Measurement surface theory

Theory: The hypothetical measurement surface.

When measuring intensity, we may isolate a source from the rest of the sound field. This is achieved by defining a measurement surface around the source, and ensuring no other sources are inside the surface.

The reason we can do this, is that Sound Intensity has directional information. When the surface is enclosing the object, sound from other sources may enter into the surface, but as long as it is not absorbed within the surface, it will exit.

The sound may either exit out because of reflection into the noise source under test, or simply because it passes directly out on the other side of the surfaces.

For this assumption to make sense, there are two main requirements:

1. There are no significant absorbers within the surface.

2. The surface is completely enclosing the device under test. In most cases, a solid and nearly fully reflecting floor will fulfill the requirement when the device under test/noise source is located on the floor.

Defining the surface

By hypothetical, we mean that the surface is not a physical object. However, it has physical dimensions. The most common surface is the cuboid (a box).

By theory, one will need to know the sound intensity at any point over the surface. In practice, it is sufficient with a certain number of averaging points, or simply by spatially averaging the surface using the scanning technique.

Note that the hypothetical surface needs some margin distance to the device under test. Typical distances from the surface to DUT is between 10 cm and 30 cm. This is to avoid collision but also to not measure in the highly reactiv field close to the DUT. The selction of distance is typically a matter of experience and review of the measurements. Standards also define guidline or boundary limitations.

Calculating sound power

As intensity is defined as power divided by area, calculating sound power from intensity values is straight-forward. Using a reference area of 1 m², the sound power in dB is the averaged intensity + ten times the logarithm of the area in square meter: L_W = I_eq + 10*log10(A).