Noise in miniature microphones

The internal noise spectrum in miniature electret microphones of the type used in the manufacture of hearing aids is measured. An analogous circuit model of the microphone is empirically fit to the measured data and used to determine the important sources of noise within the microphone. The dominant noise source is found to depend on the frequency. Below 40 Hz and above 9 kHz, the dominant source is electrical noise from the amplifier circuit needed to buffer the electrical signal from the microphone diaphragm. Between approximately 40 Hz and 1 kHz, the dominant source is thermal noise originating in the acoustic flow resistance of the small hole pierced in the diaphragm to equalize barometric pressure. Between approximately 1 kHz and 9 kHz, the noise originates in the acoustic flow resistances of sound entering the microphone and propagating to the diaphragm. To further reduce the microphone internal noise in the audio band requires attacking these sources. A prototype microphone having reduced acoustical noise is measured and discussed.     

Direct catalytic asymmetric alpha-amination of aldehydes

The first direct catalytic asymmetric alpha-amination of aldehydes is described herein. alpha-Unbranched aldehydes react in this novel proline-catalyzed reaction with dialkyl azodicarboxylates to give alpha-amino aldehydes in excellent yields and enantioselectivities.     

Intrinsic room-temperature electrophosphorescence from a pi-conjugated polymer

Electrically induced phosphorescence from a poly(para-phenylene) ladder-type polymer is observed for the first time and characterized using time resolved spectroscopy. Short-lived phosphorescence is also observed in gated fluorescence spectra and is found to be quenched reversibly by oxygen. Thermally activated triplet diffusion to covalently bound palladium sites, which are formed at a concentration of about 80 ppm in a side reaction during polymer synthesis, is believed to be the cause of this novel effect, which suggests a new approach to the design of efficient electroluminescent materials.     

Zur Beeinflussung des Hagelkornwachstums

Summary To decrease the accretion rate of a growing hailstone its deposit has to be made dry. Under such conditions a spherical form is prefered to an ellipsoidal one. The growth rate of the former is lower than that of the latter to a factor of two. The accretion rate can be slowed down further when the small cloud particles are partially frozen. In a solid state they will mostly be rejected from the surface of the (dry) hailstone and do not contribute to growth.In paragraph 4 calculations are made about the proportion of cloud particlesV which must be frozen to arrive at a dry deposit. The variables are air temperaturet _L , hailstone diameterd, free H₂O-contentw _f , and air pressurep. The results are shown in Figures 1, 2, and 3 and demonstrate the high ratio of cloud droplets which should be frozen to reduce the growth rate noticeably. Figure 4 is valuable for hailstones growing in a cloud model as postulated byLudlam.This decrease in accretion rate will normally lower the hail damage. But there exists another method which produces hailstones with a very high rate of liquid water. Such spongy particles burst as soon as they hit the ground. The conditions for such a procedure are not yet elaborated but are quite dissimilar to those described above.     

Catalytic asymmetric intramolecular alpha-alkylation of aldehydes

The development of a general catalytic asymmetric aldehyde alpha-alkylation reaction constitutes a major challenge in organic synthesis. Here, we report the first and successful approach toward its solution: (S)-alpha-methyl proline catalyzes the intramolecular alkylation of various halo aldehydes to the corresponding formyl cyclopentanes, -cyclopropanes, or -pyrrolidines in excellent yields and enantioselectivities. Most remarkably, racemization, aldolization, or catalyst alkylation do not occur to any significant extend, further illustrating the power, mildness, and profound selectivity of enamine catalysis.     

Quantum mechanical predictions of the stereoselectivities of proline-catalyzed asymmetric intermolecular aldol reactions

Quantum mechanical calculations were employed to predict the ratio of four stereoisomeric products expected from two complex reactions involving the aldol reactions of cyclohexanone with benzaldehyde or with isobutyraldehyde catalyzed by (S)-proline. Experimental tests of these predictions provide an assessment of the state-of-the-art in quantum mechanical prediction of products of complex organic reactions in solution.     

Untersuchung des Saccharins

Ohne Zusammenfassung