Measurements and listening impressions of three comparable lightweight, folding, open & semi-open portable units: Koss PortaPro (left): mfr specs 60 Ohm, 101 dB @ 1mW; familiar to many here. See some discussion here, @Bill-P 's mods and some more discussion here, and @ultrabike 's measurements of a possible fake here. Currently $42 at Amazon US for this grey/blue version. Sennheiser PX100-II (middle): 32 Ohm, 114 dB (/V?); second version of a design that’s been around a while. Currently $70 at Amazon for the plain-wire version; also available with inline mic/remote. AKG K-403 (right): 32 Ohm, 118dB/V; discontinued but available from marketplace sellers via Amazon from $34. Semi-open design, bigger drivers (~40 mm) than the other two. Each of these fold up for transport. The PortaPros have a hook and complementary slot built into the headband to keep them secure, and the K403s have the two-way pivoting-pad design shared by many AKGs. The headband on the PX100-II is in one piece with two pleather pads, whereas the other models have a two-piece sliding arrangement with the K403 having a foam pad under each slider bracket. The Senn is the most comfortable but the AKG feels the most secure on the head thanks partly to the bigger pads/drivers. Measurements Measurement setup is as described here, except that SPL calibration is improved by referencing to an external signal and SPL meter. Main shortcomings FR-wise are too much rolloff in the bass, too high around 5-5.5k, too low c. 7-11k, and with full-size phones at least (doesn’t seem to be reflected in the two smaller of these three), a hole around 3.5k. Measurements were referenced to 80 dB @ 1kHz, because no matter what I tried with software and hardware I couldn’t avoid clipping at the PortaPros’ 5k peak at anything higher. This has the effect of making my usually atrocious distortion measurements look better so for comparison I’ve included them below; but it should be borne in mind that they’d look much worse at the usual 90 or 100, and might not scale the same way relative to each other with increasing SPL. PortaPro first, PX100-II second, K403 third, then FR and impedance for all 3 together: The long ridge around 3k on the PX100-II right-channel CSD looks suspicious. I didn’t hear extra shrillness in that channel and dropping the floor on the left-channel graph down another 5dB didn’t reveal an equivalent ridge, but it did show up on repeat measurements. There’s also a longer ridge on the right K403 CSD, but that sounded ugly on both channels in that area anyway. All three seem pretty clean in the mid and upper treble (dropping the graph floor for the Senns, which are low from the start in that area). PortaPro impedance is interesting. Listening impressions Listening was done straight out of a phone to a mix of 16/44 FLACs and high-bitrate MP3s (these are cheap portables, after all). In terms of tonal balance, the PortaPros sounded the most natural. The 5k peak is there, adding a bit of excitement, but it doesn’t seem intrusive (not as high as the graph suggests). Soundstage/imaging and dynamics were easily the best of the three. Bass was a bit ‘one-note’, especially when trying to eq it up a bit to try to match the Senns. The PX100-IIs had more bass, with lower bass in particular being rendered more precisely than the PPs, and a smooth lower midrange. Attack & decay in bass & lower mids were however slow. Treble to me was too recessed and dynamics in the midrange and treble were reduced, as was detail/resolution (sorry if I’m inappropriately conflating those, but I don’t really get the distinction). Soundstage and imaging were smeared by comparison. Bassheads and fans of a more ‘laid back’ sound might like these, but in direct comparison with the PortaPros and to my tastes, they were unexciting. The K403s sounded… er… like they look on the graph. Tonal balance was just off, the wrong-way tilt through the midrange and into the lower treble being very evident. Breathy singers became screechy ones and horns grated terribly. Detail/resolution seemed better than the Senns, but likely because of this overemphasis. Soundstage and imaging were not up to the PP standards. Bass was faster than the Senn but even with eq couldn’t match the Senn down low (maybe surprising given the bigger drivers). Some careful eq tidies up the midrange imbalance somewhat, and there’s detail in the treble; but ultimately the Senn bass goes deeper and the PortaPro has better technicalities across the range. Conclusion: PortaPros all the way. Maybe bassheads would appreciate the PX100-IIs outside among background noise, but if technicalities are important they don’t really compare.