• WHY WE EXPLORE
• PLANETARY NEWS
• SPACE TOPICS
  • Contests, Events, and Awards
  • Extrasolar Planets
  • How It Works
  • Near Earth Objects
  • Our Solar System
  • Search for Life
  • Space Missions
    • 2001 Mars Odyssey
    • Cassini-Huygens
      • Mission Objectives
      • Mission Facts
      • Cassini's Tour of the Saturn System
      • Cassini's Science Instruments
      • Selected Data from Cassini's Cameras
      • The Cassini Signature Disk
      • The Huygens Art Contest
      • Links
    • Chandra X-Ray Observatory
    • Chandrayaan-1
    • Chang'e 1
    • Dawn
    • Deep Impact
    • Genesis
    • Hayabusa (MUSES-C)
    • Hubble Space Telescope
    • Kaguya (SELENE)
    • Lunar Reconnaissance Orbiter (LRO)
    • Mars Exploration Rovers
    • Mars Express
    • Mars Global Surveyor
    • Mars Reconnaissance Orbiter
    • Mars Science Laboratory
    • MESSENGER
    • New Horizons
    • Past Missions
    • Phoenix
    • Private Missions
    • Rosetta
    • SMART-1
    • SOHO
    • Spitzer Space Telescope
    • Stardust
    • Ulysses
    • Venus Express
    • Voyager
• FOR KIDS

browse projects: Apophis Mission Design Competition Carl Sagan Fund for the Future Catalog of Exoplanets FINDS Exo-Earths International Year of Astronomy 2009 LIFE Experiment: Phobos Messages from Earth Observing Earth Pioneer Anomaly Planetary Microphones SETI Optical Telescope SETI Radio Searches SETI@home Shoemaker NEO Grants Solar Sailing Space Advocacy Space Information

browse space topics: 2001 Mars Odyssey Asteroids and Comets Cassini-Huygens Chandra X-Ray Observatory Chandrayaan-1 Chang'e 1 Compare the Planets Dawn Deep Impact Earth Extrasolar Planets Genesis Hayabusa (MUSES-C) Hubble Space Telescope Jupiter Kaguya (SELENE) Lunar Reconnaissance Orbiter (LRO) Mars Mars Exploration Rovers Mars Express Mars Global Surveyor Mars Reconnaissance Orbiter Mars Science Laboratory Mercury MESSENGER Moon Near Earth Objects Neptune New Horizons Past Missions Phoenix Planetary Analogs Planetary Exploration Timelines Pluto and Charon Private Missions Rosetta Saturn Search for Extraterrestrial Intelligence SMART-1 SOHO Space Imaging Spitzer Space Telescope Stardust Trans-Neptunian Objects Ulysses Uranus Venus Venus Express Voyager Weekly Planetary Radio Trivia Contest

Space Topics: Cassini-Huygens

The Huygens "Microphone"

Why is there a microphone aboard the Huygens probe? To hear the sound of thunder! But the instrument, which is called the Acoustic Sensor Unit, or ACU, was designed to return only enough information to Earth for its science team to tell whether they had detected thunder. This is not the same as recorded sound. In order to help people around the world hear something akin to the noises of Huygens' descent, The Planetary Society offered to help the instrument team process their data into sounds from Titan, calling upon our experience with designing and building the Mars Microphone.

Why is there a microphone aboard the Huygens probe? To hear the sound of thunder! But the instrument, which is called the Acoustic Sensor Unit, or ACU, was designed to return only enough information to Earth for its science team to tell whether they had detected thunder. This is not the same as recorded sound. In order to help people around the world hear something akin to the noises of Huygens' descent, The Planetary Society offered to help the instrument team process their data into sounds from Titan, calling upon our experience with designing and building the Mars Microphone.

What's the Huygens Microphone For?

According to the instrument's science team, "The Huygens Atmospheric Structure Instrument, or HASI, designed and built by an international team headed by Marcello Fulchignoni, will measure the temperature, pressure, density, and other physical properties of Titan's atmosphere as Huygens descends to the surface. The acoustic sensor unit is part of the Permittivity, Wave, and Altimetry (PWA) subsystem of HASI and was designed, developed, and tested by the Space Research Institute (IWF) of the Austrian Academy of Sciences, Graz, Austria. Due to exposure to the extremely cold atmospheric environment of TItan, standard microphones cannot be used." Peter Falkner, who was responsible for the development of the acoustic sensor unit, explains. "Much effort was necessary to find, qualify, and calibrate a proper pressure sensor for this application. We put candidate sensors for hours into liquid nitrogen at 77 Kelvin (-196°C) to see if they were able to operate at these extreme cold conditions."

