I measured the HD 1 Frees using a G.R.A.S. Model 43AG ear/cheek simulator plus RA0402 ear simulator and KB5000 simulated pinna, a Clio 10 FW audio analyzer, and a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. I used a Sony HWS-BTA2W Bluetooth transmitter to send signals from the Clio 10 FW to the headphones. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed. Note that some of the measurements I usually perform are not included here; because the HD 1 Frees have only Bluetooth input, my analyzer was unable to measure their spectral decay; and, of course, impedance and sensitivity measurements are irrelevant for wireless-only earphones.
The frequency response of the HD 1 Frees is normal in having peaks around 3 and 6.5kHz, but unusual in that the 6.5kHz peak is so much higher in amplitude than the 3kHz peak. Normally, it’s the other way around. Although it’s tough even for experienced technicians to know precisely how headphones or earphones will sound from their measurements, I speculate from these that the HD 1 Frees will sound a bit recessed in the upper midrange and lower treble, and unusually strong in the mid-treble. The broad bump in the bass is fairly standard for dynamic earphones.
This chart shows the HD 1 Frees’ measured right-channel frequency response with Bluetooth (BT) and noise canceling (NC) on, measured with the stainless-steel coupler included with the RA0402 ear simulator, and with the addition of G.R.A.S.’s new KB5000 pinna, which I’ll eventually 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 I completely switch to the new pinna.
This chart shows how the HD 1 Frees differ from a few passive earphones I’ve reviewed. (I had no measurements of other Bluetooth earphones available.) Clearly, there’s a little extra energy around 50Hz and 6.5kHz, and a little less than usual around 3kHz.
Because of Bluetooth’s latency problem, I had to measure the total harmonic distortion (THD) of the HD 1 Frees using discrete tones in one-octave steps rather than swept tones; still, the results should be comparable to my usual distortion measurements. The distortion is very low, at less than 0.5% -- so low that I had to adjust my usual Y-axis scale down from its usual maximum value of 50%. Note that I tested a couple of other Bluetooth earphones at the same time, and got distortion numbers in the more usual range of 2-4% -- this measurement does appear to be genuinely excellent, not just a result of the different measurement technique.
In this chart, the level of external noise is 85dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. Clearly, the isolation offered by the HD 1 Frees seems less than average; if this is a concern for you, I recommend replacing the stock eartips with Comply foam tips.
The HD 1 Frees’ range of Bluetooth operation was outstanding: I measured reliable line-of-sight reception at 33’ indoors -- three times farther than some Bluetooth headphones and earphones can manage.
. . . Brent Butterworth