We studied YBa2Cu3O7-x ceramic samples during external magnetic field increase from 200 to 4000 Oe at 77 K using acoustic emission and magnetization methods. We observed a small magnetic anomaly near 2500 Oe. This observed magnetic anomaly was accompanied by acoustic emission impulses of activity Ṅ equal to ∼20 s-1. These impulses are thought to be Bean's mixed-state field Hs. Based on Bean's theory, the full penetration field H* has been calculated to be ∼1200 Oe. The possible mechanism of acoustic emission signals due to interaction between flux lines during the formation of a mixed state inside a sample is discussed.