Caracterização vibroacústica e síntese sonora da viola caipira

Orientadores: José Maria Campos dos Santos, François Gautier, Frédéric AblitzerTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica e Le Mans UniversitéResumo: A viola caipira é um tipo de viola brasileira amplamente utilizada na música popular. Ela é composta de dez cordas metálicas dispostas em cinco pares, afinadas em uníssono ou oitava. Este trabalho de tese concentra-se na análise das especificidades dos sons musicais produzidos por este instrumento pouco estudado na literatura. A análise dos sons de viola caipira mostra a presença de vibrações simpáticas de cordas, o que resulta em um halo de som, constituindo uma característica perceptiva importante. Os movimentos de cordas dedilhadas são estudados usando uma câmera de alta velocidade, revelando a existência de choques entre cordas que levam a efeitos claramente audíveis. A análise modal das vibrações do corpo realizada por um vibrômetro à laser de varredura e um martelo de impacto automático permite identificar algumas diferenças em relação ao violão clássico. As mobilidades do cavalete também são medidas usando o método do fio quebrante, que é simples de usar e de baixo custo, uma vez que não requer o uso de um sensor de força. Combinadas com uma análise modal de alta resolução (método ESPRIT), tais medidas permitem determinar as formas modais nos pontos de acoplamento entre corda/corpo e assim caracterizar o instrumento. Uma modelagem física baseada em uma abordagem modal híbrida é realizada para fins de síntese sonora. Tal modelagem considera os movimentos das cordas em duas polarizações, os acoplamentos com o corpo e as colisões entre cordas. Este modelo é chamado de modelo híbrido porque combina uma abordagem analítica para descrever as vibrações de cordas e parâmetros experimentais que descrevem o corpo. Um conjunto de simulações no domínio do tempo revelam as principais características da viola caipiraAbstract: The viola caipira is a type of Brazilian guitar widely used in popular music. It consists of ten metallic strings arranged in five pairs, tuned in unison or octave. The thesis work focuses on the analysis of the specificities of musical sounds produced by this instrument, which has been little studied in the literature. The analysis of the motions of plucked strings using a high speed camera shows the existence of sympathetic vibrations, which results in a sound halo, constituting an important perceptive feature. These measurements also reveal the existence of shocks between strings, which lead to very clearly audible consequences. The modal analysis of the body vibrations, carried out by a scanning laser vibrometer and an automatic impact hammer reveals some differences and similarities with the classical guitar. Bridges mobilities are also measured using the wire-breaking method, which is simple to use and inexpensive since it does not require the use of a force sensor. Combined with a high-resolution modal analysis (ESPRIT method), these measurements enable to determine the modal shapes at the string/body coupling points and thus to characterize the instrument. A physical modelling, based on a modal approach, is carried out for sound synthesis purposes. It takes into account the strings motions with two orthogonal polarizations, the couplings with the body and the collisions between strings. This model is called a hybrid model because it combines an analytical approach to describe the vibrations of strings and experimental data describing the body. Simulations in the time domain reveal the main characteristics of the viola caipiraDoutoradoMecanica dos Sólidos e Projeto MecanicoDoutor em Engenharia Mecânica141214/2013-999999.010073/2014-00CNPQCAPE

Acoustics of the banjo: measurements and sound synthesis

Measurements of vibrational response of an American 5-string banjo and of the sounds of played notes on the instrument are presented, and contrasted with corresponding results for a steel-string guitar. A synthesis model, fine-tuned using information from the measurements, has been used to investigate what acoustical features are necessary to produce recognisable banjo-like sound, and to explore the perceptual salience of a wide range of design modifications. Recognisable banjo sound seems to depend on the pattern of decay rates of “string modes”, the loudness magnitude and profile, and a transient contribution to each played note from the “body modes”. A formant-like feature, peaking around 500–800 Hz on the banjo tested, is found to play a key role. At higher frequencies the dynamic behaviour of the bridge produces additional formant-like features, reminiscent of the “bridge hill” of the violin, and these also produce clear perceptual effects

