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Humans can hear sound frequencies ranging from 20 Hz to 20 kHz. Many animal species can hear frequencies outside this range, so either below 20 Hz or above 20 kHz. 

A well-known example is dogs, who can hear higher frequencies than humans can- hence why we have dog whistles. The same applies to house cats, which actually have the broadest hearing range among mammals. Rodents produce high-frequency squeaks that cats can hear, allowing them to locate and catch their prey. 

Elephants and whales can produce infrasounds, or sounds with frequencies below the range humans can detect, to communicate over long distances. Bats, dolphins, some insects (like katydids), and whales (again) can produce ultrasounds, or sounds with frequencies above the range of human hearing. 

The main point is that many animals can hear and produce a wider range of sound frequencies to communicate with one another. 

So, what am I trying to suggest about demons (and the Sparda twins)? 

Going back to the question above: what if demons can also produce sounds of frequencies outside of the range humans can hear? Hypothetically, demons could create a network of information amongst each other to share general information that everyone in the Demon World needs to know, like who fought who and won, who's the leader, etc. Or for demons that are a part of packs, they can formulate plans to attack others or humans while also avoiding detection. 

Also, imagine being a demon sneaking into the Human World and putting on a perfect disguise to avoid attention and make hunting humans easier. But you decide to send a message- like the location of the portal for others to find, or a warning to other demons you're around and to avoid you (because it'll mean death for them). 

Then, a few hours later, a man in red appears and foils all of your plans. 

The ability to hear across a broad spectrum of frequencies would allow Dante (and likely Vergil) to find other demons easily and make them even more effective demon hunters. 

In addition, the twins being able to produce sounds of higher or lower frequencies due to their demonic heritage could allow them to have secret "conversations" in front of many humans. Trish, being a demon herself, would most likely hear those, though. 

(Just imagine Dante and Vergil secretly "arguing" in front of Lady and Morrison while Trish is in the background trying not to laugh out loud. Or the twins being miles apart and "sharing" their locations with each other. )

There's also a comedic aspect to this possibility with Nero-

What if Nero has no clue that demons, including his dad and uncle, can hear frequencies that humans can't?

He also has no idea he can hear those sounds, too. 

As his demonic abilities develop further, Nero starts hearing these sounds that Kyrie, the kids, and Nico can't hear. It drives him insane because: 1) Where are these sounds coming from? 2) Why is he the only one who can hear them? 

Out of desperation, he tells the twins and is nonchalantly told that he's probably been hearing a group of a particular type of demon conversing amongst themselves... which does NOT calm him down at ALL. 

The conversation then leads to an impromptu family demon-hunting trip. Plus, Dante and Vergil have to teach Nero how to control his hearing and block out certain sounds. And (to his horror) also how to produce specific sounds. 

Later on, he asks Nico to make special headphones to block out sounds of specific frequencies (with the intention of avoiding hearing the twins' "messages" to him, lol) 

Other random thoughts I had were: Dante and Vergil (plus Trish) getting annoyed whenever someone blows a dog whistle near them (which Lady takes advantage of). Or when they are near construction or machinery that produces infrasonic or ultrasonic noise.