Arturo Orozco Santillán

Spatially extended sound equalization in rectangular rooms

The results of a theoretical study on global sound equalization in rectangular rooms at low frequencies are presented. The zone where sound equalization can be obtained is a continuous three-dimensional region that occupies almost the complete volume of the room. It is proved that the equalization of broadband signals can be achieved by the simulation of a traveling plane wave using FIR filters. The optimal solution has been calculated following the traditional least-squares approximation, where a modeling delay has been applied to minimize reverberation. An advantage of the method is that the sound field can be estimated with sensors placed in the limits of the equalization zone. As a consequence, a free space for the listeners can be obtained.

A note on the use of novel thermoacoustic radiators for ultrasonic experiments: the importance of phase in a focused field

The use of a novel thermoacoustic transducer facilitates the study of ultrasonic fields; with this aim, a description of its simple construction is given. In particular, an experiment with a spherically curved source is presented, where the theoretical acoustic pressure obtained by a numerical approximation shows, by comparing with the experimental results, the importance of taking into consideration the phase of the radiated sound on the field produced by focusing sources. The type of thermoacoustic transducer presented in this paper radiates sound in the audible and ultrasonic regions, without resonances, up to at least 100 kHz. These features, along with their ease of construction, allow the use of large, planar or curved, acoustic radiators in laboratory experiments.

Space dependence of acoustic power and heat flux in the thermoacoustic effect

The dependence of heat flux with the perpendicular distance to a single plate in a stationary plane wave and of the acoustic power absorbed or generated is obtained. The analysis shows the simultaneous existence of zones near the plate where the fluid does work and others where work is done on the fluid. Depending on the mean temperature gradient one effect dominates over the other. An analogous behavior occurs with the hydrodynamic heat flux parallel to the plate. Also shown is the relation between the conductive heat flux perpendicular to the plate and the absorbed or generated acoustic power.

Acoustic power and heat fluxes in the thermoacoustic effect due to a travelling plane wave

The thermoacoustic effect in a single plate due to a travelling plane wave is analyzed with respect to the conditions for the refrigerator or prime mover cycle to occur. The relation between the conduction of heat along a normal to the plate and the absorbed or generated acoustic power is also shown.

Sound radiation from thermal non-resonant sources : Planar and nonplanar geometries

An acoustic source that radiates sound in the audible and ultrasonic regions, to at least 100 kHz, has been under development. These kinds of sources are very simple and easy to construct; they consist of a heated surface over a material substrate. Moreover, they have additional advantages: they can be made with plane, convex or concave radiation surfaces, and they have no resonances. We present, as examples of actual sources, a cylindrical source and a baffled rectangular piston. A comparison of the measured sound pressures with those calculated from theory, for the two examples, shows a very good agreement.

A resonance shift prediction based on the Boltzmann–Ehrenfest principle for cylindrical cavities with a rigid sphere

An investigation on the resonance frequency shift for a plane-wave mode in a cylindrical cavity produced by a rigid sphere is reported in this paper. This change of the resonance frequency has been previously considered as a cause of oscillational instabilities in single-mode acoustic levitation devices. It is shown that the use of the Boltzmann-Ehrenfest principle of adiabatic invariance allows the derivation of an expression for the resonance frequency shift in a simpler and more direct way than a method based on a Greens function reported in literature. The position of the sphere can be any point along the axis of the cavity. Obtained predictions of the resonance frequency shift with the deduced equation agree quite well with numerical simulations based on the boundary element method. The results are also confirmed by experiments. The equation derived from the Boltzmann-Ehrenfest principle appears to be more general, and for large spheres, it gives a better approximation than the equation previously reported.

The effect of the coupling between the top plate and the fingerboard on the acoustic power radiated by a classical guitar (L)

An experimental investigation on the coupling between the fingerboard and the top plate of a classical guitar at low frequencies is presented. The study was carried out using a finished top plate under fixed boundary conditions and a commercial guitar. Radiated sound power was determined in one-third octave bands up to the band of 1 kHz based on measurements of sound intensity. The results provide evidence that the way in which the fingerboard and top plate are coupled is not a relevant factor in the radiated acoustic power of the classical guitar in the studied frequency range.

Analysis of the resonance frequency shift in cylindrical cavities containing a sphere and its prediction based on the Boltzmann‐Ehrenfest principle

It is known that forces generated by high‐level acoustic waves can compensate for the weight of small samples, which can be suspended in a fluid. To achieve this, a standing wave is created in a resonant enclosure, which can be open or closed to the external medium. This phenomenon, called Acoustic levitation, has numerous applications in containerless study and processing of materials. Although it is possible to levitate a sample for long periods of time, instabilities can appear under certain conditions. One of the causes of oscillational instabilities is the change of the resonance frequency of the cavity due to the presence of the levitated object. The Boltzmann‐Ehrenfest principle is used to find an analytical expression for the resonance frequency shift in a cylindrical cavity produced by a small sphere, with kr < 1, where k is the wavenumber and r is the radius of the sphere. The validity of this expression has been investigated by means of the Boundary Element Method and experiments. In addition, ...