Andi Petculescu

Fine-tuning molecular acoustic models: sensitivity of the predicted attenuation to the Lennard–Jones parameters

In a previous paper [Y. Dain and R. M. Lueptow, J. Acoust. Soc. Am. 109, 1955 (2001)], a model of acoustic attenuation due to vibration-translation and vibration-vibration relaxation in multiple polyatomic gas mixtures was developed. In this paper, the model is improved by treating binary molecular collisions via fully pairwise vibrational transition probabilities. The sensitivity of the model to small variations in the Lennard-Jones parameters--collision diameter (sigma) and potential depth (epsilon)--is investigated for nitrogen-water-methane mixtures. For a N2(98.97%)-H2O(338 ppm)-CH4(1%) test mixture, the transition probabilities and acoustic absorption curves are much more sensitive to sigma than they are to epsilon. Additionally, when the 1% methane is replaced by nitrogen, the resulting mixture [N2(99.97%)-H2O(338 ppm)] becomes considerably more sensitive to changes of sigma(water). The current model minimizes the underprediction of the acoustic absorption peak magnitudes reported by S. G. Ejakov et al. [J. Acoust. Soc. Am. 113, 1871 (2003)].

A prototype acoustic gas sensor based on attenuation

Acoustic attenuation provides the potential to identify and quantify gases in a mixture. We present results for a prototype attenuation gas sensor for binary gas mixtures. Tests are performed in a pressurized test cell between 0.2 and 32atm to accommodate the main molecular relaxation processes. Attenuation measurements using the 215-kHz sensor and a multiseparation, multifrequency research system both generally match theoretical predictions for mixtures of CO2 and CH4 with 2% air. As the pressure in the test cell increases, the standard deviation of sensor measurements typically decreases as a result of the larger gas acoustic impedance.

Air-coupled ultrasonic sensing of grass-covered vibrating surfaces; qualitative comparisons with laser Doppler vibrometry

The paper addresses several sensitive issues concerning the use of air-coupled ultrasound to probe small vibrations of surfaces covered with low-lying vegetation such as grass. The operation of the ultrasonic sensor is compared to that of a laser Doppler vibrometer, in various contexts. It is shown that ambient air motion affects either system, albeit differently. As air speed increases, the acoustic sensor detects a progressively richer turbulent spectrum, which reduces its sensitivity. In turn, optical sensors are prone to tremendous signal losses when probing moving vegetation, due to randomly varying speckle patterns. The work was supported by the Office of Naval Research.

The sound of music and voices in space part 1: theory

While probes to other planets have carried an impressive array of sensors for imaging and chemical analysis, no probe has ever listened to the soundscape of an alien world. With a small number of exceptions, planetary science missions have been deaf. The most successful acoustic measurements were made by the European Space Agencys 2005 Huygens probe to Titan, but although this probe was spectacularly successful in measuring the atmospheric sound speed and estimating the range to the ground using an acoustic signal that the probe itself emitted, we still have no measurements of sounds generated by alien worlds. Although microphones have been built for Mars, the Mars Polar Lander was lost during descent on 3 December 1999, and the Phoenix probe microphone was not activated (because the Mars Descent Imager system to which it belonged was deactivated for fear of tripping a critical landing system). Instead of measuring acoustic signals that had propagated to the microphone from a distance, aerodynamic pressure fluctuations on the microphone (caused by wind on the surface of Venus in the case of the 1982 Russian Venera 13 and 14 probes, and turbulence during the parachute descent in the case of Huygens) masked the soundscape on these Venus and Titan missions. Given the lack of such data from these earlier missions, some early enthusiasts for acoustics in the space community are now skeptical as to whether it will ever have a useful role. However basing such an assessment on past performance presupposes that the sensor systems have been optimized for the environment in question.

