Nachhall

Nachhall is a German term used in acoustics to describe the persistence of sound in an enclosed space after the source has stopped. It corresponds to the phenomenon known in English as reverberation. Unlike an echo, Nachhall results from many overlapping reflections from multiple surfaces, producing a gradual decay of sound energy rather than a discrete repetition.

In a room, sound energy decays at a rate determined by volume, surface materials, geometry, and air absorption. The reverberation time, usually denoted RT60, is the time required for the sound pressure level to decay by 60 decibels. The Sabine equation RT60 = 0.161 V / A relates this time to room volume V and total absorption A, where A is the sum of area times absorption coefficient across all surfaces.

The amount of Nachhall influences acoustical quality and intelligibility. Surfaces that reflect sound produce longer reverberation, while absorptive materials such as textiles, foams, wood finishes, and diffusers shorten and diffuse decay. In concert halls the reverberation contributes to fullness and blending; in speech-oriented rooms it can reduce intelligibility if excessive. Modern spaces may use adjustable or tunable acoustics to control Nachhall.

Measurement of Nachhall typically involves recording impulse responses with a loudspeaker and microphone, then extracting RT60. Standards such as ISO 3382 describe procedures for room acoustics measurements and parameters including reverberation time.

Etymology: Nachhall combines nach- meaning after or subsequent with Hall meaning hall or echo in German. In German-language acoustics, Nachhall is the common term for reverberation, while Echo denotes a distinct, time-delayed repetition.