11,129 research outputs found
Oscillation threshold of a clarinet model: a numerical continuation approach
This paper focuses on the oscillation threshold of single reed instruments.
Several characteristics such as blowing pressure at threshold, regime
selection, and playing frequency are known to change radically when taking into
account the reed dynamics and the flow induced by the reed motion. Previous
works have shown interesting tendencies, using analytical expressions with
simplified models. In the present study, a more elaborated physical model is
considered. The influence of several parameters, depending on the reed
properties, the design of the instrument or the control operated by the player,
are studied. Previous results on the influence of the reed resonance frequency
are confirmed. New results concerning the simultaneous influence of two model
parameters on oscillation threshold, regime selection and playing frequency are
presented and discussed. The authors use a numerical continuation approach.
Numerical continuation consists in following a given solution of a set of
equations when a parameter varies. Considering the instrument as a dynamical
system, the oscillation threshold problem is formulated as a path following of
Hopf bifurcations, generalizing the usual approach of the characteristic
equation, as used in previous works. The proposed numerical approach proves to
be useful for the study of musical instruments. It is complementary to
analytical analysis and direct time-domain or frequency-domain simulations
since it allows to derive information that is hardly reachable through
simulation, without the approximations needed for analytical approach
Constraining Primordial Magnetism
Primordial magnetic fields could provide an explanation for the galactic
magnetic fields observed today, in which case they may also leave interesting
signals in the CMB and the small-scale matter power spectrum. We discuss how to
approximately calculate the important non-linear magnetic effects within the
guise of linear perturbation theory, and calculate the matter and CMB power
spectra including the SZ contribution. We then use various cosmological
datasets to constrain the form of the magnetic field power spectrum. Using
solely large-scale CMB data (WMAP7, QUaD and ACBAR) we find a 95% CL on the
variance of the magnetic field at 1 Mpc of B_\lambda < 6.4 nG. When we include
SPT data to constrain the SZ effect, we find a revised limit of B_\lambda < 4.1
nG. The addition of SDSS Lyman-alpha data lowers this limit even further,
roughly constraining the magnetic field to B_\lambda < 1.3 nG.Comment: 12 pages, 9 figure
Differential Phase Estimation with the SeaMARC II Bathymetric Sidescan Sonar System
A maximum-likelihood estimator is used to extract differential phase measurements from noisy seafloor echoes received at pairs of transducers mounted on either side of the SeaMARC II bathymetricsidescan sonar system. Carrier frequencies for each side are about 1 kHz apart, and echoes from a transmitted pulse 2 ms long are analyzed. For each side, phase difference sequences are derived from the full complex data consisting of base-banded and digitized quadrature components of the received echoes. With less bias and a lower variance, this method is shown to be more efficient than a uniform mean estimator. It also does not exhibit the angular or time ambiguities commonly found in the histogram method used in the SeaMARC II system. A figure for the estimation uncertainty of the phasedifference is presented, and results are obtained for both real and simulated data. Based on this error estimate and an empirical verification derived through coherent ping stacking, a single filter length of 100 ms is chosen for data processing application
Acoustic Holographic Rendering with Two-dimensional Metamaterial-based Passive Phased Array.
Acoustic holographic rendering in complete analogy with optical holography are useful for various applications, ranging from multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. Conventional approaches rely on a large number of active transducers and phase shifting circuits. In this paper we show that by using passive metamaterials as subwavelength pixels, holographic rendering can be achieved without cumbersome circuitry and with only a single transducer, thus significantly reducing system complexity. Such metamaterial-based holograms can serve as versatile platforms for various advanced acoustic wave manipulation and signal modulation, leading to new possibilities in acoustic sensing, energy deposition and medical diagnostic imaging
Tomorrow's Metamaterials: Manipulation of Electromagnetic Waves in Space, Time and Spacetime
Metamaterials represent one of the most vibrant fields of modern science and
technology. They are generally dispersive structures in the direct and
reciprocal space and time domains. Upon this consideration, I overview here a
number of metamaterial innovations developed by colleagues and myself in the
holistic framework of space and time dispersion engineering. Moreover, I
provide some thoughts regarding the future perspectives of the area
An Investigation of Energy Transmission Due to Flexural Wave Propagation in Lightweight, Built-Up Structures
A technique to measure flexural structure-borne noise intensity is investigated. Two accelerometers serve as transducers in this cross-spectral technique. The structure-borne sound power is obtained by two different techniques and compared. In the first method, a contour integral of intensity is performed from the values provided by the two-accelerometer intensity technique. In the second method, input power is calculated directly from the output of force and acceleration transducers. A plate and two beams were the subjects of the sound power comparisons. Excitation for the structures was either band-limited white noise or a deterministic signal similar to a swept sine. The two-accelerometer method was found to be sharply limited by near field and transducer spacing limitations. In addition, for the lightweight structures investigated, it was found that the probe inertia can have a significant influence on the power input to the structure. In addition to the experimental investigation of structure-borne sound energy, an extensive study of the point harmonically forced, point-damped beam boundary value problem was performed to gain insight into measurements of this nature. The intensity formulations were also incorporated into the finite element method. Intensity mappings were obtained analytically via finite element modeling of simple structures
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