The sound of music and voices in space part 2: modelling and simulation

As is shown in the paper, atmospheres affect both the generation and the propagation of sound. The effect on sound generation, depending on the actual source mechanisms that we exemplify by organ music and speech is two-pronged—the acoustic characteristics are altered not only by the nature of the gas but also by mass loading of the source. For the propagation of sound, the atmospheres act as frequency-dependent “filters,” characteristic of the composition and ambient conditions of each planet. The media files associated with the article are organized as follows. Table 1 sets the stage. Table 1 contains a calibration tone at 97 dB re 20 µPa. A short clip of the organ solo (played on the organ in St. Margarets Church, East Wellow, Hampshire, United Kingdom), the words, Earth, Mars, Titan and Venus are then spoken and are used in the next three tables to illustrate how each might sound at the distances indicated and on each of the locations. The last example in Table 2 illustrates how all the organ clips would sound if played together. Note: If the sound becomes inaudible, it is due to the attenuation of the particular atmosphere. Do not continually adjust the volume to hear the sound.

Infrasonic attenuation in the upper mesosphere–lower thermosphere: A comparison between Navier–Stokes and Burnett predictions

This paper presents the results of a pilot study comparing the use of continuum and non-continuum fluid dynamics to predict infrasound attenuation in the rarefied lower thermosphere. The continuum approach is embodied by the Navier-Stokes equations, while the non-continuum method is implemented via the Burnett equations [Proc. London Math. Soc. 39, 385-430 (1935); 40, 382-435 (1936)]. In the Burnett framework, the coupling between stress tensor and heat flux affects the dispersion equation, leading to an attenuation coefficient smaller than its Navier-Stokes counterpart by amounts of order 0.1 dB/km at 0.1 Hz, 10 dB/km at 1 Hz, and 100 dB/km at 10 Hz. It has been observed that many measured thermospheric arrivals are stronger than current predictions based on continuum mechanics. In this context, the consistently smaller Burnett-based absorption is cautiously encouraging.

Guest editorial: Acoustic and related waves in extraterrestrial environments

Recent years have seen a resurgence of acoustic sensing in planetary exploration, complementing the prevailing electromagnetic techniques. For outreach purposes mostly, some attention was paid to converting electromagnetic sensor pickup into audio playback signals. Such examples are the effects of Saturn’s lightning or bow wave on the Cassini spacecraft or those of pulsar emissions on Earth-based sensors. Lately the potential of genuine acoustical information has been increasing steadily. Sound waves carry information on the properties of the propagation medium, which we can access: from the ratio of the stiffness to the density in the sound speed, to the interplay of chemistry and relaxation processes in the frequency-dependent absorption and phase speed. Sound and vibration interact with matter intimately, in a way that complements—and in many ways exceeds— the electromagnetic interactions conventionally used on probes. These interactions occur in the atmospheres of Venus, Titan, and Mars, in the under-ice oceans of Europa, and in the lakes of Titan, and can reach to the cores of planets. They can be seen in the acoustic sensor that monitors the gentle fall of dust onto the surface of a moon, the seismic waves detected by Apollo missions to the Moon, in the oscillations of gas giants and stars (that can indicate the presence of orbiting planets), and in the oscillations of vast dust clouds, as density fluctuations which, on very large scales, have heralded the eventual formation of stars and planets. Furthermore, our own visceral interactions with sound in daily life provide opportunities for outreach and education from studies of acoustics in extraterrestrial environments. Acoustic exploration in planetary science started by making rudimentary measurements in challenging environment, one of the first attempts being a passive instrument accompanying the final Venera landers on Venus. The microphones, looking for evidence of thunder, were only able to measure sounds generated aerodynamically by air flowing past the lander. Capacitive foil microphones had actually been used before during some Apollo Moon landers, to determine the statistics of dust raised by the landing impacts. Berg et al. describe an efficient technique to determine the velocities of dust particles and micrometeorites that relies on analyzing acoustic waveforms produced by particle impacts on impact plate microphones. The technique was successfully used on Apollo missions to the Moon and, more recently, on the Rosetta mission to the Comet 67 P/Churyumov-Gerasimenko. A somewhat larger collision, that of Comet Shoemaker-Levy 9 with Jupiter, added significantly to the body of knowledge about the range of mechanical waves that can exist in the atmosphere of Jupiter and the ice giants Uranus and Neptune. We have never recorded the natural soundscape of another world. There are rare data from microphones, but it is likely that the pressure fluctuations that are attributed to “the sound of wind” are aerodynamic pressure fluctuations on the surface of the microphone (i.e., they are not acoustic, and do not propagate to distance at the local speed of sound). Use of multiple microphones to distinguish such fluctuations from acoustic signals has not been employed to date, and the windscreens commonly used to shield microphones from this on Earth would present challenges for extra-terrestrial use (e.g., in decontamination to prevent the possibility of introducing microbes from Earth to other worlds). Acoustic instrumentation has tended to be based on common usage on Earth, rather than being specifically designed for an extraterrestrial environment. In 1999, a substantially “off-theshelf” microphone was flown onboard the ill-fated Mars Polar Lander, which crashed during descent. The Mars Descent Imager system of the 2008 Phoenix lander had a microphone, designed to record descent sounds as well as any post-landing acoustic event. However, the plans to turn the microphone on were scrapped in order to avoid a technical problem that might have been potentially dangerous to the mission. The Mars2020 rover will carry a customdesigned microphone to record ambient sounds. Perhaps the most carefully thought-out acoustics suite deployed to date was that carried by the Huygens probe that landed on Titan in January of 2005. Beside a microphone for recording the ambient sounds of Huygens’ descent in Titan’s thick atmosphere, the Huygens Atmospheric Structure Instrument had an active ultrasonic sensor that measured the speed of sound over the last 12 km before the landing. Moreover, analysis of ultrasonic signal attenuation obtained immediately after landing seems to indicate the presence of volatile gases such as ethane, acetylene, and carbon dioxide. The Surface Science Package had an acoustic transmitter-receiver configuration commonly used to assess distance to ground (e.g., in depth sounders). This Sound Detection and Ranging system, called the Acoustic Properties Instrument–Sounder, was used to assess Electronic mail: andi@louisiana.edu

