What is sound?
From a physical point of view, sound is nothing but the vibration of air, which in your ears is converted into nerve signals that are interpreted by the brain.
The symphonies of Mozart, the noise of a drill or the cry of a bat - how you experience the sound is completely determined in your inner ear and brain.
How does sound arise?
Whether you clap your hands, sing a song, or play a song through a speaker, you set the molecules in the air in motion and produce sound waves. The density of the molecules (the air pressure) varies, and that determines how the sound is recorded.
How do we hear sound?
The principle of form follows function applies to your ears - they are 'designed' to absorb sound waves. The ears consist of three parts: the outer ear, the middle ear and the inner ear.
1. The outer ear picks up the sound.
The sound waves that you pick up reach the outer ear, which leads the waves through the ear canal to the eardrum in the middle ear.
2. Middle ear amplifies the sound.
The sound waves collide with the eardrum, which starts to vibrate. The movement activates three tiny bones, which amplify the sound and send it through a membrane to the inner ear.
3. Inner ear converts the sound.
The inner ear is filled with fluid, which is set in motion by the sound waves. As a result, the fluid pushes against thousands of microscopic, sensitive hair cells. These create electrical signals that are recognized by the brain as sounds thanks to the auditory cortex.
What sounds do we hear?
The ear can only pick up certain sounds. Which they are, depends on the number of sound waves per second or the frequency, which is expressed in Hertz (Hz). The frequency determines the pitch.
On average we can hear sounds with a frequency between 20 and 20,000 Hz, while many animals can hear sounds with higher frequencies (ultrasound). Sound with a frequency below 20 Hz is called infrasound.
How can you measure noise?
Noise is measured in different ways, depending on the characteristic that matters.
As stated, the pitch is measured by registering the number of waves per second. This is expressed in hertz.
The loudness is measured in decibels (dB). The decibel scale is logarithmic: every 10 dB increase means a power increase by a factor of 10. If the sound pressure is 10 times as high as at 0 dB, the sound intensity is therefore 10 dB. If the sound pressure is 100 times as high, the sound volume is 20 dB. At around 85 dB, the sound level is so high that you need hearing protection. The pain threshold is around 125 dB.
The velocity of the molecules in the air that transmit the vibrations is expressed in kilometers per hour, but depends on pressure, humidity and especially temperature.
At normal air pressure at the sea surface and a temperature of 15 degrees Celsius, noise moves at a speed of 1225 km / h. In the water, however, sound is about four times faster, and in solid matter, the sound propagates even faster.
The speed at which an object moves in relation to the sound is expressed in mach. The fastest aircraft in the world, the Lockheed SR-71 Blackbird, flew at speeds above mach 3 - three times as fast as the sound.
What is sound used for?
Since sound is an indication of vibrations - or waves - in the air and is therefore a physical quantity, it influences the environment in all kinds of ways. Especially sound whose frequency is outside the reach of our ears has various applications:
Animals hunt with sound
Some animals use very high sounds - so ultrasound - to navigate or hunt prey. This applies, for example, to the bat and to toothed whales such as the porpoise and the dolphin. These animals use echolocation: they transmit ultrasound and listen to its echo. The ultrasound tells the animal what the ultrasound has hit and at what distance.
Sonar imitates animals
Humans have found ways to use the inaudible high-frequency sound. For example with sonar technology, in which short pulses of ultrasound are transmitted under water, after which the reflection is collected and recorded.
For example, ships can determine the distance to the seabed or schools locate fish, submarines or shipwrecks.
Ultrasound destroys cancer
Ultrasound can also be used in the medical world, because different types of tissue reflect the sound in different ways. When making echoes, sound is used with frequencies of millions of hertz, making it possible to look into internal organs.
Most pregnant women receive an ultrasound to check whether the pregnancy is going well. With modern ultrasound techniques, extremely detailed 3D photos of the fetus can be made.
Ultrasound can also be used to destroy certain cancerous tumors.
Doctors and prospective parents give ultrasound pictures of the development of the baby in the womb.
Infrasound detects explosions
Infrasound, with a very low frequency, is also inaudible to humans. But if the sound is powerful enough, you can feel it. Some whales communicate with the help of infrasound, and the low sounds can also be used to map earthquakes and search for oil wells.