Over the last decades and since the’60 s, civil aircraft noise has been mitigatedsignificantly:theacousticalenergyemittedbyaveragemoderncivilaircraft—directlyrelated to the noise it is responsible for—is somehow only 10% of an aircraft of thesame size in the’60 s. This success was made possible thanks to specific researchefforts which focused on reducing the aircraft noise at its source. wenty years ago, at the beginning of the twenty-first century, the InternationalCivil Aviation Organisation (ICAO) promoted the “balanced approach” for morestringent international regulations on air traffic noise. In Europe, the AdvisoryCouncil for Aeronautics Research in Europe (ACARE) targeted a further 50% reduc-tion by 2020 and 65% by 2050 of the perceived noise, with respect to the year 2000as reference.To understand the basics of aviation noise and its challenges, this chapter aims todescribe simply and in short the key knowledge related to it.Acoustics is a scientific discipline, which combines the physical aspects fordescribing mechanical phenomena and the physiological aspects for characterisingthe auditory sensation. Therefore, physical mechanics and neurosciences are insep-arable for understanding and interpreting the auditory perception induced by noiseand, finally, for evaluating the annoyance. To deal with this last aspect, which goesbeyond physics, it is necessary to understand other sciences, human, psychologicaland sociological.Many noise sources surround us, like the manufacturing industry, automotive,railway, and, of course, aviation; they all generate annoyance. The main physical quantities of acoustics are sound pressure, -power and -intensity.The physical units of these quantities, in the International System, are respectivelyPascal (Pa), Watt (W) and Watt per square meter (W / m2). The acoustic power of asmall drone is about 10–6W, while the one of a jet-aircraft is about 106W. Such a largerange of values to consider brings the acoustician to express the physical quantitiesusing the Logarithm function and a value which serves as a reference because thisapproach compresses the scales into a meaningful interval.