Ultrasonic wave propagation in reticulated foams saturated by different gases: High frequency limit of the classical models

International audienceTransmission experiments are performed on high porosity reticulated polyurethane foams saturated by different gases at ultrasonic frequencies up to 800 kHz. An excess attenuation is observed at high frequencies, when the wavelength is not sufficiently large compared to the lateral dimensions of the fibers. At lower frequencies, these experiments lead by using classical models of equivalent fluids, to a fast and reliable method for determining the characteristic length $\Lambda$

Measuring the dynamic shear modulus of poroelastic foams in the audible frequency range

International audienceThe prediction of acoustical properties of multilayered systems including poroelastic layers using the full Biot theory is in principle possible but in practice limited by the absence of material data. One of the parameters that is difficult to measure is the dynamic rigidity of the porous frame. Current experimental methods are limited to the lower part of the audible frequency range 1 (typically below 400 Hz) and require special shapes of the sample (cube, cylindrical rod or very thin samples). Since most sound absorbing plastic foams are viscoelastic, the elastic moduli may vary strongly with frequency, a measuring technique in the full audible frequency range is needed. Recently 2 a new method for the measurement of the dynamic shear modulus of the frame of poroelastic foams in the medium and high audible frequency range (1 to 4 kHz) has been presented This method is based on the measurement of the velocity and the damping of a Rayleigh-type surface wave on sample with thickness larger than the Rayleigh wave penetration depth. The Rayleigh wave was excited through direct mechanical excitation of the frame or the porous material and detected using a laser-doppler vibrometer. The velocity of this wave is closely related to the shear velocity, which is directly linked to the shear modulus. The damping of the Rayleigh wave can be used to determine the imaginary part of the shear modulus.In this work a first attempt is made to measure the dynamic shear modulus on a layer of finite thickness. In this way there is no requirement whatsoever concerning the shape of the sample under investigation

Study of Circumferential Waves on a Poroelastic Cylinder

International audienceThe dispersion relations of acoustic modes in poroelastic cylinders with and without elastic shell coating are determined and solved. The influence of elastic frame parameters and the Biot parameters on the dispersion curves is studied in the configuration with and without coating. The dispersive Rayleigh and whispering gallery waves are highly sensitive to the density and the shear modulus of the skeleton, opening aperspective for the evaluation of the mechanical parameters of poroelastic materials confined in cylindrical tubes during the manufacturing process. The predicted dispersion curves are validated with experimental results obtained by use of different experimental setup in the case of aporous circular cylinder

Guided elastic waves in porous materials saturated by air under Lamb conditions

International audienceThe propagation of guided elastic waves in porous materials saturated by air under Lamb conditions is studied theoretically and experimentally. The modes are derived from expressing the boundary conditions on the normal and tangential stresses and the displacements at the interfaces between the porous layer and the surrounding fluid. The stresses and the fluid pressure inside the porous medium are obtained from Biot's equations of poroelasticity. Symmetrical and antisymmetrical modes are found when the porous layer is loaded by the same fluid on both sides. Damping mechanisms include viscous and thermal exchanges between the solid and the fluid, in addition to the classical structural damping. Using an experimental setup based on the generation of standing waves in the layer and taking the spatial Fourier transform of the displacement profile, the phase velocities of three modes were measured for two porous materials in a frequency range between 80 Hz and 4 kHz. The measurements confirm the theoretical predictions and provide information on the shear modulus of a sound-absorbing material in a wide frequency range

Temperature dependence of the electrical conductivity of imidazolium ionic liquids.

The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10^7 Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.ionic liquids; molten salts;

Correlation of room acoustic parameters and noise level in eating establishments

This article addresses the impact of the occupancy level, the average acoustic absorption and the so-called acoustic capacity of a space, which is proportional with the volume and inversely proportional with the reverberation time, on the behavior of talking people in an eating establishments. Four different settings were compared: two casual dining restaurants, a self-service student canteen and a small faculty club. The Lombard effect was observed in all cases. In a restaurant with an average amount of absorbing surface of 2.4 m2 or more per person, the sound pressure level increased with more than 3 dB per doubling of the number of people. Results for the student canteen show that people started to communicate less when the number of people present was so high that the absorbing surface dropped under 1.5 m2/person (80 people). The level even stopped to increase with increasing occupancy from 150 people present and beyond, corresponding with 0.8 m2 of absorbing surface per person. This is roughly consistent with an estimated value for the acoustic capacity of that space, which was 189 people (corresponding with a table occupancy of about 72%). In the latter circumstances, the background noise level, as expressed by LA,95 was as high as 69 dB. Overcoming this level for oral communication would require a not sustainable vocal effort. In the tests performed in other restaurants, the observed occupancy was below 60%, which, thanks to the higher number of absorbing surfaces in those restaurants, was well below the acoustic capacity

Visualization of ultrasonic wave field by stroboscopic polarization selective imaging

A stroboscopic method based on polarization selective imaging is proposed for dynamic visualization of ultrasonic waves propagating in a transparent medium. Multiple independent polarization parametric images were obtained, which enabled quantitative evaluation of the distribution of the ultrasonic pressure in quartz. In addition to the detection of optical phase differences δ in conventional photo-elastic techniques, the azimuthal angle φ and the Stokes parameter S2 of the polarized light are found to be highly sensitive to the wave-induced refraction index distribution, opening a new window on ultrasonic field visualization

“Photoacoustics” and “acousto-optics”: listening to light and looking at sound

This paper briefly introduces the physics behind photoacoustic (listening to light) and acousto-optic (looking at sound) phenomena and describes several applications for fundamental or applied material research. The exploitation of a photoacoustic cell for the thermal characterization and depth profiling of liquid crystals is illustrated. It is illustrated that a photoacoustic cell can be used to study the magnetocaloric effect, and thus to investigate thermal and magnetic material properties. It is shown that optically excited and detected acoustic waves in materials can be very helpful to investigate their microscopic structure. In the impulsive stimulated scattering technique the mechanisms of converting light into thermal, acoustic, orientational and structural modes is exploited to simultaneously obtain information about the many physical properties of relaxing liquids.status: publishe

Calculation Of Human Echolocation Cues By Means Of The Boundary Element Method

Presented at the 19th International Conference on Auditory Display (ICAD2013) on July 6-9, 2013 in Lodz, Poland.Some visually impaired people are able to recognize their surroundings by emitting oral sounds and listening to the sound that is reflected at objects and walls. This is known as human echolocation. The present paper reports the calculation of objective auditory cues present in human echolocation by means of the boundary element method using a spherical model of the human head in the presence of a reflecting disc at different positions. The studied frequency range is 100 Hz to 8 kHz. The results show that frequencies above 2 kHz provide information for localization of the object, whereas the lower frequency range might be used for size determination. It is also shown that stationary sound signals in echolocation can provide relevant acoustic cues, so as displacements in the proximity of a reflecting object become frequency-dependent amplitude modulations. Further calculations in a higher frequency range and with a realistic model of a human head could bring more light to the current knowledge in human echolocatio