In-Vitro and Numerical Investigations of the Influence of a Vocal-Tract Resonance on Lip Auto-Oscillations in Trombone Performance

International audienceControlling the acoustic impedance of the upstream airways in brass instrument performance may bean important factor influencing the efficiency of the sound production process. Because of the complexcharacteristics of the lip-valve oscillator, the conditions under which a vocal-tract resonance may be favorableto the sustain of lip auto-oscillations are not easy to determine. In order to investigate this aspect ofbrass performance, an experimental method based on an active control approach is applied to an artificialtrombone player system in order to simulate the influence of a vocal-tract resonance at the playing frequency.By varying the amplitude and phase characteristics of this upstream impedance load, we investigatethe acoustical influence of this resonance on lip vibrations and on the acoustic pressure generated in theinstrument. The observations reveal that variations of the phase difference between the downstream andupstream impedance induce significant variations of the playing frequency. An optimal phase tuning pointcharacterized by a maximum of downstream acoustic pressure at the input of the instrument, and uncorrelatedto a maximum of downstream input impedance, is identified. These experimental results are comparedwith numerical simulations, both of which produce similar findings. The optimal tuning point appears tobe partly related to the displacement of the playing frequency close to a mechanical resonance of the lips.This induces a greater “efficiency" of the lip-valve system, hence maximizing the acoustic flow generatedinto the instrument while other control parameters (quasi-static mouth pressure, lip tension) are maintainedconstant. In addition to an exploration of acoustical influence of the vocal tract, this experimental methodhence offers promising perspectives for the study of artificial lips under playing conditions

Validation of registration techniques applied to XRD signals for stress evaluations in titanium alloys

To estimate stresses near specimen surfaces, X-ray diffraction (XRD) is applied to titanium alloys. Some of these alloys are difficult to study since they are composed of various phases of different proportions, shapes and scales. For millimetric probed volumes, such multi-phase microstructures induce shallow and noisy diffraction signals. Two peak registration techniques are introduced and validated thanks to tensile tests performed on two titanium alloy samples

1100 days in the life of the supernova 2018ibb -- The best pair-instability supernova candidate, to date

International audienceAbridged - Stars with ZAMS masses between 140 and $260 M_\odot$ are thought to explode as pair-instability supernovae (PISNe). During their thermonuclear runaway, PISNe can produce up to several tens of solar masses of radioactive nickel, resulting in luminous transients similar to some superluminous supernovae (SLSNe). Yet, no unambiguous PISN has been discovered so far. SN2018ibb is a H-poor SLSN at $z=0.166$ that evolves extremely slowly compared to the hundreds of known SLSNe. Between mid 2018 and early 2022, we monitored its photometric and spectroscopic evolution from the UV to the NIR with 2-10m class telescopes. SN2018ibb radiated $>3\times10^{51} \rm erg$ during its evolution, and its bolometric light curve reached $>2\times10^{44} \rm erg\,s^{-1}$ at peak. The long-lasting rise of $>93$ rest-frame days implies a long diffusion time, which requires a very high total ejected mass. The PISN mechanism naturally provides both the energy source ($^{56}$Ni) and the long diffusion time. Theoretical models of PISNe make clear predictions for their photometric and spectroscopic properties. SN2018ibb complies with most tests on the light curves, nebular spectra and host galaxy, potentially all tests with the interpretation we propose. Both the light curve and the spectra require 25-44 $M_\odot$ of freshly nucleosynthesised $^{56}$Ni, pointing to the explosion of a metal-poor star with a He-core mass of 120-130 $M_\odot$ at the time of death. This interpretation is also supported by the tentative detection of [Co II]$\lambda$1.025$\mu$m, which has never been observed in any other PISN candidate or SLSN before. Powering by a central engine, such as a magnetar or a black hole, can be excluded with high confidence. This makes SN2018ibb by far the best candidate for being a PISN, to date