Abstract: To reveal the mechanism of ultrasonic multiple scattering by densely packed scatterers,the two-dimensional(2D) problem of wave transmission through densely packed and parallelly oriented cylinders is examined in water.By analyzing the characteristics of the ballistic wave packet and the scattered wave,the contributions of multiple scattering are identified.A group of non-contact cylinders,whose sizes are evenly distributed,locations are randomly determined,and has a density of~100/cm² together with a surface fraction of~0.53,is considered.To better understand the characteristics of the transmitted signals where temporal randomness is expected,signals obtained from multiple models are averaged for analysis.As 2.5 MHz broadband ultrasonic pulses incident upon cylinder groups,the transmitted ballistic packets are centered at 0.5 MHz in the frequency domain,while temporal coherence of these packets are observed for different cylinder groups.The scattered waves arrive after the ballistic wave and their frequencies are similar to those of the incident waves,while temporal incoherence is found for different cylinder groups.The frequency dependent attenuation coefficient of the ballistic pulse qualitatively agrees with that determined from the independent scattering approximation.For the scattered wave,its temporal variation in acoustic intensity can be described using a diffusion model.