Reverberation is the sum of the reflections (LF + HF → “WET”) of a direct signal (“DRY”) off various obstacles in an environment.
Understanding Reverberation
- Sources of Impulsive Sound: Reverberation is particularly associated with impulsive sound sources, which have a duration that is practically negligible.
- t0 = moment of perceiving the direct sound (DRY)
- t1 = moment of perceiving the first reflection, which is from the nearest obstacle
- ΔtPRE-DELAY = t1 – t0 = the time between the perception of direct sound (DRY) and the perception of the first reflection from the closest obstacle (e.g., wall). This time provides the first “spatial” information about the dimensions of the recording environment.
The Haas Effect
The Haas Effect represents the absolute threshold in the perception of ECHO or REVERBERATION:
- ΔtPRE-DELAY < 30 [msec] → REVERBERATION
- ΔtPRE-DELAY > 80 [msec] → ECHO
Times exceeding one-tenth of a second (0.1 [sec] = 100 [msec]) are definitely perceived as ECHO, meaning they are interpreted as a repetition rather than a temporal prolongation.
Early Reflections
- Early Reflections: These are the first reflections that are perceived and, more importantly, those that, after few bounces off walls, have an energy content comparable (but still less than) to the source (DRY).
Coda (Tail)
- Coda: Reflections with energy content significantly lower than the source (DRY) due to either numerous bounces or because they impacted highly sound-absorbent surfaces.
High Damping
- High Damping: This leads to a decay of high frequencies (HF), which can be simulated using equalization (EQ).
Impact on Sound Sources
- For Impulsive Sources:
- Reverberation results in a temporal prolongation of the source.
- For Stationary Sources: In the case of sound sources that have a non-negligible duration, reverberation leads to three modifications of the signal:
- a) Temporal Prolongation
- b) Non-linear Amplification
- c) Timbre Modification: This is because different materials can absorb various frequencies differently, and reflections can combine with the direct signal at different phases, resulting in constructive and destructive interference.
Conclusion
Understanding reverberation is crucial for sound engineers and producers, as it significantly influences the audio quality in any recording or listening environment. By managing reflections and the balance of direct and reverberated sound, one can achieve a more controlled and pleasant listening experience.