The frequency of sound has characteristics that are also characteristic of a number of other phenomena propagating by means of a wave. This is true, for example, for light or X-rays. Sound frequency 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 time period during which they occur. For example, the frequency of a sound determines the pitch we hear. Or we do not hear if the vibrations are beyond the limit 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 the quantity under consideration is called the Doppler effect (after the discoverer). It can also be observed using light waves as an example, but the speed of light propagation is very high (about 300 thousand kilometers per second), and this makes it very 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 main road anda car with a working siren is approaching from afar. When he is still far away, the siren's roar will seem deaf to you. This means that the sound frequency is low. But as it gets closer, it will grow more and more.
You will be able to hear a higher and higher pitch, which will peak as the car passes you. When the object passes you and begins to move away again, the wavelength of the sound will again decrease (literally, smooth out, if it is depicted on a graph). This happens for the reason that the sound of the siren is first in some way “caught up” by the machine, 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, as it were, “smoothes out”. Actually, this is called the Doppler effect.
Effect value
However, one should not assume that the Doppler effect is some dry fact from the world of electrodynamics. It is this knowledge that is widely used in modern sound radars, which are based on measuring wave frequencies. And in the same way, traffic police officers determine the speed of vehicles, and other relevant services determine the speed of aircraft, river flows, etc. Burglar alarms that respond to movements in the room also work on this principle.
Discovery of Edwin Hubble
But perhaps the most significant discovery related to this effect is the Hubble law. Back in 1929, US astronomer Edwin Hubble sent histelescope into the starry sky. By observing distant galaxies, he discovered an interesting thing. Many of these galaxies were shrouded in some halo of red haze. Just as the sound of a receding object is heard to us at a higher pitch, so the color of a 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, the faster it is receding. This observation greatly contributed to the most popular idea among modern astrophysicists about the expanding Universe and the Big Bang as its beginning.
