HIGH-FREQUENCY ATMOSPHERIC ACOUSTIC SOUNDERS

The use of a relatively high frequency for atmospheric sounding is discussed. Details are given of two acoustic sounders operating at frequencies of 4.7kHz and 6.5kHz. Conventional acoustic sounders usually operate at frequencies below about 2kHz. This is primarily because the atmospheric attenuation of sound at these frequencies is relatively small and greater acoustic output powers can be obtained from a given size of antenna in this frequency range. However, there are advantages to be gained from the use of sounders operating at somewhat higher frequencies.For example, the performance of a sounder can often be degraded by environmental noise, such as that produced by traffic and aircraft. This was found to be a limitation in the case of a sounder operating at 1730Hz (Asimakopoulos and Cole 1977) at University College London. A high frequency sounder operating at 4.7kHz is found to be less adversely affected by these noise sources because the acoustic spectra of both traffic and aircraft noise contain less energy at higher frequencies. The reverberation time of the acoustic antenna after the transmission of a pulse determines the minimum height from which an echo can be received. This time is shorter for a physically smaller high frequency antenna and hence a region of the atmosphere much closer to the ground can be probed. © 1979.