Sound waves are fluctuations in air pressure
If they are audible to the human ear we talk about audible sound. The fluctuations in pressure occurring with audible sound are extremely low. At a normal pressure of 1013 mbar even changes in the μPa range can stimulate the human ear. A suitable pressure sensor with the appropriate sensitivity is the microphone.
Sound level meters have been designed for measurements in the free field. There is also a free field if the level falls by 6 dB per duplication of the distance from the source. This is the case in most rooms.
Office with carpet, curtains and partitions = Free field
Cellar with concrete walls, without furniture, highly reverberant = Reverberant field
Measuring conditions are ideal if there are absolutely no interfering objects in the sound field. This would be the case, for example, on top of a mountain. Because there are no walls or ceilings on which sound can be reflected, free dispersion is guaranteed (free field). In a closed room there is usually a wall opposite the noise source. This can cause reflections which distort the measured result (reverberant field).
With the Testo 815 you get superior microphone sensitivity and precise sound measurements year after year.