Can a single sonic scream wipe out an entire city? | sciences

Sound, in its simplest sense, is a mechanical vibration transmitted through a medium, whether this medium is air, water, or even a solid body.

When you hear the sound of feet moving in your neighbors’ apartment, even though they left the house years ago, this sound is a vibration that moves in the cement walls, until it reaches you.

The two cups and string experiment, which we always played with as children, is one of the simplest and most beautiful ways to understand how sound is transmitted. When someone speaks into the first cup, its base vibrates due to the sound waves produced by his voice, and these vibrations are transmitted to the taut string connected to the second cup.

As the string is stretched, it vibrates in the same pattern that started from the first cup, sending these vibrations to the second cup, which in turn vibrates and transmits them to the air inside it, so it turns again into a sound that can be heard clearly.

This simple experiment clearly shows that sound is nothing but vibrations, or waves, that move from one place to another, and everything we hear in our daily lives, from business conversations to music, works in the same way: a vibration that starts somewhere, travels through a physical medium, and then reaches our ear to be translated from a physical vibration into an auditory sensation.

In this context, sound is therefore (mechanical) energy that comes to our ears in the form of a pressure wave. When it reaches it, it presses on the eardrum, and these vibrations are transmitted through bones and nerves to the brain, which interprets them as sound.

Sound intensity

To measure the intensity of sound, scientists use a unit called the decibel, which is a smart way to express large numbers in a simplified way, because the difference between the weakest sound and the strongest sound that we can hear is very huge, and therefore scientists use logarithms to convert these large differences into an understandable measure.

In other words, every small increase in decibels means a huge increase in sound energy. For example:

• 30 dB: a quiet whisper.
• 60 decibels: normal conversation between two people.
• 85 decibels: The noise of heavy traffic and the average human screaming next to you.
• 95 dB: Electric drill next to you
• 129 decibels: the record for the loudest human scream (British Gil Drake)
• 130 dB: A loud siren or a very loud party.
• 150 dB: The sound of an airplane engine near you.
• 180 dB: A strong, striking pressure wave.

Every increase of about 10 decibels means that the sound has become approximately 10 times stronger in energy, so do not be fooled that the difference between 90 and 100 decibels is small, it is in fact a huge difference in the intensity of the sound and its effect on the ear.

Effect on the body

The louder the sound, the more the body is affected. For example, from 85 to 120 decibels, gradual damage begins in the hair cells inside the ear responsible for hearing. If the noise continues, chronic tinnitus or temporary hearing loss may appear after prolonged exposure.

In this case, the body may feel anxious or slightly dizzy due to the strong vibrations.

As for 120 to 140 decibels, there is immediate severe pain in the ear, with a possible rupture of the eardrum, an accelerated heartbeat, and an increase in blood pressure as a result of the “acoustic panic” response.

Then, when the sound rises to the limits of 150 to 180 decibels, permanent damage to the inner ear and irreversible hearing loss occurs, with bleeding in the auditory canal or inside the middle ear, and vibrations in the internal organs that may cause nausea, loss of balance, or even micro-rupture of tissue.

In this case, the person feels that the sound is no longer “heard,” but rather felt as pressure hitting the entire body.

This sound pressure is also sufficient to break weak glass windows and cause minor internal damage, and if it rises above 180 decibels, it causes major structural damage to light buildings, and the collapse of parts of the facades.

As for the sound closer to 190 decibels (but not exceeding it), it causes strong vibrations that make the air itself seem to push the body, which enters a state of “disequilibrium” due to the intense alternating pressure on the inner ear, and a person may faint from the intensity of the auditory pressure alone.

Beyond this level, the intensity of the sound leads to severe damage to most buildings, widespread collapse of weak buildings, and at the highest level almost complete destruction of the residential area in the location of the loud sound.

Krakatoa volcano

Now let’s talk about the explosion of the Krakatoa volcano, which occurred on August 27, 1883 in Indonesia. It was one of the most violent volcanic explosions in recorded history, as it destroyed most of the island of Krakatoa and generated massive tsunami waves that exceeded 30 meters in height and claimed the lives of more than 36,000 people.

The explosion was so powerful that its sound was heard more than 4,800 kilometers away, in places as far away as Australia, India and Madagascar. Scientists estimated its intensity at up to 194 decibels, making it the loudest sound in history. Does this mean that at those limits the sound could destroy a city as the volcano did?

Not exactly, at about 194 dB, the sound reaches the maximum possible in air.

When sound travels through the air, it creates compressions and rarefactions, that is, the air molecules through which the sound flows are compressed and brought closer together, and rarefied, that is, they move away from each other.

But when the sound becomes very strong, it reaches the point that the vacuum region becomes completely empty of particles, that is, a small void within the wave itself, then the air molecules cannot move away from each other any further, because there is nothing left there.

At this moment, the air stops “vibrating” as a sound wave, and begins to move as a whole as a single mass, and here the sound turns into a shock wave, that is, a real physical explosion, and not sound in its ordinary sense.

So, 194 decibels is the upper limit of sound in the air, beyond which the sound does not become “louder”, but rather explodes, so the energy of the Krakatoa volcano was definitely greater than the upper sound limit, creating a loud sound that reached the maximum sound limit, and also proceeded to be a destructive shock wave.

What about an entire city?

Now let us ask: Can sound erase a city, for example? The short scientific answer: Yes, but there is a necessary caveat, because that effect will not occur as a human or natural “sound” that can be generated in the atmosphere. The matter has gone beyond the stage of sound, and has become a very powerful pressure wave capable of destroying a city, or more.

Sound – as we know it – is widespread pressure waves, as we mentioned above, and even if it is loud, it faces restrictions in available energy, propagation and absorption that make the idea of ​​affecting a city impractical, but when we talk about an explosion, or a shock wave, this is the cause of destruction, as happened in the case of the aforementioned volcano.

To destroy a city requires enormous energy that produces a pressure wave whose energy is equivalent to the energy of a large explosion, which is much greater energy than any possible natural or human sound source. We did not talk about thermal or radiation effects that have an impact in this context.

In the case of the Hiroshima nuclear bomb (15,000 TNT equivalent), for example, massive destruction occurred in the city centre, on a scale of hundreds to thousands of square metres.

Most of the buildings within the first kilometer of the epicenter of the explosion were destroyed or severely damaged. As for the farthest areas, they witnessed burns, broken glass, and partial damage, but he was able to put them out of operation. Hiroshima was not completely wiped out, but its center was reduced to vast ruins.

Historical reports and testimonies of survivors, who were kilometers away from the center of the explosion, indicate that what they heard was “a single, short sound resembling a crackling sound or a sharp whistle, then complete silence.” The reason for this is that the explosion occurred at an altitude of about 600 meters above the city (an air explosion), so the sound wave and the thermal and light shock arrived in fractions of a second, almost together.

Some witnesses further away described the sound as resembling a “deep metallic explosion” or a “massive thunderclap” followed by darkness and dust.