Lmax & Peak should not be confused when measuring noise levels. Lmax is the RMS (root mean squared) maximum level of a noise source or environment where peak is the maximum level of the raw noise source.
RMS, or root mean squared, is a measure of the magnitude of the varying noise source quantity. The name derives from the calculation, from either a series of discreete values or a continuous varying function, of the square root of the mean of the squares of the values.
The reason for using RMS values gives a clearer understanding of a noise level, making calculations and measurements easier for a noise source waveform which is changing constantly in its magnitude.