Propagation Of Sound Signals In Quasi-porous Media And Analyze The Sound Properties Of Pests

The technique of detection acoustic signals of pests in stored grain represents the newly emerging field of Insect Acoustics in recent years. Compared with traditional inspection methods, the newly technique is a quick and practical method which is attracting increasing attention. It already can quantify the infestation in USA by detection the acoustic signals of pests that feed inside grain kernels. But there have been relatively few research efforts in identification the species of grain pests by the acoustic signals. To detect the sound produced by pest activity inside the grain it is necessary to understand the propagation properties of sound in various types of grain. This paper describes some experimental investigations of propagation properties of sound in various types and thickness of grain samples using the method of standing wave tube for the first time, extends the sound propagation theory in porous media and develops the sound propagation model in grain, analyzes and identifies the creeping sound signals of five species of pests. These researches provide us a basis for forecasting the pests in grain in earlier stage, accurately utilizing pesticides, improving the efficiency of prevention and reducing the loss of stored grain.The sound absorption properties of various types and thickness of grain are measured by means of the standing wave tube technique. The following conclusions can be drawn: For the same thickness the larger grains has a smaller sound absorption coefficient than the smaller grains. The kernel in spheroid has a smaller sound absorption coefficient than in non-spheroid. The thickness of grain sample is an important factor effecting on the sound absorption coefficient. The average sound absorption coefficients of larger grains are smaller than the small grains in general. The properties of sound absorption coefficient of grain are in close relationship with the shape and size of kernel, and have no relation with the texture of solid grain.We consider there are two kinds of mechanism causing the attenuation of sound in grain. One kind of mechanism regards the grain as quasi-porous media. It was the viscous resistance and heat conduction in the narrow passageways between the grain kernels that convert the sound energy into heat energy and lose it. The other kind ofmechanism regards the grain as a combined resonator. We regard every layer of grain as a lot of Helmholtz resonators in parallel, and regard different layer of grains as the paralleling resonators in series. The grain is a combined resonator in series and parallel.Based Biot's theory of acoustic propagation in porous media this paper applies Johnson's model to grain media. Considering the viscous interaction between the kernel and the air inside the grain the paper shows the dynamic tortuosity function in grain. Considering the thermal exchanges between the kernel and the air the paper shows the dynamic permeability function in grain. We consider the pores in grain both cylindrical pore and parallel slits. Further there has been the formula of the sound absorption coefficient of grain. The agreement between calculation and measurement is good.This paper analyzes and identifies the creeping sound signals of five species of pests. We collect the signals using a recording. The signals are transformed into digital signals and sent to a computer. Digital signal processing includes amplifying the sound signals, low-pass filtering, wavelet transform for de-noise and analyzing power spectra using MATLAB software. For different species of pests the acoustic characteristics are different in time domain and frequency domain. In time domain it is different in the number of sound pulse. In frequency domain it is different in the amplitude and main peak frequency range. These differences may be caused by the weight and creeping speed of different species of pests.