Wednesday, December 13, 2017

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I measured the Quad Drivers using a G.R.A.S. Model RA0045 ear simulator (plus a Model 43AG ear/cheek simulator for isolation measurements), 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 headphone amplifier. I used one of the smaller silicone eartips because that’s what best fit the RA0045. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.

Frequency response

Although the Quad Drivers’ frequency response might look a little bumpy, this is actually a fairly by-the-book earphone response -- the kind of response that’s generally thought to deliver subjectively flat sound. The only thing that’s slightly anomalous is that the 3kHz peak, a typical characteristic in measured earphone response, extends to higher frequencies than usual, up to about 5kHz. To attempt to confirm the Quad Drivers’ Hi-Res Audio certification, I set the Clio FW 10 for a 96kHz sampling rate instead of the usual 48kHz, to see how the Quad Drivers’ ultrasonic response compared to those of a few other earphones I had on hand -- and the 1Mores do seem to deliver a substantial amount of energy at 40kHz.

Frequency response

Adding 70 ohms output impedance to the V-Can’s 5 ohms, to simulate the effects of using a typical low-quality headphone amp, shows how the Quad Drivers differ from most balanced-armature earphones, many of which show radically varying responses on this test because of their large impedance swings in the treble. There’s almost no difference here at all -- just a boost of about 1dB above 10kHz. This leads me to speculate that the Quad Drivers use their dynamic drivers for all of the bass and midrange and even the lower part of the treble, and restrict their balanced armatures to the mid and upper treble. But you can’t argue with success.

Frequency response

This chart shows the Quad Drivers’ frequency response compared with some other multidriver earphone models: the Audiofly AF1120, the PSB M4U 4, and 1More’s Triple Driver. While the Quad Drivers aren’t as flat in measured response as the AF1120s, their response is more typical of what’s considered subjectively pleasing in earphones. Note that the Triple Drivers have more bass and treble output relative to their midrange.

Waterfall

The Quad Drivers’ spectral-decay (waterfall) chart shows no noteworthy resonances, and the resonance in the bass is somewhat less than I’m used to seeing with high-quality earphones.

THD

The Quad Drivers’ total harmonic distortion (THD) is insignificant even at very loud listening levels. Even at 100dBA (measured with pink noise), distortion is only about 2.3% at 20Hz, which will be inaudible.

Isolation

In this chart, the external noise level is 75dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. For comparison, I’ve also included the isolation plots of: the Audiofly AF1120s, a model with over-ear cable routing, which tends to produce better isolation; the Sony XBA-H1s, which, like the Quad Drivers, use standard cable routing; and the noise-canceling Bose QC20s, which offer the best isolation of any earphones I’ve measured. The Quad Drivers deliver measured isolation that’s OK, but somewhat below average for earphones, though I didn’t notice a lack of isolation when wearing them. As always with earphones, your results may vary.

Impedance

Most earphones with balanced-armature drivers show a huge impedance swing in the treble. The Quad Drivers don’t -- they’re almost dead flat at 31 ohms up to about 8kHz, and drop to about 23 ohms at 20kHz. The electrical phase is also surprisingly flat. This is why the Quad Drivers’ frequency response remains consistent, even when they’re used with amps with a high output impedance.

The sensitivity of the Quad Drivers, measured between 300Hz and 3kHz with a 1mW signal calculated for the rated 32 ohms impedance, is 107.6dB. That’s above average -- any source device will easily drive these earphones to high levels.

. . . Brent Butterworth
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