Diffuse Field

Knowledge Base


The diffuse field is a sound field that contains sound from all directions.

What is the Diffuse Field?

A diffuse sound field, or "diffuse field" for short, occurs in reverberant rooms in which the direct sound is  extended by many additional reflections from the walls. In an ideal diffuse field, the proportion of sound energy (represented by the "sound velocity vector") is the same on average from every direction. The opposite of a diffuse field is called "free field". This consists only of the direct sound and has no reflections from other directions.

In Microphone Practice

In addition to the free-field transmission coefficient or free-field frequency response of a microphone (usually referred to only as "frequency response"), the diffuse field frequency response is a crucial factor for evaluating the sound color of a microphone. This graph indicates an average of the transmission of a microphone for all spatial directions. The following diagram shows both the free field and the diffuse field frequency response for the vocal microphone V4 U from SCHOEPS:

[Fig. 1] Free-field and diffuse field frequency response of the vocal microphone V4 U

As you can see, the diffuse field response for this cardioid microphone proceeds always below the free-field frequency response. This is typical for transducers with directivity (see polar diagram), since the sound averaged from all directions provides less output signal energy compared to the main recording direction. For the V4 U, the two curves remain at an almost constant distance through almost the entire frequency range. Only in the heights above about 7 kHz, the directivity of the microphone increases somewhat, which leads to the fact that the diffuse field frequency response deviates further from the free-field frequency response. This leads in the example of the V4 U to a sound characteristic in which the diffuse field sounds slightly softer due to the lower heights in the signal and less bright than the recorded sound from the main recording direction. This is a desired feature of this capsule in this case. In general, however, care should be taken when purchasing a microphone that a description of the sound characteristics such as "light" or "dark" depends not only on the free-field frequency response, but also on the transmission of the diffuse field. If the diffuse field frequency response is not provided, it can also be calculated from the free-field frequency response and the polar diagram. For this purpose, the IEC 60268-4 standard provides a conversion formula.

Particularly for microphones with omnidirectional characteristics, the knowledge of the different behavior in the free field and the diffuse field is important when selecting the right capsule type (see diffuse field equalization).