Noise belongs among the negative phenomena in our environment and can have a negative effect on human health. Excessive noise occurs in many areas, e.g. in transport, aeronautics, and during manufacturing processes. It is often necessary to meet specified noise limits. Acoustic measurements are carried out in many cases, e.g. in order to evaluate the sound absorption properties of investigated materials and acoustic conditions in a given environment. Measurements of extremely low sound pressure levels can be used to describe the acoustic environment in an acoustic laboratory or to measure noise emissions from quiet machines and equipment. Furthermore, measurements of extremely low noise levels are limited by the thermal noise of the measurement chain. If the sound pressure levels are lower than the thermal noise of the measurement chain, then the measurement is not correct. This paper describes the measurement of extremely low sound pressure levels using an improved correlation function method.