October 5, 2022

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What is the speed of sound?  In the case of Mars, this question takes on new meaning

What is the speed of sound? In the case of Mars, this question takes on new meaning

The persistent rover recorded various sounds propagating through the Martian atmosphere. It turns out that sound travels slower than on Earth, but this is the least interesting result about the propagation of sound waves on Mars.

Let’s start with what is the speed of sound? This, by definition, is the speed of propagation of the sound wave in the medium. This means that in order to talk about the speed of sound, one must have any medium in which that sound can propagate. The sound wave is created as a result of compression and dilution of the material that is made of this medium. It is a longitudinal wave, that is, the vibrations have the same direction of propagation.

Therefore, in a vacuum, where there is practically nothing, it is difficult to talk about the propagation of sound at all. For a life form found in such conditions, installations such as the best microphones or bluetooth speakers would be meaningless. Even taking into account the fact that even in space there is no complete vacuum, since we have interplanetary or interstellar matter, its density is usually so low that the propagation of sound is out of the question.

The speed of sound on Earth depends mainly on…

Our logical conclusion from this definition is that the speed of sound may be related to the density of the medium and its other properties. On Earth, the speed of sound in air when it comes to low-intensity sound (and this is the case in nearly every case we deal with) does not depend on the frequency of the vibrations. That is why the sound, consisting of different frequencies, reaches the listener at the same time as a whole (I omit the effect of obstacles and possible reflections that make up the sound). Instead, the speed of sound depends on the air temperature.

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The standard adopted speed of sound is 340 m/s, that is, the speed of sound is up to 15 degrees. On a hot day, the sound travels a little faster, and in cold weather the speed is much slower. At -40 ° C it is already 306 m / s, while at 40 ° C it is 355 m / s.

Of course, we are only talking about air, because, for example, in wood, metal and diamond, the speed varies. Diamonds reach a length of 18 km / s, while in an atmosphere consisting only of carbon dioxide, it is only 259 m / s.

Mars and the sources of sound on its surface

Speaking of carbon dioxide, it is worth returning to this Mars, since the atmosphere is now filled with this gas. Carbon dioxide makes up 95% of Mars’ atmosphere. It is usually much cooler there than on Earth, and perhaps most importantly, the density of the atmosphere is negligible compared to the density of the atmosphere on Earth. Even at the summit of Mount Everest, the air is still 50 times denser than the average air density on Mars where the rover conducted its experiments measuring the speed of sound.

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Many different audio sources were used for testing. For example, innovative helicopter rotors rotating at 2500 rpm produce sound with a frequency of 84 Hz, while the sound wave generated by the evaporation of rock fragments after firing a SuperCam laser has a frequency of 2000 Hz. These sound sources are located very far from the rover, unlike the rover wheels, which also make noise when rolling on the surface of Mars. The sound is recorded by perseverance microphones.

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Sound on Mars has a different speed and depends on the frequency

Based on these measurements, it was possible to determine the speed of sound in the Martian atmosphere. As expected, this speed is similar to a medium made entirely of carbon dioxide, but it interestingly depends on the frequency of the sound.

The speed of sound on Mars depends on frequency, the higher the frequency, the faster the sound

Thus the lower tones travel at a speed of 240 m/s, while the higher tones reach a speed of 250 m/s. It is possible that the higher-frequency vibrations that the perseverance cannot record propagate faster. Now imagine the sound of a machine made up of different frequencies, or several instruments of different pitches being played on Mars. What do our ears hear? We ignore the difficulty of human work on Mars without a suit or a room in which the air has a density comparable to that of Earth.

perseverance microphones
Two microphones on board the perseverance wagon

The above dependence, according to scientists, is the result of a very rare, cold, often carbon dioxide atmosphere. This was all before in the realm of theoretical conjecture, but now even critics won’t deny it because we have actual measurements.

The whole thing is not so simple, of course, because when listening to music on the ground, the interference caused by obstacles in the environment also plays an important role, and this also changes the sound compared to the natural one. On Mars, variable speed would be an additional problem.

The silence of Mars is a term that can be treated as a truism today

The environment on Mars has another advantage. Although the sound can spread out there, it gets dull very quickly. After only 8 meters, it can be noticeably muted, and at higher frequencies it can be completely muted. This is a distance 8 times shorter than on Earth. For this reason, scientists suggest that Mars is very quiet most of the time. So quiet that the mission console felt the rover microphones were broken and not working.

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Low pressure in the Martian atmosphere causes the surface of Mars to muffle sound very quickly

It is likely that the conditions are similar to those in the anechoic chamber, which in theory is an ideal place for audiophiles to listen (in practice, the nature of the room can have a beneficial effect and such a room is not always comfortable). Just like the make up of the atmosphere there that’s lethal to our lungs.

Source: NASA, inf. King