Vibroacoustics of the piano soundboard: Reduced models, mobility synthesis, and acoustical radiation regime

In string musical instruments, the sound is radiated by the soundboard, subject to the strings excitation. This vibration of this rather complex structure is described here with models which need only a small number of parameters. Predictions of the models are compared with results of experiments that have been presented in Ege et al. [Vibroacoustics of the piano soundboard: (Non)linearity and modal properties in the low- and mid- frequency ranges, Journal of Sound and Vibration 332 (5) (2013) 1288-1305]. The apparent modal density of the soundboard of an upright piano in playing condition, as seen from various points of the structure, exhibits two well-separated regimes, below and above a frequency flim that is determined by the wood characteristics and by the distance between ribs. Above flim, most modes appear to be localised, presumably due to the irregularity of the spacing and height of the ribs. The low-frequency regime is predicted by a model which consists of coupled sub-structures: the two ribbed areas split by the main bridge and, in most cases, one or two so-called cut-off corners. In order to assess the dynamical properties of each of the subplates (considered here as homogeneous plates), we propose a derivation of the (low-frequency) modal density of an orthotropic homogeneous plate which accounts for the boundary conditions on an arbitrary geometry. Above flim, the soundboard, as seen from a given excitation point, is modelled as a set of three structural wave-guides, namely the three inter-rib spacings surrounding the excitation point. Based on these low- and high-frequency models, computations of the point-mobility and of the apparent modal densities seen at several excitation points match published measurements. The dispersion curve of the wave-guide model displays an acoustical radiation scheme which differs significantly from that of a thin homogeneous plate. It appears that piano dimensioning is such that the subsonic regime of acoustical radiation extends over a much wider frequency range than it would be for a homogeneous plate with the same low-frequency vibration. One problem in piano manufacturing is examined in relationship with the possible radiation schemes induced by the models.Comment: Research highlights: - Synthetic modelling of a piano soundboard overa broad-frequency-range (several kHz). - Quantitative agreement between predicted and measured apparent local modal density. -Modal density of a plate with non-special orthotropy and arbitrary contour. -Similar characteristic impedance between comparable pianos, over several kHz. -Extension of the subsonic regime of acoustical radiation over a wide frequency range. Journal of Sound and Vibration (2013) http://dx.doi.org/10.1016/j.jsv.2013.03.01

Single electron tunneling through high-Q single-wall carbon nanotube NEMS resonators

By first lithographically fabricating contact electrodes and then as last step growing carbon nanotubes with chemical vapour deposition across the ready-made chip, many potential contamination mechanisms for nanotube devices can be avoided. Combining this with pre-defined trenches on the chip, such that the nanotubes are freely suspended above the substrate, enables the formation of highly regular electronic systems. We show that, in addition, such suspended ultra-clean nanotubes provide excellent high-frequency and low-dissipation mechanical resonators. The motion detection mechanism of our experiment is discussed, and we measure the effect of Coulomb blockade and the back-action of single electron tunneling on the mechanical motion. In addition data on the mechanical higher modes is presented.Comment: 7 pages, 6 figure

Acoustics of the banjo: theoretical and numerical modelling

A previous paper [Woodhouse et al., Acta Acustica 5, 15 (2021) https://doi.org/10.1051/aacus/2021009] showed acoustical measurements of an American 5-string banjo alongside similar measurements on a guitar, revealing a strong contrast in bridge admittance. Theoretical and numerical modelling is now presented to probe the physics behind this contrast. Without the bridge and strings, the banjo membrane has a rising trend of admittance associated with its modal density, and it has a distinctive pattern of sound radiation because an ideal membrane has no critical frequency. When the bridge and strings are added to the banjo, three formants shape the amplitude envelope of the admittance. One is associated with local effects of mass and stiffness near the bridge, and is sensitive to bridge mass and the break angle of the strings over the bridge. The other two formants are associated with dynamical behaviour of the bridge, analogous to the “bridge hill” in the violin