Whistling in the clavichord.

Sympathetic string vibration plays an essential role in the clavichord's sound quality and tonal identity. Sympathetic vibration comes from the undamped string segments between the bridge and tuning pins. Under some conditions, a specific note, a whistling tone, stands out of the reverberation halo due to sympathetic vibration. It is hypothesized that this whistling tone comes from resonance between played and sympathetic segments of strings that are coupled through the bridge. Vibratory measurements for three pairs of excited and sympathetic strings are conducted on a copy of a historical instrument built by Hubert in 1784. The influences of bridge mobility and tuning on sympathetic string frequency and damping are studied. The results show a significant increase in vibratory amplitude, frequency veering, and damping increase in the string segments when tuning approaches frequency coincidence. Numerical simulations of a reduced clavichord model corresponding to the experiments are conducted using the modal Udwadia-Kalaba formulation. Simulation gives a more accurate picture of the veering phenomenon. Simulation and experimental results are in good agreement, showing that whistling in the clavichord comes from string resonance. It is favored by frequency coincidence between excited and sympathetic string segments and by higher bridge mobility.

Temperature-controlled spectrophotometry: a simultaneous analysis of phase transition, thermal degradation and optical properties of semi-transparent composites from 20 °C to 450 °C.

So far, optical and effective radiative properties of polymer matrix based composites were investigated at temperatures well below their degradation temperature. At the same time, polymers exhibit temperature dependent physical properties and may undergo structural changes as their temperature raises. In this work, we employ the "Temperature-Controlled Spectrophotometry", a new method enabling to identify simultaneously phase transitions, thermal degradation and radiative properties of semi-transparent composites over a large temperature range. The method consists of measuring simultaneously the normal-normal and the normal-hemispherical transmittances and reflectances of the sample subjected to a laser irradiation with tuneable wavelength while the temperature is rised from room temperature up to 450 °C by means of a CO2 laser. Physical changes of the sample are identified from the temperature variation of normal-normal transmittance and specular reflectance measurements. Most of the results here are presented at a specific wavelength of 1070 nm but the proposed method is suitable over the semi-transparency spectral domain of the material by changing the wavelength of the probe laser. An inverse method for parameter identification based on normal-hemispherical measurements is employed to determine the transport effective radiative properties of the sample, namely the transport extinction coefficient and the transport scattering albedo from room temperature to 325 °C.

Toward a physical model of the clavichord.

String excitation by the tangent in the clavichord is a unique mechanism. The tangent, keeping in contact with the string after the initial strike, continuously controls the string tension. Four main flexible subsystems are considered in the clavichord: the tangent/key subsystem, the string subsystem, the bridge-soundboard subsystem, and the string damper subsystem. A modal description of the dynamics of these subsystems is proposed. Parameters of the subsystems are estimated on a copy of a historical instrument by Hubert (1784). The different subsystems and their couplings are modeled using a modal Udwadia-Kalaba formulation. The string-tangent interaction is modeled via the intermittent contact dynamics, using the Kirchoff-Carrier string model. Realistic string, soundboard, and tangent motions are obtained using a time-domain synthesis scheme that computes the dynamics of the uncoupled subsystems and the constraints resulting from coupling between them. Simulated motions of the model in response to a driving force on the key are analyzed. The results are consistent with experimental measurements and published data on the dynamics of the clavichord. The model is able to reproduce the main acoustic features of the instrument: force on the key for intonation control, key velocity for dynamic nuances control, and constant spectral slope for varying dynamic nuances.

Circumventing the miscibility gap in InGaN nanowires emitting from blue to red.

Widegap III-nitride alloys have enabled new classes of optoelectronic devices including light emitting diodes, lasers and solar cells, but it is admittedly challenging to extend their operating wavelength to the yellow-red band. This requires an increased In content x in In x Ga1-x N, prevented by the indium segregation within the miscibility gap. Beyond the known advantage of dislocation-free growth on dissimilar substrates, nanowires may help to extend the compositional range of InGaN. However, the necessary control over the material homogeneity is still lacking. Here, we present In x Ga1-x N nanowires grown by hydride vapor phase epitaxy on silicon substrates, showing rather homogeneous compositions and emitting from blue to red. The InN fraction in nanowires is tuned from x = 0.17 up to x = 0.7 by changing the growth temperature between 630 °C and 680 °C and adjusting some additional parameters. A dedicated model is presented, which attributes the wide compositional range of nanowires to the purely kinetic growth regime of self-catalyzed InGaN nanowires without macroscopic nucleation. These results may pave a new way for the controlled synthesis of indium-rich InGaN structures for optoelectronic applications in the extended spectral range.