Mars Microphone 2020

A brief comparison between normal conditions, 5 mbar and -40°C

2015-03-11T11:44:54Z

Here is a comparison between tests under normal conditions, 5 mbar and -40°C.
It shows that :

a lower pressure decreases the sound power received by the microphone while temperature has no effect on it
the microphone is well qualified at least above 2000 Hz in extreme conditions

Test at -40°C

2015-02-09T16:14:35Z

As we had already tested the microphone at low pressure, the objective of this test was to determine the frequency response of the microphone under a temperature of -40°C (Martian surface temperature), still under with an Earth-like atmosphere for now.
The test consisted in recording 60 samples of 1 second at a sampling frequency of 100 kHz, for each frequency. The shift of frequency and the record of samples were automatically done by the Matlab script. The Matlab script made all the work and tested 38 frequencies (16 between 100 Hz and 1000 Hz, 17 between 1000 Hz and 10 kHz, and 5 between 10 kHz and 15 kHz, all evenly spaced). The goal of this test was also to analyse the effect of the distance between the emitting source and the microphone, so we changed the gauge.

What did we retrieve ?

We obtained 60 noise records, plus 60 records for each of the 38 frequencies that we tested.
Although we used a square wave below 1000 Hz (in order to have stronger power at low frequencies, since the previous tests have shown that measures at low frequencies were hard to extrapolate), the buzzer was then supplied by a sine signal whereas the datasheet's advice was to use a square one. That was done to prevent harmonics inherent to a square wave from appearing.

Analysis

We used the same algorithm to reduce the noise background on our graphs.
In previous tests, when measuring low frequencies inferior to 1000 Hz, the signal emitted by the buzzer was still too weak for the microphone to provide a significant signal at the expected frequency. The same phenomenon tends to appear at low temperature.
We observe directly on the microphone recordings that the amplitudes of the voltage due to the sound are quite the same at -40°C as at 20°C. One can notice a peak around 6500 Hz, another one at 11 kHz.
We obtained the following graph:

: PSD of the peaks agains its frequencies,
d = 4.5 cm

It is a bit less regular than before but it may be due to the several manipulations of the buzzer. The distance still does not affect the microphone sensitivity.

Conclusion

The microphone is well qualified above 1 kHz. The two components have quite well resisted to the long stay at low temperature but we will probably have to change the buzzer for future experiments. Moreover, we will try to find another component or system to test the low frequencies because it has not been done yet.

Test with a 5 mbar pressure

2015-02-09T15:32:24Z

Introduction

We made the first tests at IRAP, in a vacuum chamber. The goal of these tests was to qualify the microphone under a 5 mbar pressure (Martian surface pressure) but still with an Earth-like atmosphere composition.
The test consisted in recording 60 samples of 1 second at a sampling frequency of 100 kHz, for each frequency. The shift of frequency and the record of samples were automatically done by a Matlab script that used the LabView software. The Matlab script was supposed to make all the work and test 30 frequencies. However, a license problem forced us to do the tests manually… That's research work! Thus it modified the outputs: number of samples, frequencies…

What did we retrieve?

We tested 17 different frequencies in a range from 100 Hz to 15 kHz. We also did noise background in order to eliminate (or at least reduce) the noise in the records.
Let's note that we did two tests: the first one with a 4.3 cm gauge, the second one with a 27.5 cm gauge, in order to analyse the influence of distance on the signal amplitude.

Analysis

Deleting the noise background

The first part of the analysis was to delete the noise background of the vacuum chamber from the records for each frequency. To do so, we assumed that the background noise is not correlated with the sound produced by the magnetic buzzer. Therefore, the PSD of the measured signal is the addition of the PSD of the noise and the PSD of the desired signal. By having a lot of recordings for the noise background and for each frequency that we want to test, we were able to subtract the noise from the records.

Results

We obtained graphs such as the following one (the buzzer emits a 5 kHz sinusoid):

As the microphone is less sensitive at low frequencies, the deleting noise algorithm is more efficient at those frequencies. Higher, we still have some noise which is not deleted.
What the tests show is that under 1000 Hz, the power of the emitted sound by the buzzer is not strong enough to be clearly heard by the microphone, or the microphone has too low sensitivity, thus one cannot distinguish it from the noise background. This behaviour is similar to the one at 1 bar. Above 1 kHz however, the results can be taken into account.
We observe directly on the microphone recordings that the amplitudes of the voltage due to the sound is the same at 5mbar as at 1bar. Nonetheless, the buzzer was fed with a sine tension whereas its datasheet gives the emission power against frequency for a square tension. In addition, we do not know the emission power of the buzzer at 5mbar. Therefore, the normalized curves might not be correct.
Let's note that the distance does not drastically affect the signal amplitude at this scale.

: PSD of the peaks against its frequencies,
d = 4.3cm

Conclusion

Globally, the microphone seems to be well qualified above 1 kHz. Moreover, once back to normal pressure, it appears that both the microphone and the buzzer have successfully resisted to the stay at low pressure.

What can be heard on Mars ?

2015-02-09T14:27:28Z

The microphone will be sensitive to the same frequencies as the human ear : between 20 Hz and 20 kHz. A first problem occurs with that : the Martian atmosphere is very thin and those frequencies do not travel for long distances. Nevertheless, the wind on Mars can be strong enough to generate noise and the rover too. So, here is a list of possible noise sources that could be heard by the microphone :

Dust storms and dust devils : those can be dangerous for the rover. Only small size dust devil might be audible. Bigger ones make sound in lower frequencies.
Saltation : Martian particles of dust which are in the air will hit the rover. This might produce a
noise which can be heard by the microphone. Data may give the information of how many
particles hit the rover, which can help to know better the Martian environment for future
missions.
Wind over the rover : as the wind blows over the MSL2, it might produce sounds. In addition, we should also hear sounds made by the laser SuperCam.


Dust devils seen by SPIRIT: July 13, 2005
Credits : NASA/JPL/Texas A&M

Mars Microphone 2020 objectives

2015-02-09T14:27:26Z

The Mars Microphone for Mars 2020 is a very simple and exciting experiment, whose primary objective is to achieve a world premiere : retrieve sounds from Mars, in particular the sound generated by its "mother" payload, the SuperCam LIBS laser. In addition to this primary objective, it will contribute to basic atmospheric investigation by analysis of the acoustic environment and help put constraints on wind vortex or dust devils properties.

Secondary objectives will strongly depend on favourable circumstances. The microphone may be able to retrieve Martian atmosphere electrical activity related sounds and possibly retrieve the sound of bolides entry.

The mass of this experiment is about 50g, and its consumption 150 mW. Depending on the operational scenario, the energy need may be very small.

While built in Europe, with students involvement, the Mars Microphone will also strongly rely on the heritage of the previous Mars Microphone experiments, led by Berkeley SSL and the Planetary Society for the Phoenix, Mars Polar lander and NetLander missions. This experiment will therefore feature a unique combination of outreach, educational initiative and scientific objectives, .

About us and our work

2015-02-09T14:27:23Z

We are second year students at ISAE-SUPAERO, the French Aerospace Engineering School. Highly interested in space, we want to work in this field later. Our mission is to qualify the microphone, implement a method to record data for the vibration and temperature tests. We will also have to work on how to integrate it with SuperCam and how it should be implemented to have good quality recordings, since the Martian atmosphere is quite different from ours (most of it is C02 at a 5 mB pressure...). This project is highly interesting insofar as we will test the microphone in space and Martian conditions. It is also a good opportunity to meet scientists and engineers of this field.