Influence of loudspeaker directivity and measurement geometry on direct acoustic levels over façades for acoustic insulation tests with the International Standard ISO 140-5

The International Standard ISO 140-5 on field measurements of airborne sound insulation of façades establishes that the directivity of the measurement loudspeaker should be such that the variation in the local direct sound pressure level (ΔSPL) on the sample is ΔSPL < 5 dB (or ΔSPL < 10 dB for large façades). This condition is usually not very easy to accomplish nor is it easy to verify whether the loudspeaker produces such a uniform level. Direct sound pressure levels on the ISO standard façade essentially depend on the distance and directivity of the loudspeaker used. This paper presents a comprehensive analysis of the test geometry for measuring sound insulation and explains how the loudspeaker directivity, combined with distance, affects the acoustic level distribution on the façade.The first sections of the paper are focused on analysing the measurement geometry and its influence on the direct acoustic level variations on the façade. The most favourable and least favourable positions to minimise these direct acoustic level differences are found, and the angles covered by the façade in the reference system of the loudspeaker are also determined. Then, the maximum dimensions of the façade that meet the conditions of the ISO 140-5 standard are obtained for the ideal omnidirectional sound source and the piston radiating in an infinite baffle, which is chosen as the typical radiation pattern for loudspeakers.Finally, a complete study of the behaviour of different loudspeaker radiation models (such as those usually utilised in the ISO 140-5 measurements) is performed, comparing their radiation maps on the façade for searching their maximum dimensions and the most appropriate radiation configurations.     

The Universal Vassiliev Invariant for the Lie Superalgebra

We compute the universal weight system for Vassiliev coming from the Lie superalgebra applying the construction of [13]. This weight system is a function from the space of chord diagrams to the center Z of the universal enveloping algebra of , and we find a combinatorial expression for it in terms of the standard generators of Z. The resulting knot invariants generalize the Alexander-Conway polynomial. Received: 4 July 1996 / Accepted: 21 August 1996     

69,71Ga NMR studies of a superconducting Ga 84 cluster compound

We present ^69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound Ga ₈₄ [ N ( SiMe ₃ ) ₂ ] ₂₀ Li ₆ Br ₂ ( thf ) ₂₀ ^. 2 toluene, which is composed of ligand-coordinated Ga₈₄ metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below T _c ~ 7.2 K. Our preliminary results between 10-300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate T ₁ ^-1 follows the Korringa law ⁶⁹ ( T ₁ T ) ^-1 = 0.36 s ^-1 K ^-1 , but with a relaxation rate approximately three times smaller than in bulk -Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to ~ 10 ³ K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates.     

Importance of acoustic shielding in sonochemistry

It is well known that sonochemistry is less efficient at high acoustic intensities. Many authors have attributed this effect to decoupling losses and shielding of the acoustic wave. In this study we investigate both phenomena for a 20 kHz ultrasound field with an intensity ranging from 40 to 150 W/cm2. Visualization of the bubble cloud has demonstrated that the void fraction below the ultrasound horn increases more than proportional with increasing power input. Nevertheless, the energy coupling between the horn and the liquid remains constant; this implies that decoupling losses are not reinforced for larger bubble clouds. On the contrary, microphone measurements have shown that due to the larger bubble cloud a substantial part of the supplied energy is lost at high power inputs. In striving towards more efficient sonochemistry, reduction of shielding appears as one of the major challenges.     

Rota–Baxter Algebras and New Combinatorial Identities

The word problem for an arbitrary associative Rota–Baxter algebra is solved. This leads to a noncommutative generalization of the classical Spitzer identities. Links to other combinatorial aspects are indicated.      

Spectroscopy of the hyperfine structure of antiprotonic 4He and 3He

The spin magnetic moment $\mu^{\overline{p}}_{s}$ of the antiproton can be determined by comparing the measured transition frequencies in $\overline{p}^4$ He^?+? with three-body QED calculations. A comparison between the proton and antiproton can then be used as a test of CPT invariance. The highest measurement precision of the difference between the proton and the antiproton spin magnetic moments to date is 0.3%. A new experimental value of the spin magnetic moment of the antiproton was obtained as $\mu^{\overline{p}}_{s} = -2.7862(83)\mu_{N}$ , slightly better than the previously best measurement. This agrees with $\mu^{p}_{s}$ within 0.24%. In 2009, a new measurement with antiprotonic ³He has been started. A comparison between the theoretical calculations and experimental results would lead to a stronger test of the theory and address systematic errors therein. A measurement of this state will be the first HF measurement on $\overline{p}^3$ He^?+?. We report here on the new experimental setup and the first tests.