Leo Kärkkäinen

The Equation of state for two flavor QCD at N t = 6

We calculate the two flavor equation of state for QCD on lattices with lattice spacing a = (6T) −1 and find that cutoff effects are substantially reduced compared to an earlier

On the sound field of an oscillating disk in a finite open and closed circular baffle

Equations describing the radiation characteristics of a rigid disk in a finite open baffle are derived using a method similar to that used by Streng for a circular membrane based upon the dipole part of the Kirchhoff–Helmholtz boundary integral formula. In this case, however, a power series solution to the radiation integral is derived in order to eliminate the need for numerical integration. Hence, a set of simultaneous equations is obtained by simply equating the coefficients of the power series, which leads to two mathematical functions, one real and one imaginary, that can be applied to any radial velocity distribution. This provides an alternative method to obtain the sound scattered by a disk or the complementary hole in an infinite resilient screen according to Babinet’s principle. Using the principle of superposition (or Gutin concept), it is shown how the sound radiation characteristics of a disk radiating from just one side can be obtained by combining the radiation field of a disk in a finite b...

On the sound field of a circular membrane in free space and an infinite baffle

An enhanced method for calculating the radiation characteristics of a tensioned circular membrane in free space is presented using an analytical solution to the infinite integral in the free-space Green’s function in cylindrical coordinates. This enables direct calculation of the surface pressure series coefficients by equating the coefficients of the resulting Bessel series in a set of simultaneous equations. Eliminating both numerical integration and least-squares minimization improves calculation speed and accuracy. An infinite baffle is introduced to provide an indication of what the theoretical limit of the bass performance would be using a very large enclosure. Furthermore, analytical solutions to the pressure field integrals are presented. A force transmission coefficient is introduced, which is the ratio of the total radiation impedance to the motional impedance. The motional, radiation, and diaphragm impedances of the damped membrane are calculated, together with the near- and far-field pressure ...

On the forces in single-ended and push-pull electret transducers

The equations for the electromechanical force conversion in single-ended and push-pull electret transducers are derived. Traditionally, the charge distribution has been modeled as a concentrated layer at an arbitrary distance from the surface of the dielectric. For the purpose of this analysis, a negative charge is assumed to be evenly distributed throughout the dielectric. The membrane has a conductive coating in which a positive charge is induced, giving an overall dipole charge. The resulting formulas are used to derive the voltage sensitivity of a microphone and the equivalent electrical circuit for the electromechanical transduction part of a microphone or loudspeaker. An equivalent external polarizing voltage is then derived that would produce the same driving force in a conventional electrostatic loudspeaker without a stored charge. The condition for the static stability of a circular electret membrane is also determined.

Thermodynamics for two flavor QCD

Abstract We conclude our analysis of the Nt = 6 equation of state (EOS) for two flavor QCD, first described at last years conference. We have obtained new runs at amq = 0.025 and improved runs and amq = 0.0125. The results are extrapolated to mq = 0, and we extract the speed of sound as well. We also present evidence for a restoration of the SU(2)×SU(2) chiral symmetry just above the crossover, but not of the axial U(1) chiral symmetry.

SIMULATION OF THE TRANSFER FUNCTION FOR A HEAD-AND-TORSO MODEL OVER THE ENTIRE AUDIBLE FREQUENCY RANGE

In this study, a method for simulating the transfer function of a head-and-torso model over the entire audible frequency range is introduced. The simulation method uses the ultra-weak variational formulation (UWVF) which is a finite element type method tailored for wave problems. In particular, the UWVF uses plane wave basis functions which better approximate the oscillatory field than a polynomial basis used in the standard finite element methods (FEM). This leads to reduction in the computational complexity at the high frequencies which, accompanied with parallel computing, extends the feasible frequency range of the UWVF method. The accuracy of the new simulation tool is investigated using a simple spherical geometry after which the method used for preliminary HRTF simulations in the geometry of a widely used head-and-torso mannequin.

On the sound field of a shallow spherical shell in an infinite baffle.

A method is presented for calculating the far field sound radiation from a shallow spherical shell in an acoustic medium. The shell has a concentrated ring mass boundary condition at its perimeter representing a loudspeaker voice coil and is excited by a concentrated ring force exerted by the end of the voice coil. A Greens function is developed for a shallow spherical shell, which is based upon Reissners solution to the shell wave equation [Q. Appl. Math. 13, 279-290 (1955)]. The shell is then coupled to the surrounding acoustic medium using an eigenfunction expansion, with unknown coefficients, for its deflection. The resulting surface pressure distribution is solved using the King integral together with the free space Greens function in cylindrical coordinates. In order to eliminate the need for numerical integration, the radiation (coupling) integrals are solved analytically to yield fast converging expansions. Hence, a set of simultaneous equations is obtained which is solved for the coefficients of the eigenfunction expansion. These coefficients are finally used in formulas for the far field sound radiation.

A dipole loudspeaker with a balanced directivity pattern.

Analytical equations describing radiation characteristics of an oscillating ring in a circular finite baffle are derived, including the limiting case of a dipole point source at the center. An oscillating sphere would represent the ideal dipole source, having a constant directivity pattern at all frequencies, but would be inconvenient to realize especially in portable devices. It is found that a planar piston with uniform surface velocity but variable phase arranged to emulate the sphere does not have such a smooth on-axis response as the sphere. Instead a planar piston with the same phase distribution but uniform pressure represents an ideal planar source with a smooth on-axis response and near constant directivity. The surface velocity is plotted and it is then shown that a similar response can be achieved using a finite number of concentric rings based on this velocity distribution.

Comparison of spheroidal and eigenfunction-expansion trial functions for a membrane in an infinite baffle

The on-axis far-field pressure response of a circular membrane in an infinite baffle when driven by a uniformly distributed electrostatic force is calculated using two different trial functions for the surface velocity distribution. The first is an expansion based upon a solution to the free space wave equation in oblate spheroidal coordinates, which has already been derived in a previous paper [J. Acoust. Soc. Am. 120(5), 2460-2477 (2006)], and the second is a membrane eigenfunction expansion (or Bessel series), which is rigorously derived in this letter. Although the latter can be used as a basis for calculating a number of different radiation characteristics such as the radiation impedance or directivity, etc., only the on-axis far-field sound pressure is considered here. The results are compared and discussed.

Deep convolutional neural networks for estimating porous material parameters with ultrasound tomography

The feasibility of data based machine learning applied to ultrasound tomography is studied to estimate water-saturated porous material parameters. In this work, the data to train the neural networks is simulated by solving wave propagation in coupled poroviscoelastic-viscoelastic-acoustic media. As the forward model, a high-order discontinuous Galerkin method is considered, while deep convolutional neural networks are used to solve the parameter estimation problem. In the numerical experiment, the material porosity and tortuosity is estimated, while the remaining parameters which are of less interest are successfully marginalized in the neural networks-based inversion. Computational examples confirm the feasibility and accuracy of this approach.