Passive Mode: In passive mode, the sound of ATH-ANC9 changes significantly. The center of the midrange remains fairly similar to what you would hear with the ANC9’s noise-cancellation circuitry engaged, but the upper midrange and lower treble ranges both become more subdued, as do some lower midrange/upper bass frequencies. The upshot of this is that in passive mode the ANC9’s sound takes on a “hooded” or “closed-in” quality. The effect is somewhat like hearing a person speak from within the pulled-up hood of a big winter parka: you can still recognize the person’s voice, but the sound is definitely a little “off.” For this reason, I regard the ATH-ANC9’s passive mode primarily as a fail-safe option—a mode to be used only if the battery fails and you don’t have a spare one handy.
Noise Reduction Capabilities: In general terms, the ATH-ANC9 offers the most effective and most flexible noise reduction capabilities I’ve head from any active noise-cancelling headphone to date. Though I would never have wished this upon myself, I had a chance to test the ATH-ANC9’s absolute noise reduction capabilities in the aftermath of an incident where my office flooded during a torrential Texas downpour. As part of the clean up effort, a big, industrial “axial air mover” had to be placed under my desk to help dry things out, and it made my whole office sound like the wrong end of a wind tunnel (the racket was unbelievable). But I put on the ATH-ANC9s, flipped on the most aggressive noise reduction mode (Mode 1 or “Airplane Mode”), and to my surprise immediately experienced a reasonable facsimile of peace and quiet. With the Audio Technica 'phones in play, the turbine-like roar of the air mover sounded more like quiet “whish” of an air conditioning fan. That’s how effective the ATH-ANC9’s noise reduction capabilities can be. Now, let’s look at the specific effects of each of the ANC9’s noise reduction modes.
Mode 1 (“Airplane Mode”): Mode 1 yields substantial noise reduction, yet preserves a sound that leaves most of the midrange, upper midrange and high frequencies untouched. Because Mode 1 tackles noise that reaches upward to higher frequencies than the other two modes do (up to 300Hz for Mode 1 vs. 200Hz for Modes 2 and 3), Mode 1 had more impact on bass, upper bass, and some lower midrange frequencies, with the result that the ANC9’s slightly bass-forward character became even more pronounced. I suspect this might even be a deliberate strategy on the designers’ parts, where the intent might to pull down noise (via active noise cancellation) while also allowing a subtle level of bass boost to help lower frequencies rise above residual background noise.
Mode 2 (“Office Mode”): Mode 2 is in many ways similar to Mode 1 and offers about the same amount of noise reduction, but with this difference: Mode 2’s noise reduction effects do not reach as high up into lower midrange as Mode 1’s do (up to 200 Hz for Modes 2 and 3 vs. 300 Hz for Mode 1). The sonic upshot of this is twofold: flower midrange frequencies sound just a smidgeon more balanced with Mode 2 in play, while perceived bass emphasis (as noted for Mode 1) is less pronounced. Mode 2 is meant for use in office environments and in that context it provides a sound that is a little more accurately and naturally balanced than Mode 1 (though Mode 1 would no doubt be superior in noisier environments).
Mode 3 (“Study Mode”): Mode 3 is very similar to Mode 2, but with less aggressive noise reduction setting applied—a change that yields surprisingly audible benefits if you like to listen in already quiet, study hall-like environments. Mode 3 gives less dramatic noise reduction effect than Mode 1 or 2, but in exchange its sound tends to be significantly more open, transparent, and naturally balanced. To hear the ATH-ANC9 at its best, then, get to a reasonably quiet room and use Mode 3.
Some readers might ask if the differences between these modes are things only finicky audiophiles would notice, and answer is that the differences are easy to discern for experienced listeners and complete neophytes alike. In fact, it’s fun to watch people try the ATH-ANC9 for the first time; you’ll see them switching between modes until their eyes suddenly light up when they find the just-right mode for the environment at hand. The general rule is that Mode 1 (“Airplane Mode”) works best for noisy environments, while Mode 2 (“Office Mode”) or Mode 3 (“Study Mode”) is better for quieter environments. Let your ears be your guides.