No abstract

SUMMARY In an eventual aim to detect background long‐period acoustic waves in the atmosphere that have to be excited incessantly by the same sources as for background free oscillations of the solid Earth, we installed a cross array of barometers in a 10‐km‐wide university forest in central Honshu. The array has 28 microbarometers employing quartz crystal resonator technology with a station spacing of about 500 m. We analysed 1‐s continuous sampling records in a time period from 2002 March to 2003 November to obtain 1‐D frequency–wavenumber spectra and 2‐D frequency–slowness spectra to detect three kinds of waves in the seismic band from 1 mHz to 0.5 Hz. We measured the dispersion curves of these waves with an assumption of stochastic stationary plane waves. The waves we detected are (1) background internal gravity waves travelling from northwest at frequencies from 1 to 5 mHz with a phase velocity of about 50 m s−1, (2) background acoustic waves traveling from northwest at frequencies from 0.01 to 0.1 Hz with a frequency‐dependent phase velocity of about 400 m s−1 at 0.1 Hz and about 800 m s−1 at 0.01 Hz and (3) microbaroms travelling from southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m s−1. Internal gravity waves (1) dominate over the random component of atmospheric turbulence in the relevant frequency range. At higher frequencies the random component with coherent length shorter than 500 m is dominant. Background acoustic waves (2) show clear annual variation with maxima in winter. The annual variation and their incident azimuths suggest their possible origins in mountain regions. Microbaroms (3) are most likely to be excited by standing ocean waves in nearby coastal regions. The power spectra of these acoustic waves were compared to the seismic spectra obtained in the same university forest. Their spectral shapes are mutually similar and change commonly with variable weather conditions, indicating that the microbaroms and microseisms have their common origins.

Infrasound from Space Shuttle II and Skylab I recorded at Palisades, New York had characteristics strikingly different from all previous signals recorded from large rockets launched at Cape Canaveral. In the case of Space Shuttle, the reentry signal is unusually weak and lacking normal low frequency components. This anomaly is traced to the horizontal attitude of the reentry boosters and their slower speed. In the case of Skylab, the reentry signal arrives unusually early and the ratio of launch to reentry signal amplitude is inverted from past values. A combination of a different launch direction and weak stratospheric sound channel is responsible for the signal anomalies.

Continuous observation of the infrasonic wave has been recently started at Aichi University of Education in Kariya City. Major interest is in the infrasounds frequently radiated into the atmosphere by the summit explosion of the volcano Sakura-jima.In detection of the infrasonic signal in the turbulent boundary layer, it is often indispensable to reduce the irregular noise caused by the turbulent fluctuation of the wind. In the present study, a new type of wind noise reducer has been designed. It is a multi-pipe line microphone, having a number of branches of pipes of equal diameter. The function of it is to take the equally-weighted spatial average of the pressure field along a straight line, and is essentially the same as the Daniels' line microphone (1959). However, the manufacturing of the multi-pipe line microphone is practically easier than the Daniels' type. Another merit of the new device is in that it is as easy to take the two-dimensional average if necessary.From several field tests, it has been shown that the wind noise is remarkably attenuated by the use of multi-pipe line microphone, although the measured noise attenuation has been smaller than the theoretical estimation based on the turbulence model given by Priestley(1965) by about 5dB in average.