Sound is a very simple phenomenon to explain. Zeno of Citium said: «we hear because the air between the speaker and the hearer is struck in a spherical manner; and is then agitated in waves, resembling the circular eddies which one sees in a cistern when a stone is dropped into it». In other words, any vibrating object is the origin of sound, like the stone that falls into a pond and produces oscillation in the water. The water itself then becomes a transmitting medium: each point of the water transmits that vibration.
The same happens when we hit the table or talk, vibrating our glottis. Any object that vibrates transmits that vibration to the air next to it and so on. The air does not move but transmits the vibration. Let us imagine a line of people next to each other, and someone pushes the next person. None of those people will move, but they will transmit that push to the person next to them. The same thing happens with sound. We can transmit sound through the air, but also through a solid medium, like railway tracks, through which we can learn that a train is coming. Sound can also be transmitted through other media, such as water, and then some properties change. The speed of sound in air is about 340 m/s, whereas in water it is four times faster, about 1,500 m/s. This means that sounds are transmitted much better in water and that we can hear more distant and low-pitched sounds much more easily.
But we must bear in mind that sound is a human perception. If we move our hand, there is also a vibration, but it cannot be heard. We can only perceive frequencies between 20 and 20,000 hertz. This would be our window of hearing, while some animals can hear what we call ultrasounds, frequencies above the range we can perceive.
Answered by Chantal Ferrer, director of the Department of Applied Physics and Electromagnetism of the University of Valencia.
Question submitted by Núria Bofill Roca.
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