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 Green's 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.

Transfer of angular momentum to matter from acoustical vortices in free space.

An experimental demonstration of the mechanical transfer of orbital angular momentum to matter from acoustical vortices in free field is presented. Vortices with topological charges l=+/-1 and l=+/-2 were generated and a torsion pendulum was used to study the angular momentum transfer to hanging disks of several sizes. This allowed us to make a comparative study of the effective acoustical torque in terms of topological charge of the vortex, the disk radius, and its position along the main propagation axis. A theoretical discussion of the generated sound fields is also provided.

[Noise conditions inside waiting rooms of primary care clinics]. / Ruido en salas de espera de unidades médicas.

OBJECTIVES: to explore the acoustic noise conditions of the waiting rooms in primary care facilities and to evaluate whether the ambient noise level was within the recommended levels for these facilities. METHODOLOGY: the waiting rooms of four primary care clinics located in the south of Mexico City were tested. Several recording points homogeneously distributed and close to the seats of the users were chosen. In each recording point, four recordings of the ambient noise were carried out in intervals of 20 to 40 minutes; this was done during the provision of care. The day in which the recordings were made was randomly selected. The values of the specifications used in the analysis of the ambient noise levels were determined in the laboratory. RESULTS: the ambient noise level of the waiting rooms in each of the four clinics was above 55 dB-A. This level is considered as the threshold of annoyance. In one of the clinics, the level exceeded 65 dB-A. The chief contributors to the noise were the conversations of the people that were at the waiting room or in adjacent rooms, and in some cases the sound from the TV sets. CONCLUSIONS: the study showed that the ambient noise level passed the upper limits indicated by The American National Standard Institute. It is appropriate to define criteria to guarantee suitable noise conditions inside health care facilities and to revise current Mexican standards.