2.1       The physical terms "sound" and "noise"

A thorough and well-founded analysis of noise as an environmental hazard starts with the explanation of the physical umbrella term "sound":

According to DIN 1320 "Acoustics - Terminology", sound is defined as mechanical vibrations and waves in an elastic medium. These sound events can be produced by the membrane of a loudspeaker, the vocal cords in the larynx, the string of a musical instrument or the case of a machine. They also include vibrations in liquids and gases, what explains the noise emission of the jet blast from jet planes and rockets.

In any case, sound propagation is bound to a transmission medium. The present work deals with what is called "airborne sound".

In the following, the notion of sound always implies "audible sound", i.e. the sound that can be perceived by human ears. According to DIN 1320, this is sound with a frequency within the range of human hearing. A sound event only causes an auditory event with a person when the sound intensity lies above what is called the threshold of hearing. Sound intensities above the threshold of pain additionally create sensations of pain (see section 2.4.1).

The terms "tone", "ring", "sound impulse" and "bang" specify particular sound signals and their temporal course.

According to DIN 1320, a "noise" is a non-specific and undesired sound signal, e.g. machinery and vehicle noises. This definition reveals the random and orderless character of noises, which are composed by numerous discrete tones.

This shows that a noise is an acoustic signal with numerous frequencies which have no regularities, other than rings for example.

Due to periodic events, noises can also have discrete tones, which are superposed to the noise component characterized by a broad frequency band. These discrete tones increase the interference effect of noises considerably and this is why they are considered in the measurement result with an addition for discrete tones (see section 2.4.3).

Figure 2/1 illustrates the difference between a tone and a ring on the one hand and a noise on the other hand. The fourth example represents "white noise", which is characterized by frequency components distributed uniformly just above the audible range.

This kind of noise is familiar to us from natural phenomena like the rustling of trees or the breaking of waves.