Even if it's not a haunted place, just being in a very dark warehouse or an empty hallway....

There's definitely no one around, but you feel a chill behind you, and for some reason, your chest feels tight as if someone is watching you... Have you ever experienced that?

The culprit behind this eerie phenomenon, which is often dismissed with, "There must be a ghost here," is likely not a ghost at all, but rather 'infrasound'!

But why are scientists only now revealing this interesting fact?

The answer is surprisingly simple. It's not that they didn't know, but measuring it was tedious and complicated.

The 'ghost frequency' that you can't hear, but your body reacts to

Infrasound is sound that is very low, below $20\text{Hz}$.

It's something that the human ear cannot hear at all. However, our organs and eyes respond to these vibrations.

British engineer Vic Tandy frequently witnessed a gray ghost in his lab.

Feeling creeped out, one day he discovered that an experimental blade was vibrating on its own.

The culprit was a recently installed fan.

The $18.9\text{Hz}$ infrasound emitted by the fan matched the vibration frequency of the human eye, creating an optical illusion (a ghost).

"In the past, we couldn't prove it because we didn't have the equipment!"

There were certainly strange and unsettling spaces in the past.

Places that felt scary, made you not want to stay long, and felt even stranger at night.

Back then, most people just brushed it off. "It must be my imagination." Or they went a step further and called a shaman, saying, "The energy here is bad."

Because it was invisible, it was hard to explain or prove.

However, these days, the situation has changed a bit. Technology has started to explain what our senses perceive.

With sensors that detect subtle vibrations, and brainwave monitors or heart rate sensors becoming increasingly precise, things we only thought were 'feelings' are now being recorded as actual data.

When exposed to infrasound environments, stress levels rise, heart rates fluctuate, and patterns of increased anxiety are repeatedly observed.

The important reason for this is simple. In the past, we were scared because we didn't know why; now, it's changed to "there is a cause, but we don't know it, which is even scarier."

Ironically, the reason this research has become more active recently is that we live in such environments.

We are not living in quieter conditions than before; rather, we are living in even more infrasound. The outdoor units of air conditioners on the rooftops of large buildings, ventilation systems in underground parking lots running all night, vibrations from subways running through the city, and the low-frequency sounds from wind turbines in the distance. These are noises that are not easily heard but our bodies continue to react to.

In fact, there have been cases where residents of specific apartment complexes collectively reported issues like "I can't sleep," "I feel tight in my chest," and "I feel anxious for no reason," which were later traced back to large ventilation fans in underground parking lots, rooftop cooling units, or subtle vibrations amplified by the building's structure. The problem is that these sounds are inaudible. So until the cause is found, the space itself becomes labeled as 'strange.' It's the perfect condition to be treated like a haunted place.

In the end, the strange chills, the inexplicable anxiety, and the unexplainable tightness we felt in the past.

It's likely that the true nature of these feelings was not the whispers of spirits, but rather physical signals that were continuously resonating in a realm we could not hear.

It's a bit bittersweet to think that the past belief in ghosts was unfounded, but on the other hand, it was the screams of machines saying, "Please turn me off!"