Reviewed on: SoundStage! Solo, May 2021

I measured the DALI IO-4 headphones using laboratory-grade equipment: a GRAS Model 43AG ear/cheek simulator/RA0402 ear simulator with KB5000/KB5001 simulated pinnae, and an Audiomatica Clio 12 audio analyzer. For isolation measurements, I used a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. For most measurements, the headphones were fed signals from an MPOW BH259A Bluetooth transmitter. For some wired measurements, the headphones were amplified using a Musical Fidelity V-CAN amplifier; I used a Schiit Magnius amplifier for distortion measurements. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed. If you’d like to learn more about what our measurements mean, click here.

Frequency response

This chart shows the IO-4s’ frequency response. This is somewhat flatter than usual, which suggests that they may sound a little elevated in the low- and mid-midrange region.

Frequency response

This chart shows the response in wired mode with power on and off, compared with Bluetooth mode. Not surprisingly, the response is similar with the power on; the wired connection seems to have a little more bass output than the Bluetooth connection. (Note that the Bluetooth measurement is a gated response, because of Bluetooth’s latency, and thus may not be 100% comparable with the measurement in wired mode.) The wired mode boosts the treble, which will probably be heard as a reduction in bass response.

Frequency response

This chart shows the IO-4s’ right-channel response compared with the DALI IO-6 headphones (with NC on), the Marshall Monitor II A.N.C.s (NC on, Rock mode), and the AKG K371s (closed-back headphones that come very close to the Harman curve response). Again, the IO-4s’ response is flatter than usual. As with the IO-6 headphones, there’s some bass roll-off, but the relatively low level of the treble output should keep the sound balanced.


The IO-4s’ spectral-decay plot (measured in wired mode) looks pretty clean, with a mild resonance around 1.9kHz, but it’s well-damped and gone within about two cycles.


The total harmonic distortion of the IO-4 headphones (measured here in Bluetooth mode) is near zero at the very loud level of 90dB (level set with pink noise), but at the extremely loud level of 100dBA, it runs between about 2% and 6%.


In this chart, the external noise level is 85dB SPL, and numbers below that indicate the degree of attenuation of outside sounds. The lower the lines, the better the isolation. As you can see, the isolation of the IO-4s is comparable to that of other dynamic-driver, closed-back models, like the AKG K371s, and the benefit of the IO-6es’ noise canceling is negligible. I also threw in the open-back HiFiMan Deva headphones, as I mentioned them in the review.


In wired mode, the IO-4 headphones’ impedance curve is surprisingly flat for a dynamic-driver design; it barely deviates from the rated 25 ohms, and shows very little phase shift through the entire audioband.


In Bluetooth mode, latency measures 220ms with the MPOW BH259A transmitter. This is a typical result with the SBC codec.

Sensitivity, measured between 300Hz and 3kHz, using a 1mW signal calculated for 25 ohms rated impedance, is 103.2dB. So these headphones should play pretty loud when plugged into an inflight entertainment system.

Bottom line: The IO-4 headphones measure just like comparable HiFiMan models. Other than the low sensitivity, there are no concerns.

. . . Brent Butterworth