Not to speak of the wellknown ripple tank method, various methods for model experiment on room acoustics have been designed to meet the purpose of respective investigations. Here, we propose a convenient method for the study of steady state sound field. A Two dimentional model of a room, of for example 40×50×5 cm, covered with a thick glass plate is made, and the air in the model is excited by pure tone with a loud-speaker mounted in its corner. When the frequency of sound from the loud speaker is equal to the resonant frequency of the model, scattered cork dust on the model floor move intensively and make a figure formed of groups of plaits. The figure varies its shape depending on the frequency, that is to say the mode of vibration of the air in the model. By observing the cork dust figure, one can visualize the sound pressure distribution of standing waves in the model, in another words the position of pressure maxima and pressure nodal lines, just as reading a map. As regards to the particle velocity of the air, one can compare its magnitude and direction at each point in the model, from the height and the direction of cork dust plaits. In this paper, some fundamental natures of dust figure and one of the applications of this method are reported. Relations between fineness of cork dust and space of dust plaits, height of plaits and sound particle velocity measured in a one dimentional model. Using two dimentional models, pressure contours are measured and compared with dust figures in various cases of frequency and different shapes of models. In the last section of this paper, the deformation of the pressure distribution and the frequency shift of normal modes from the rectangular case, when some types of projection are planted on one side of the rectangular model are reported. Mounting several loud-speakers apart, separate observations on each one of the degenerated modes of vibration were made possible. The dust figure method was very convenient for instant discrimination of vibration in the model room.
