WARNING: The following article contains spoilers for Superman: Son of Kal-El #3 on sale now.
Superman and Jon Kent unveiled a new power that Kryptonians have: the ability to speak under a human level of hearing. With this power, Kryptonians under a yellow sun can have full conversations undetectable to the average human being. This is incredibly useful, albeit sneaky, power that allows Superman to have total privacy if he wants. Of course, such a power comes with the potential for misuse, not that Superman would do that. The only reason he unveiled this power was to not frighten a civilian they were trying to help. But at the same time, having the ability to hide whatever he says seems to fly in the face of the transparency he's been so adamant about upholding.
In Superman: Son of Kal-El #3 by Tom Taylor, John Timms, Gabe Eltaeb, Dave Sharpe, the father and son encountered another artificially created superhuman, a young woman named Faultline who could set off earthquakes just by taking a few steps. While both Clark and Jon carried her to a safe place, they had a conversation beneath the human hearing range so as not to scare her and potentially set off her powers. Faultline's density was so great that it took two of them to lift her. If she dropped from that height because she was panicking she could have leveled the city. So that justifies the use of such a sneaky power.
There is obviously the potential for misuse here. The ability to mask your conversations from people would be a major weapon to be used in espionage. And given recent problems with Leviathan and Checkmate, this could be a power that could come in handy to hide things from both organizations. How fortunate then, that the power only seems to work when speaking to another Kryptonian. It all but guarantees that only people Superman trusts could be spoken to in this way. Discounting people like General Zod of course.
This latest development in voice based powers isn't something new to comic books. Characters like the Marvel mutant Siryn have altered her voice powers so that she could hypnotize people. More recently, it was revealed that even speedsters had some type of vocal abilities. Reverse Flash could run at high speeds and then whisper suggestions into people's ears. Because he moved so fast no one would know it was him speaking and they would assume it was their own idea, regardless of how horrible it may be.
Fortunately, it's not a power that forces people to act against their own wills. It is one that shields their privacy however. This is a great irony considering the recent steps Superman has taken to be more transparent than ever, revealing his secret identity to the world so he isn't living a lie anymore. For him to have this power means he could lie anytime he wants or even hide information that he deems too dangerous for others to know. Both of them must understand this duality when using the power, but given that it's only used for good, perhaps that assuages any concerns that their attempt to maintain the truth is a sham.