Beat 4 : A wave phenomenon which occurs when two or more waves of different frequencies become superimposed. The resultant wave has amplitude maxima ("beats") at the frequency equal to the difference of the frequencies of the initial waves
Intergovernmental Oceanographic Commission IOC technical series 37, Tsunami Glossary (A glossary of terms and acronyms used in the tsunami literature - English), , United Nations Educational, Scientific and Cultural Organization UNESCO, Paris, France 1991. http://unesdoc.unesco.org/images/0008/000885/08853 8eb.pdf, (ND/ Tsunami Sci - Brit) {590}