PLANNING FOUNDATIONS
2.2 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 our 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 a jet blast from jet planes and rockets. In any case, sound propagation is bound to a transmission medium.

A distinction is made between structure-borne sound, water-borne sound and airborne sound. A particular case of airborne sound is the audible sound, which covers the frequency range of airborne sound audible for humans. In the following, this manual shall deal with "audible sound".

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", "bang" and "noise" specify particular sound signals and their temporal course.

A "noise" is a sound signal, which is composed of numerous tones of any frequency and level.

It is therefore 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. The occurrence of discrete tones significantly increases the disturbing effect of noises (see section 2.4.3). Sudden and strong variations of a noise (intermittent noise), which occur only temporarily, are called impulsive noises. Sound can also transmit information (music, speech), which may disturb others. Depending on which directive is applied, these increased disturbances are taken into account in an assessment by means of an addition.

Figure 2/1 illustrates the difference between a tone and a ring on the one hand and a noise on the other hand. The forth example is white noise, which consists of elements of all frequencies within a particular frequency range.

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