Laboratory measurement of sound absorption of occupied pews and standing audiences

Places of worship, as well as other performing spaces or large arenas are characterized by lightweight pews or seats, with moderate or negligible upholstery, leading to very low absorption coefficients. Consequently, the audience becomes the most important sound absorbing element, capable of playing a fundamental role in determining the acoustic characteristics of the space. Consequently accurate knowledge of its acoustic properties is required for any design purpose. Several studies have been carried out with reference to audiences seated on upholstered theatre seats but there is a considerable lack of information about occupied pews. The well known difficulty of taking into account edge effects during such measurements poses further questions as well as the effect of the density of occupation, and the seasonal variations due to clothing. This paper presents the results of a series of laboratory measurements aimed at clarifying such aspects. The measurements showed that the edge effects are negligible and that total absorption is better related to the number of persons present than to the area they cover. Nonetheless, as the density grows, or when the audience is seated, there is a reduction in absorption which may be explained by the reduction in exposed body surface. Lightweight clothes show a considerable reduction in sound absorption over all the frequency bands, suggesting that significant seasonal fluctuations in reverberation time should be expected in places where the audience is the only sound absorbing surface.     

Spatial characterization of laser-induced plasmas: distributions of neutral atom and ion densities

Spatial distributions of temperature and relative number densities of Fe neutral atoms and ions in a laser-induced plasma have been measured by time- and space-resolved emission spectroscopy. The deconvolution of the intensity spectra has been carried out to obtain the local emissivity of neutral atom and ion lines as a function of the axial (z) and radial (r) coordinates. The plasma was generated with a Fe-Ni-Al alloy in air at atmospheric pressure using a Nd:YAG laser. The distributions present a dark region with negligible neutral atom and ion emissivities and densities, situated close to the sample surface at axial distances z1 mm. In the emitting region, the temperature has its maximum at the plasma axis (16000 K) and shows a monotonous decrease towards the border (6000 K). The neutral atoms and ions occupy regions corresponding to a great extent to different radial positions. At an axial distance z=1.8 mm, at which the maximum emissivities are produced, the maximum of the ion density is at r0.6 mm, whereas the neutral atom density maximum is at r1.3 mm. PACS 52.70.kz; 52.50.Jm; 52.38.Mf     

Spatial distributions of the number densities of neutral atoms and ions for the different elements in a laser induced plasma generated with a Ni-Fe-Al alloy

Spatially-resolved emission spectroscopy, including spatial devonvolution of the spectra, has been used to determine the three-dimensional distributions of the relative number densities of neutral atoms and ions of the elements present in a laser-induced plasma generated with a Ni-Fe-Al alloy. The method is based on the precise measurement of the local electronic temperature from Saha–Boltzmann plots constructed with Fe I and Fe II lines. The plasma was generated in air at atmospheric pressure using a 1064-nm Nd:YAG laser, and the emission was detected in the time window 3.0–3.5 μs. The ionization fraction was very high (above 0.9) for the three elements in the sample, only decreasing behind the expanding plasma front. The relative number densities were obtained from the emissivities of selected elemental lines as well as the temperature. The error in this procedure was estimated, and it was found that it is largely due to the uncertainties in the transition probability values used. The spatial distributions of the total relative number densities of the three elements were shown to coincide within the error, a result which is relevant to the development of models of plasma emission used in analytical applications. The ratios of the total number densities of the elements in the plasma were compared to their concentration ratios in the sample; however, the relatively high errors in the relative number densities did not permit any definitive conclusions to be drawn about the stoichiometry of the laser ablation process.