FOCUSED ACOUSTIC BEAM IN NON LINEAR MEDIUM N.S. BADHVALOV, R.V. KHOKHLOV, E.A. ZABOLOTSKAJA and I.M. ZHILEIKIN J. Phys. Colloques, 40 C8 (1979) C8-56-C8-61 DOI: 10.1051/jphyscol:1979812