an over-ear NC model, and somewhere between the class-leading Bose QC35 II headphones and the more average PSB M4U 2 headphones in the critical “airplane cabin noise band” of 50-1200Hz.
This chart shows the isolation of the PXes in their various NC modes. Office mode provides a mild amount of NC, comparable to that of typical cheap NC headphones. City and Flight modes offer far stronger and more useful NC, although City mode passes along much more sound between 800Hz and 5kHz, presumably to allow the wearer to hear voices and approaching vehicles. The chart also shows the effect of pushing the NC-level adjustment to the maximum Amplified setting in Flight mode, which appears simply to reduce the level of NC.
The PXes’ impedance magnitude is very high at an average of 910 ohms, which is to be expected, considering that the headphones can’t be used in passive mode with the power off. This means that the input impedance you see is that of the internal drive electronics rather than of the headphone driver. It also means that, in wired mode, the sound will not vary significantly depending on the source device, because the headphones’ impedance will dominate the circuit. Phase shift is negligible, as is to be expected from the nearly flat impedance.
The sensitivity of the PXes in wired mode with NC off, measured between 300Hz and 3kHz with a 1mW signal and calculated for the specified 22 ohms impedance in wired mode, is 105.3dB. You should have no problem getting adequate volume when you plug the Bowers & Wilkins PXes into an airplane seat’s headphone jack.
. . . Brent Butterworth
brentb@soundstagenetwork.com