Noise levels

Noise levels

Noise means any unwanted sound. Noise is not necessarily random. Sounds, particularly loud ones, that disturb people or make it difficult to hear wanted sounds, are noise. For example, conversations of other people may be called noise by people not involved in any of them; any unwanted sound such as domesticated dogs barking, neighbours playing loud music, portable mechanical saws, road traffic sounds, or a distant aircraft in quiet countryside, is called noise.


Airborne sound that is reflected against hard surfaces, for example, a hood over a machine or the walls of a room, can be reduced by covering the surfaces with absorbents on the inside. Materials such as plastic foam, textile fibres, or metal. The sound absorbents convert the airborne sound into small amount of heat energy.

The absorption factor denotes how much of the incoming sound that will be absorbed.


One way to reduce the propagation of noise is to insulate the noise source using a wall or hood. The noise insulation capability of a wall increases as it weight increases. The insulation capabilities can also be further improved by using a double wall system. Insulation pads are available with and without a wear resistant surface.

The Transmission Loss DTL denotes the reduction expressed in dB as a wall passage.


Structure borne sound is vibration in solid materials, generated by a machine, for example. It can be damped by applying a pad on one side or by the sandwich method. In both cases the vibration energy is converted into heat.

The loss factor denotes how much of the vibration energy is absorbed.


Frequency is measured in Hertz (Hz, the number of vibration cycles per second) and denotes the pitch of the sound. 100 Hz is a bass tone with a wavelength of 3 m whereas the wavelength for a frequency of 3,000 Hz is only 1 dm.

Decibel (0,1 Bel = 1 dB) is a unit used to express the ratio between two levels of sound. Besides sound pressure (LP), the following magnitudes are expressed in dB: sound intensity (LI), sound power (LW), equivalent average value (Leq). Zero Bel is defined as the lowest limit of audibility.

6 dB higher sound level means a doubling of the sound level.

Two independent sound sources of equal power, result in a sound level that is 3 dB higher
than a single source.

LP dB(A) is an average value across all frequencies in the audible range, adjusted to the sensitivity of the ear to the different frequencies.