On the basis of the 'sound fluctuation level' concept, a new technique and circuitry for rating environmental sound fluctuations has been developed. The proposed instrument is a properly modified and improved version of a part of a recent microprocessor-based apparatus, which directly measures both mean energy and fluctuation level of noises. The changes made to the previous apparatus concern the physical descriptor of the noise fluctuation level (Lphi ) and are based on preliminary results of physical and subjective tests in the laboratory, carried out in order to optimize the averaging time constants, which have a very important role in measurements involving sound fluctuation assessment.