Gasdynamic modeling of strong shock wave generation from lightning in Titan’s troposphere.

In an effort to predict the characteristics of thunder on Titan, a model is being developed for the formation of the initial strong shock wave by a short cylindrical lightning discharge “segment.” (Such small discharge segments are later used to synthesize a tortuous cloud‐to‐ground 20 Km‐long lightning channel in Titan’s troposphere.) The shock wave is obtained numerically as a solution of the coupled gasdynamic equations of state and conservation of momentum, mass, and energy. The relevant acoustic quantities are the pressure, density, particle velocity, and specific internal energy of the shock wave immediately following the discharge. Different scenarios for the initial deposition of energy from the discharge into the shock wave are investigated. The altitude‐dependent ambient conditions in Titan’s lower atmosphere—temperature, pressure, and density—are extracted from Cassini‐Huygens data, while specific heats and transport coefficients of the main constituents of Titan’s troposphere (N2, CH4) are obt...

Non-Hertzian behavior in binary collisions of plastic balls derived from impact acoustics

This paper presents slight deviations from Hertzs impact law, inferred from acoustic signatures of polypropylene ball collisions. An impact acoustics model is used to fit the acoustic data. The model is built upon a generalized relationship between impact force (F) and deformation (xi) of the form F=kappaxi(alpha). Agreement with experiment is reached when alpha and kappa differ from Hertzs values by -6.25% and +1%, respectively. The difference is ascribable to non-idealities such as slight material inhomogeneities, impact-point asymmetry, plasticity etc. Also, the collision energy released as sound, which is usually dismissed as negligible, is derived from data fitting. The acoustic-to-incident energy ratio, dependent on impact duration, is constrained to be on the order of 100 ppm.

Doppler ultrasonic detection of targets buried in grass‐covered soil

An ultrasonic Doppler vibrometer (UDV) is used outdoors to detect vibrating targets buried in grass‐covered soil. The sensor head uses two solid‐dielectric transducers, in a pitch–catch configuration. A first set of measurements is done using calibrated vibrational sources (shakers), whose vibrational characteristics are known and/or easily predictable. Then, the system is put to the test of detecting a landmine buried in a realistic environment. The target (landmine) is excited either by a mechanical shaker or by a loudspeaker, through acoustic‐to‐seismic coupling. The wind speed was monitored continuously. Since it is known that wind degrades the UDV signal, efforts were made to perform the experiments in a still environment. The UDV results are compared with those obtained with a laser Doppler vibrometer.