The Zenfone 9's fingerprint sensor is embedded in the power button, and it's among the best side-mounted units we've ever used. It's not quite on par with the one in the Poco F4 GT, which is our most recent champion, but very close (closer in fact than the Poco F4's). Recognition accuracy on the first try was ever so slightly above 95%, which is great performance in our book.
It's also very fast, so we have absolutely no complaints. As with most side-mounted sensors, you can choose to only have it engage upon a press of the power button, which is always our preferred setting. Having it respond to touches results in a lot of accidental unlocks in our experience. On devices running MIUI this is an either/or selection, but in ZenUI you get to pick between only pressing or both.
The skin provides a very interesting and useful feature too - swiping down over the fingerprint scanner to bring down the notification pane. This is less needed on a small device like this, since swiping down from the top of the screen requires less painful finger gymnastics than on bigger phones, but the feature is still very welcome, and once you get used to it, it's sorely missed on devices, which don't have it.
The swipe can also be set to refresh the current webpage or app, go to the top/end position of a webpage quickly, skip media forward or back, and play/pause. While all of these sound good, we thought the ability to quickly show notifications trumped all. Of course, you may have a different preference, and that's why it's good that there are more options.
As on any other Android phone, you can use face unlock too, and as on any other Android phone, it's less secure than the fingerprint sensor. If you're okay with that, the experience is pretty seamless, with fast unlocks - though the speed is pretty much on par, if not slightly slower than unlocking with a fingerprint, so you'll probably only want to use this if you're philosophically opposed to fingerprint scanning, or if you don't like pressing (or touching) the power button to unlock and want to only go the double-tap route.
The Zenfone 9 employs the usual dual-speaker setup with a dedicated grille at the bottom and the earpiece pulling double-duty as the secondary channel. These are among the best speakers on the market on anything that isn't a gaming phone, and it makes sense that Asus would engineer great speakers, considering it's the company making the ROG series smartphones, which probably have the best speakers in the entire gaming phone niche.
We'd rate the Zenfone 9's speakers as being on par with Samsung's best in terms of quality, and that's high praise, as we're talking cream of the crop here. This achievement deserves to be praised even more when you consider that because of its petite stature, the phone doesn't really have a lot of room on the inside to house big speakers - and yet, somehow, it manages to. Don't get us wrong, however: we're not saying these are great speakers for a phone this size. We're saying these are great speakers for a phone, period.
We want to point out that thanks to clever psychoacoustics, the bass reproduction is outstanding in the context of how small phone speakers are. You'll never compare it to anything produced by a decent Bluetooth speaker, but for bass on phones, it is very good.
Volume-wise, these speakers may not be the absolute loudest we've heard, but they're very close, and loud enough that you will never find them wanting in that respect unless you're in a very noisy environment. The quality also remains unchanged up to the maximum volume.
Even in moderately noisy settings, they perform admirably, but of course, past a certain level of ambient noise, you will have to bring the phone closer to your ears to hear what's going on. If you feel like you absolutely do need more, there's a dedicated Outdoor mode that sacrifices some quality to bring you even louder sound.
The Zenfone 9's vibration motor is very good, again (like the speakers), punching way above what you'd expect for the size. It's not the best one we've ever used, but it's clearly among the best-feeling ones out there at the moment. While not topping the depth-feeling chart, it does have plenty of depth, and you can even hear it a bit more than most these days - though not as much as the ones that used to be fitted to Samsung's flagships back in the day (not anymore).
It's unlikely you'll be disappointed by this vibration motor, and we especially like that ZenUI is using gentle nudge-like vibrations throughout the UI. This reminds us of MIUI, although the two skins do use vibrations differently, and sometimes for different things, but regardless - this is a great feature to have as it enhances the perceived physicality of your interactions with the slab of glass on the screen.
As always, we wish more skins would do this, and even take it a step further, varying vibration timing and intensity even more depending on the interaction model. Perhaps that will come in the future, but for now, we have to commend Asus for going this route and making ZenUI more of a joy to use than it otherwise would have been without these vibrations.