Professional Definition

Critical Radius (denoted as rc) is the distance from the sound source at which the sound pressure level of the direct sound equals the sound pressure level of the reflected (reverberant) sound. Within this distance, the free field (direct sound) predominates, whereas beyond it – the reverberant field.

“The value of the critical radius depends on the acoustic absorption of the room and the directionality of the sound source. The more damped the room, the larger the critical radius.”

Acoustics in Simple Words

Imagine you are talking to someone in a crowded, loud restaurant. If you stand close to the interlocutor (within the critical radius), you hear them clearly and louder than the general noise. It's like warming your hands directly over a fire. However, if you move too far away, the interlocutor's voice gets lost in the overall noise of the room (“sound soup”). At this point, the overall “temperature” of the environment becomes higher than the warmth coming directly from the source. In lecture halls, we strive to ensure that listeners are within the critical radius so they can understand speech effortlessly.

Summary

The critical radius defines the boundary of good speech intelligibility and music clarity in a given interior. Knowing it allows one to decide whether to add acoustic devices or rather change the arrangement of speakers.