What curious ears you have!

About the book:

We make sense of the world around us through our senses: hearing, sight, smell, taste, and touch. Thanks to our ears we can locate ourselves in space, communicate, determine where a sound comes from, keep our balance, and so much more! In order to hear, we use our ears, which are the only organs in our body that allow us to do that.

How do we hear what we hear? Can all animals hear? Do they hear the same things we do? Can they hear because they have ears or do they do it differently? Let's open our ears and listen carefully!

Excerpt:

Imagine we're at school during recess. We hear our friend Anna calling our name and our friend John asking us to join the game. We hear the Music teacher chatting with the History teacher, the ball bouncing on the floor, and our feet landing on the ground after a big jump. But the bell rings, bringing the recess to an end, and from stillness comes silence.

Why do we need to talk, touch, hit, or move something to produce sound? When we hit a table, even if we don't realize it, it vibrates. The vibrations of the table cause changes in air pressure. These air pressure changes travel in all directions. The sounds we hear are those vibrations reaching our ear. We know that sounds travel through air, but they also travel through water and other materials, like wooden doors or concrete walls.

 

Sounds for All Tastes.

Sounds can have different intensities. A sound can be soft, like the sound of pages flipping, or powerful, like the sound of a door slamming shut by the wind. The harder we touch, hit, or move an object, the more the air around it will vibrate, and the greater the intensity of the resulting sound. The intensity of a sound is often referred to as volume and is measured in decibels (dB).

But what makes a lion's roar sound so deep and a cat's meow so high-pitched? The difference is based on the number of vibrations per second when a sound is made. A roar (a deep sound) produces fewer vibrations per second than a meow (a high-pitched sound). The number of vibrations produced per second determines the frequency of the sound and is measured in Hertz (Hz). Deep sounds have a low frequency, and high-pitched sounds have a high frequency.

If you're ever near a piano, run your fingers across the keys from left to right to check how the sound becomes more and more high-pitched. In scientific terms, check how the frequency of the sound increases as your fingers move to the right of the keyboard.

 

Can We Hear All the Sounds?

Our ears are among the most sensitive in mammals because, compared to others, we can hear very low intensity sounds. The softest sound a person can hear is 0 dB; from that reference value, an increase of 1 dB represents a tenfold increase in sound intensity. For instance, the sound of rustling leaves is only about 15 dB, a whispered secret is around 30 dB, normal conversation reaches 60 dB, a scream is 80 dB, rock concerts dB levels are 90-100 dB. Sounds become unpleasant at around 120 dB and painful at around 140 dB, which is actually harmful for us. Be careful when you turn up the volume of the music!

And what about frequency? Humans can only hear sounds with frequencies above 20 Hz. Below that level, they are called infrasounds—sounds with such low frequencies that they are undetectable to our ears. The higher the frequency of a sound, the higher its pitch. When they surpass 20,000 Hz, they become extremely high-pitched and inaudible to the human ear. These are what we call ultrasonic sounds.

In other words, people can only hear a small portion of the existing sounds. There's a whole world of sounds we don't know anything about.

 

Higher and Higher...

Our Faithful Companion

Dogs have an incredible ability to communicate with people. They recognize human voices and other “human noises,” like the sound of a key turning in a lock or the clattering of plates on the table. They identify our moods through the tone of our voice and react differently depending on how we speak to them. Generally, they respond better to higher-pitched tones and, with some training, they can recognize between 200 and 1,000 words.

Dogs are capable of hearing ultrasonic frequencies of up to 45,000 Hz. That's why they can be trained with silent whistles. As you must have guessed, these can’t be heard by people as they emit sounds with frequencies above 23,000 Hz. They also hear very low intensity sounds, below 0 dB, like distant footsteps or voices, long before they can be heard by humans.

 

Super-Grasshoppers. 

Just like crickets, male grasshoppers attract females with a sound they make by rubbing their front wings together. A few years ago, three new species of grasshoppers were discovered, all of which “sing” at a frequency of around 150,000 Hz, way above our limit of 20,000 Hz. That’s the highest pitch ever known!

When it came to naming them, they were called Supersonus. And that makes sense! It is a supersonic song indeed.

Silent Croak

If you ever get the chance to venture into the dense jungles of Borneo, you'll probably be surprised by the constant noise of water, which can be deafening. You'll also be amazed if you bump into a Huia cavitympanum, a frog that lives on the island and opens its mouth to croak, but doesn't seem to make any sound.

Can you guess what's happening here? You're right! These frogs emit ultrasonic sounds, and their song is so high-pitched that we can't hear them. It's believed that this feature allows them to communicate with each other. Otherwise, their voice would be masked by the surrounding noise.

 

Deeper and Deeper...

Calls in the Forest

Siberian tigers are large hunters that are found in the vast forests of Eastern Russia. They're solitary animals, except during the mating season. How do they manage to keep competitors away from their territory? How do they find a mate in the lush landscape and often over great distances? While roaming their territory, they emit a wide range of grunts and roars—mostly at very low frequencies—the type of sound that travels best through air and is also best detected by their ears.

Females use these infrasounds to navigate their surroundings and, when in heat, to approach a male. If they're nurturing their cubs, they'll move away to protect them.

 

Meteorologist Birds

In April 2014, a flock of small birds called golden-winged warblers completed their migration from South America to the Appalachian Mountains and began their breeding season. They were expected to stay there for a few months, as they did every year. Surprisingly enough, just a few days after arriving, they flew southward and traveled over 400 miles away. Why did they flee from their nesting site after such a long journey? The mystery was solved a few days later when a devastating storm struck the area with 84 tornadoes. The little warblers are believed to have heard the infrasounds produced by the approaching storm, which allowed them to predict its danger and seek shelter. Once the storm had passed, they returned to the Appalachians and raised their chicks just like any other year.

Carrier Pigeons

Years ago, when there were no emails, smartphones, or even landline phones, many communications were carried out with the help of carrier pigeons. These pigeons are known for having extraordinary navigational abilities and always returning to their birthplace. It is said that the Greeks used them to announce the winners of the Olympic Games, and some even played a significant role in the World Wars. Some pigeons have even received medals for their work!

There are many theories on how carrier pigeons navigate. According to one theory based on their infrasonic hearing, pigeons—which can detect frequencies as low as 0.05 Hz—can hear the ever-present infrasounds in the atmosphere. Since these infrasounds vary from place to place, the pigeons create a “mental sound map” of the territory they're navigating, which allows them to return home.

 

Translated by Sebastián Gutiérrez - Edited by Laura Estefania