The purpose of this standard is to give, in table form, the quantities and units of acoustics. When two symbols are given for the same quantity, without special dis inction, they can be used indifferently. SI units are given in bold; other units are given in common characters. The abbreviations "ni, de, en" mean "Dutch, German, Englis " respectively (is 639:1988). The definitions of the units are given in the NBN X 02-001 standard. SPECIAL REMARKS The explanations in the “Definitions and remarks” column for the quantities generally presuppose that the systems are linear. When it is necessary to use subscripts to avoid confusion between similar symbols in different fields, the subscript "a" is recommended in the case of acoustics. In this standard, the level difference is defined as the natural logarithm of the ratio of two quantities of the same nature. It is a dimensionless magnitude. Although the coherent unit corresponding to this definition is the number 1, it is convenient to use, in practice, the special name "néper". The expression of the level difference as a function of decimal logarithms results from the definition. The relationship between the neper and the decibel, units ordinarily used to express the difference in level, is written, ? In 10.. 1dB=-------Np 20 The relationship between the bel and the decibel is written as 1 dB = 10"1 B, but only the decibel is given in this standard, the bel not being not often used. This standard also uses the symbol Ib which means "binary logarithm" or "base 2".