The frequency of sound has characteristics that are also characteristic of a number of other phenomena propagating through the wave. This is true, for example, for light or
x-rays. The frequency of sound is a certain physical quantity, which is characterized by a constant number of repetitions. It is determined by the ratio of the number of waves to the period of time during which they arise. For example, the frequency of a sound determines its pitch, which we hear. Or we don’t hear if the vibrations are outside the range of our auditory capabilities - infra- or ultrasound. If we are talking about light radiation, then depending on its frequency and wavelength, we see different colors of the spectrum: from red to blue.
Sound Frequency and Doppler Effect
An interesting phenomenon associated with this quantity is called the Doppler effect (by the name of the discoverer). It can be observed with the example of light waves, but the speed of light propagation is very high (about 300 thousand kilometers per second), and this makes it difficult to observe it in everyday conditions. And the speed of propagation of sound waves is noticeably lower. So what is the Doppler effect? Imagine that you are on the side of a highway and a car with a working siren is approaching from afar. When it is still far away, the roar of a siren will sound deaf to you. This means that the sound frequency is low. But as it approaches, it will increase more and more.
You will be able to hear a higher and higher sound tone, which will reach a maximum when the car passes you. When the object passes you and begins to move away again,
the wavelength of sound will again decrease (literally, smooth out if this is plotted). This happens because the sound of the siren is first “caught up” by the machine in some way, which shortens the distance between the troughs (crests) of the wave and makes the tone higher, and then, on the contrary, “runs away”, as a result of which the wave is “smoothed out”. Actually, this is called the Doppler effect.
Effect value
However, one should not assume that the Doppler effect is a dry fact from the world of electrodynamics. It is this knowledge that is widely used in modern sound radars, which are based on the measurement of wave frequencies. And in the same way, the traffic police determine the speed of cars, and other relevant services determine the speed of airplanes, rivers, etc. Security alarms that respond to indoor traffic also work on this principle.
Discovery of Edwin Hubble
But, probably, the most significant discovery related to the indicated effect is Hubble's law. Back in 1929, a US astronomer Edwin Hubble directed his telescope into the starry sky. Watching distant galaxies, he discovered an interesting thing. Many of these galaxies were shrouded in some halo of red haze. As the sound of a receding object is heard higher by us, so the color of the receding body appears reddish to the human eye. This literally meant that the galaxies were flying away from us. Interestingly, the farther away a galaxy is located, the faster it moves away. This observation contributed significantly to the idea of ​​the expanding Universe and the Big Bang, as its beginning, most popular among modern astrophysicists.