The Acoustic Sensor Unit (ACU) is located on the STUB, one of the HASI instrument booms that extend out of Huygens' protective shell. Two of the booms remain tucked close to the probe until they are deployed during the descent, but the STUB sticks out from the side of the probe permanently. In the diagram below, elements of the HASI instrument are highlighted in blue. The STUB is at the bottom of the diagram.

Click to enlarge > Diagram of Huygens' HASI instrument components The STUB (at the bottom of this image) is a short boom that holds Huygens' air temperature and pressure sensors out into Titan's atmosphere during the probe's descent. The acoustic sensor or microphone is not labeled on this diagram, but is located at the base of the STUB where it attaches to the spacecraft. During the descent it is exposed to the cold Titan atmosphere. Credit: HASI Instrument Team

Click to enlarge > The HASI ACU The ACU (inset) is a tiny instrument that sticks out of the base of the STUB. Credit: HASI-PWA Team (ESA / ASI / UPD / IWF)

The Acoustic Sensor is a tiny instrument weighing only 20 grams (less than an ounce). It is intended to have a supporting role to an electric fields experiment, which (scientists hope) could detect Titanian lightning. If there is any lightning near the spacecraft, the microphone should hear the thunderclap!

Key people involved in the development, test and programming of PWA ACU and the calibration and processing software were Konrad Schwingenschuh, project management; Peter Falkner, instrument development; Irmgard Jernej, hardware tests and software development; Roland Trautner, calibration and software development; Werner Wesselak, calibration; Wolfgang Koren, cryogenic testing; and Robin Hofe, modelling.

Just a Few Bits of Data

Unfortunately, Huygens was not able to transmit recorded sound directly back to Earth. Unlike the Cassini orbiter, Huygens didn’t have the luxury of saving lots of data for later transmission. Every bit of data that Huygens produced was transmitted live to Cassini, because Huygens was only expected to live about two and a half hours once it hit the top of Titan's atmosphere. That means that all of Huygens' instruments were competing for bandwidth, and all of the instrument data had to be very efficiently compressed in order to maximize the science return from the mission. For the microphone, the bandwidth was limited to only 480 bits per second. For comparison, music files that you can download from the Internet are often recorded at 128 kilobits per second; that's 267 times as much data!

So instead of sending out the sounds of Titan's wind, HASI-PWA used the microphone's data to calculate some numbers that described the sounds. For each two seconds of Huygens' descent, HASI-PWA recorded the average sound power at different frequencies (pitches). The data product that the science team can get back from the microphone is a sonogram, a diagram of time versus sound power and frequency.

This is enough information for the HASI-PWA team to determine whether they had detected thunder. However, it's not the same as recorded sound. The Planetary Society offered to help the HASI-PWA team process this sparse data into sounds so that people around the world can hear something akin to the noises of Huygens' descent. We could make Sounds from Titan with the help of our microphone scientist colleague Dr. Greg Delory of the University of California, Berkeley, who designed and built The Planetary Society's Mars Microphone.

Making the Huygens Sounds

Making the Huygens sounds is a bit of a technical challenge, because of the low time resolution of the data that HASI-PWA returned, one sonogram for each two seconds. To perform the reconstruction of the sound, Delory developed a computer program. The program examines the amount of power measured at each frequency, and generates a set of sine-shaped sound waves using the power measured by the Acoustic Sensor for the wave amplitude, and the frequency for the pitch. The program then sums all these waves together to produce the sound. This process is repeated for each sonogram, one for every two seconds of the descent.

The conversion resulted in more than two hours of recorded sound, most of which sounds like the hiss and roar of wind rushing past the spacecraft. Delory then used a variety of other processing techniques to make samples of sound that tell us something interesting about the Huygens descent and Titan's sky. Click here to listen to the sounds.

For instance, it is possible to compress the time scale on the recorded sound without changing its pitch, so that when you play back the sound you can listen to the whole descent in a few moments and figure out where in Titan's atmosphere there are unusual sounds. The "whole descent in 10 seconds" sound was made in this way.

Further processing could make the data sound more like what we might hear if we were riding along with Huygens. This sort of work requires some assumptions, some guesses, and sometimes a little artistic license. For the sounds that we have posted on our website, Delory has made some assumptions about what frequencies in the sonograms represent random noise and selectively suppressed those frequencies. Applying more creativity, he could search through the recording for interesting sonic events, and manually amplify them to bring their features out.