VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

18 CHAPTER 1. FUNDAMENTAL CONCEPTS OF ACOUSTICSThis property is used in active noise control. E.g. : anti-noisetechnique in some headphones to reduce background noise as wellas for reducing noise generated by aircrafts in the cabin of theSAAB2000 plane.1.5.4 Types of soundBased on the frequency spectrum some types of sound can be distinguished :Pure tone : A sound characterized by only one frequency and can onlybe generated, approximatively, by a tone generatorMusical tone : (Figure 1.4-above) : It consists of a fundamental withovertones and are together called partials. (Harmonics are partials).The number and the nature of the harmonics define the so-called ’tonecolor’.Chaotic or stochastic sound (Figure 1.4-middle) : (noise, hiss, etc.) Itcovers a wide frequency spectrum (where sounds with a specific frequencyand higher amplitudes than the other, are typical for the machineinvolved), think of ventilators, traffic noise, factory noise, jets,and so on. In acoustics a very detailed analysis will not be performed(cost) but an analysis in 1/1 octave band or 1/3 octave band. Theseare normalized. More detailed analysis e.g. 1/12 octave or even moredetailed are sometimes used in research.Impulse noise (Figure 1.4-below) : is a type of sound which is of veryshort duration, mostly generated by an impact.Sound can also be classified in other ways like considering the change ofamplitude in function of time. This classification will be described in Part IIIof the text where the legislation regarding environmental noise is discussed.1.6 The acoustic intensity1.6.1 DefinitionA sound source delivers energy in the form of kinetic and potential energy,that is transported by the sound wave. Assume we have a ’free field’, i.e. noreflection possible. Consider a plane of 1 m 2 perpendicular to the directionin which we want to determine the intensity of a traveling plane wave.