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• We can hear sounds because our ears turn sound vibrations from the air, into signals that are sent to our brain.

• We can’t hear all levels of sounds. Sound waves with very high frequencies are called ultrasound and our ears can’t detect them.

• Ultrasound waves have multiple uses including: detecting medical conditions, treating kidney stones and cleaning delicate objects.

We detect sounds because inside our ears we have parts that work together to turn sound waves into a signal that is sent to our brain.

The components of the ear that make this possible are:

• The eardrum: A thin flap of skin that is stretched tight like a drum.

• The ear bones: Three small bones called the hammer, anvil and stirrup.

• The cochlea: A spiral shaped part of the ear that looks a bit like a snail shell.

• The auditory nerve: The nerve that carries signals from the cochlea to the brain.

• The Pinna: The visible portion of the outer ear.

When a sound reaches us, the air particles inside our ear canal vibrate and hit the eardrum.

The eardrum then starts vibrating and these vibrations are passed to three small ear bones – called the hammer, anvil and stirrup.

The stirrup bone hits the cochlea, which turns the vibrations into an electrical signal that is sent to our brain via the auditory nerve.

When the signal reaches our brain, our brain translates the signal into the sound we hear.

The highest pitch sound humans can hear has a frequency of 20, 000 Hz(20 kHz). If the vibrations of a sound wave have a frequency that is higher than 20 kHz our ears can’t detect them and it is called ultrasound.

• If the vibrations of the air particles are slower, we hear a lower pitch sound.
• If the vibrations of the air particles are faster we hear a higher pitch sound.

Many animals have ears that are able to detect ultrasound. Dogs are able to hear sounds with frequencies up to 60 kHz, while some species of bats can detect frequencies of 200 kHz.

A dog training whistle makes sound waves that have a frequency of 40,000 Hz. Dogs ears can detect the sound, humans can’t because the frequency is above 20 kHz.

Even though we can't hear ultrasound, it has many other uses for humans.

Ultrasound waves can move physical objects by resonating with their natural frequency and are used by doctors for breaking up kidney stones .

Watch the video below to see how sound travels through, and affects, different materials.

Ultrasound can be used to clean delicate objects like jewellery or a pair of glasses. The machines that make use of this ability are called ultrasonic cleaners.

When the ultrasonic cleaner is switched on, ultrasound waves travel through the water, disturbing it and creating tiny bubbles that vigorously shake the dirt from the object.

Ultrasound scans are used to view the inside of our bodies. A small device called an ultrasound probe is used, which gives off high-frequency sound waves.

We can't hear these sound waves, but when they reflect off different parts of the body, they create echoes that are picked up by the probe and turned into an image called an ultrasound scan.

Low power ultrasound is used so that doctors can see images of inside the body without causing damage. Ultrasound waves are used to take pictures of unborn babies, to monitor vital organs such as the heart, or to assist in diagnosis of gall or kidney stones.

Ultrasound waves are used to check if railway tracks and oil pipes have hidden faults, like a crack inside that is not visible to the naked eye. The waves are aimed at the surface and their reflections are received by a probe. If there is a crack or fault, the ultrasound wave reflects differently from how it does when there isn't a fault, and the probe produces a different noise to indicate that a fault has been found.