Reviewed on: SoundStage! Solo, July 2021

I measured the ISOtunes Free earphones using laboratory-grade equipment: a GRAS Model 43AG ear/cheek simulator with the RA0402 high-resolution ear simulator with KB5000/KB5001 simulated pinnae, and a Audiomatica Clio 12 QC audio analyzer. For isolation measurements, I used a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. An Mpow BH259A Bluetooth transmitter was used to send signals from the Clio 12 QC to the earphones. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed. Note that my usual impedance and sensitivity measurements are irrelevant for wireless earphones, and impossible to do without disassembling them, and are thus not included here. If you’d like to learn more about what our measurements mean, click here.

Frequency response

The above chart shows the Frees’ frequency response measured with the RA0402 ear simulator. They’re not too crazy—actually, they’re well within sight of “normal.” There’s definitely some bass response lacking below 100Hz, but the total sum of treble energy is right about where it needs to be, although with a little extra zip around 5.5kHz.

Frequency response

This chart shows the Frees’ right-channel response compared with other true wireless earphones: the KEF Mu3s (which are the true wireless earphones I’ve found come closest to the Harman curve), the Status Audio Between Pros, and the Grado GT220s. Other than the relative lack of low bass, the Frees’ response is remarkably similar to that of the GT220s.


The Frees’ spectral-decay (waterfall) response shows notable resonances only at 3 and 5.5kHz, which correspond exactly with the response peaks in the frequency response. As I sometimes see with true wireless earphones, there’s some very high-Q “hash” up in the mid-treble, much as I usually measure with planar-magnetic headphones, but I’m not sure if it’s a measurement artifact, a Bluetooth thing, or an actual acoustical effect, or whether its effects are audible.


The Frees’ distortion is negligible at 90dBA, which is a pretty loud listening level. The highest level I could get out of them is 98dBA (rather than my usual 100dBA measurement), but this seems to push them into clipping. I never heard distortion in my testing, for what it’s worth.


This chart shows the Frees’ isolation with the silicone and foam tips, compared with the Campfire Comet earphones (with foam tips) and the Bose QC Earbuds, which have active noise canceling. The Comets, if I remember correctly, have the best passive isolation of any earphones I’ve measured—until now. The Frees’ isolation in the “airplane cabin noise band” (about 100Hz to 1.2kHz) is better than that of most noise-canceling headphones and earphones. Very impressive.


Latency with the Frees connected to the Mpow BH259A Bluetooth transmitter typically measures about 317ms. That’s about 100ms more than I typically measure with Bluetooth headphones, which suggests these don’t use the latest and greatest Bluetooth chips. So depending on the latency of your display, there’s a good chance you’ll notice lip-sync problems when watching videos.

ISOtunes states that the maximum volume of the Frees is 85dB, although the measurement technique is not specified. Using the same measurement technique I use for Wirecutter’s measurements of kids’ headphones’s measurements of kids’ headphones, I got a maximum of 90.5dBA with -10dBFS pink noise.

Bottom line: The Frees lack some low bass, and—by design—don’t play as loud as most other true wireless earphones I’ve tested, but I see no red flags here. And the passive isolation is really impressive.

. . . Brent Butterworth