Ricardo R. Boullosa

Vibration measurements in the classical guitar

Abstract Some measurements are described that pertain to the high frequency behaviour of the vibrations of the string and soundboard of the guitar.Three isolated soundboards of different woods were compared with respect to their early decay times (by means of measurements of their vibrational impulse responses), in order to assess its possible use as an objective measure of the reverberation time of the plates. The results of all these measurements show that high frequency vibrations and radiation occurs significantly from 1 to 14 kHz, but that the sound radiation has harmonics at frequencies up to 20 kHz. This report does not produce any experimental evidence as to the importance of the high frequency region from the subjective point of view, or to the correlation of the subjective impression of reverberation of the plates with measured data of early decay times, as these aspects were not addressed at this point.

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.

An exploratory study of community noise levels in Mexico City

Abstract A community noise survey undertaken in a small area of Mexico City is described. The data was collected at 23 sites for three general time periods during a day. The sampling was done in a quasi-random way within the three time periods and at different dates during February–March 1985. Several statistical analyses were done on all data. The results are presented in terms of the average of several statistical descriptors, and L dn for different classes of site.

Radiation efficiency of a guitar top plate linked with edge or corner modes and intercell cancellation

This paper was based on a theoretical framework to determine strong and weak radiation by a guitar top plate, vibrating through deflections hard to analyze: multipolar mode shapes. The air-structure interaction was examined in terms of edge modes or corner modes, and considering even or odd number modes. A numerical model was implemented and experimentally calibrated, exhibiting several advantages exploring the coupling between vibratory and acoustic waves in a top plate. Two analyses were applied detecting high or low radiation efficiency for the structure. First, the addition of volume velocity for odd numbers of poles and cancellation for even numbers were examined. In fact, both behaviors can happen at the same time, as it was shown for a corner radiator case used as an example. Second, the ratio between bending and acoustic wavenumbers was explored. To illustrate the importance of this ratio, some theoretical features of a more efficient radiator than the corner mode were exposed in an edge mode example. Labeling multipolar mode shapes as efficient or inefficient radiators showed to be a useful alternative analyzing the top plate behavior. It can be applied knowing the nodal lines of the vibration pattern and estimating the bending and acoustic wavelengths.

Some acoustical properties of the anechoic chamber at the Centro de Instrumentos, Universidad Nacional Autónoma de México

Abstract Some of the basic acoustical properties of an anechoic chamber recently brought to operation are described. The background noise levels and sound insulation of doors were measured. The free field conditions in different directions and frequencies were determined using nondirectional sources.

A simple measuring system for vibrations at low amplitudes

Abstract A simple laser system for the measurement of low amplitudes of vibration is presented which can be used for calibration purposes in vibration pickups. It is based on the amplitude measurement of the fundamental frequency component in the photocurrent spectrum (the so-called J 1 Bessel method for the measurement of very small amplitudes of vibration) using a differential optical configuration. The dynamic range in the upper region can be extended up to the micrometer region by judicious choice of the beam crossing angle and the wavelength of the laser used. In this way a region of low amplitudes normally excluded in the aforementioned method is covered. The stability of the system during the measurement is assisted by means of a digital controller.

Long-term average spectrum of Spanish

Abstract The long-term average spectrum of Spanish, as spoken in Mexico, was obtained for thirty subjects—fifteen men and fifteen women. To obtain the average spectrum, use was made of a digital system comprised of an FFT analyzer interfaced to a microcomputer. All data acquisition, storage and processing was thus digital. The main features of the average spectrum are: peaks in the region of 240 Hz and 500 Hz, a −20 dB/octave slope from 0·5 kHz to 1 kHz; a flat region between 1 kHz and 2 kHz, a further decline of −10 dB/octave to ≈3 kHz and, finally, a large flat region (within 5 dB) from 3 kHz up to 10 kHz. These results agree reasonably well with some other studies for English. The technique employed here is thus adequate for this kind of study, being relatively fast and easy to implement.

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.