*** Be warned: There will be spoilers for Stranger Things Season 5 episode 3 ahead ***
Eleven is undoubtedly one of the strongest characters in the entire Stranger Things universe, as she has single-handedly saved Hawkins and all of its citizens on multiple occasions. So far, El's only weakness has been her own limitations when using her powers. The toll of incarcerating Henry Creel in the Upside Down proved to be so great that Eleven went into a coma and woke up only to lose her memories altogether. Furthermore, her nose bleeds frequently when using her powers—a trait that has become synonymous with her character. El's biggest weakness used to be her own mastery over her psychic abilities... until the release of the latest season of Stranger Things, that is.
Stranger Things Season 5 Episode 3 opened up with the revelation that the military has figured out Eleven's true weakness: a huge conical device called "hedgehog" that seemingly emits a sound capable of stunning Hawkins' champion and no one else. So, how did the military figure out the key to defeating Eleven and, more importantly, how does it really work? Well, it's time to put our detective hats on and try to figure it all out.
Eleven's mutated blood could allow her to hear higher frequencies, making high-pitched sounds her weakness
Let's start with a bit of a recap, shall we? We know Henry Creel was the very first person in the Stranger Things universe to possess psychic powers, and due to his impressive abilities, the MKUltra experiments were founded. Doctor Martin Brenner transfused Henry's blood to pregnant women in the hopes that their children would be born with powers, and the results were certainly positive, as people like Eleven prove.
Now, thanks to Stranger Things: The First Shadow, we know that Henry Creel has a unique blood type different from any other human being, which is the true source behind his powers. In turn, it means that Eleven also carries this "mutated" blood, which gives her special abilities like telekinesis and telepathy. With that in mind, it wouldn't be too far-fetched to assume that El's blood allows her to hear higher frequencies than any normal human being, rendering her weak to very high-pitched sounds.
An average human can't normally hear above a frequency of 20,000 Hertz. However, dogs can hear frequencies up to 65,000 Hz—an evolutionary ability that was developed to ensure they could be better hunters by hearing the high-pitched sounds of prey, such as mice. In a sense, Vecna and Eleven are also "hunters" who are telepathically able to locate any person they wish, see through their eyes, and, more importantly, hear what they are saying. Thus, they would need to have a trained ear as well.
In more scientific terms, perhaps Eleven's mutated blood allows her to have more sensory cells in the inner ear's cochlea. As a result, any strong high-pitched sound could stun her, while normal humans around her wouldn't feel any kind of pain. But how exactly did the military figure out the science behind this? Well, before we answer it, be warned: There will be major spoilers for Stranger Things Volume 1, Episode 4 ahead. If you don't want any surprises ruined, it's best to turn back for now.
Still here? Then chances are you know the military has held captive Eleven's "sister", Eight, for who knows how long now. Chances are that Eight has been subjected to no shortage of experiments at this point, likely revealing that all of the MKUltra experiment test subjects are susceptible to high-pitched sounds. With that information, the military could've proceeded to create the "hedgehog" sound devices that are now Eleven's new Kryptonite.
While Eleven has a new obstacle to overcome, chances are that she isn't the only one. Stranger Things Season 5 Episode 4 revealed that Will now has powers of his own (as well as the iconic nosebleed), so he could be weak to high-pitched sounds as well. On the bright side, this would also mean that Vecna himself shares this weakness, so the Stranger Things gang might look to exploit it.
Maybe combining a little dose of fire with a high-pitched sound could be enough not only to defeat Vecna but the Mind Flayer itself. However, we'll have to wait until the finale of Stranger Things Season 5 to see what the key to destroying the horrors that lie in the Upside Down is.
Stranger Things Season 5 Volume 2 will be released on Dec. 25 on Netflix.