The properties of a soil structure may be examined in various manners. As well as a study of the stability, a knowledge of the geometry of the volume of air filled pores is often needed. The most common measurements, like those of porosity and flow resistance to gases do not permit a detailed description of this pore volume. Since wave phenomena are characterized by three independent variables, viz. frequency, amplitude and phase, with frequency chosen freely, the measurement of acoustical characteristics of the air in the soil offers new opportunities. Also a determination of the acoustical properties of a porous material is non-destructive.

In chapter 1, a description is given of an interferometric method of measurement following the derivation of the wave equation. The propagation velocity of sound in air and the specific mass of air are the important physical quantities. The change in these quantities is studied from variations in the experimental conditions, such as temperature and humidity. Next the principles of the propagation of sound in porous materials are presented. For a sample of thickness l and having a rigid backing, the specific acoustic impedance Z at the free surface is given by Z = W _{m} coth(γ _{m} l), where γ _{m} is the propagation constant for acoustical waves in the sample and W _{m} is the specific acoustic wave impedance. Z, W _{m} and γ _{m} are complex quantities. Z may be measured in an interferometer and W _{m} and γ _{m} characterize the sample material. γ _{m} and W _{m} considered as functions of frequency give more information on pore geometry than may be obtained from static measurements. The loci of the function in two types of a complex plane is studied. Finally the behaviour of this function in the complex planes is shown with some examples.

Chapter 2 contains a discussion of the measuring equipment used and of the calibration of the measuring set-up. After a discussion of the measuring techniques, the sources of error are evaluated.

Chapter 3 deals with the propagation of waves in porous materials. Independent determination of W _{m} and γ _{m} proves impossible for soil samples. A method for this, described in the literature, is rejected on the grounds of inadequate accuracy. An alternative approach is followed: the material is described by a mathematical model and the parameters in the model are considered as the characteristic quantities for pore geometry. The models assume comparatively simple geometries and may be considered an extension of the work of previous authors. In addition a new projection plane for the determination of γ _{m} and W _{m} by a graphical method is discussed. Use of the plane is confined to cases where the sample thickness may be varied. Also, formulas are derived with which the acoustical properties of prismatic of structures soils can be studied. Finally, the applicability of scale rules and the possibility of an electric- acoustical equivalent network are examined for the sample material. Neither approach seems promising.

Chapter 4 starts with a discussion of the problems to be expected on the com parison of calculated and measured curves for Z. Somes series of measurements are discussed. The mathematical models selected yield a reasonably good relation ship between the theoretical and measured values. A short critical discussion is given on the feasibility of an extension of the mathematical model.

In conclusion a brief discussion is devoted to measurements on layers whose solid phases can no longer be considered as rigid, such as layers of mulch and straw. Some results obtained with straw are dealt with.