Echo threshold
The echo threshold or the echo perception threshold describes the minimum "delay" (time difference) that reflections of a sound may still have in order to be perceived as a separate auditory event ( echo ) after the direct sound (primary signal).
The echo threshold depends on various parameters :
- The echo threshold is heavily dependent on the "signal type", ie on the characteristics of the sound. It can be between 20 ms (for "clicks") and several seconds (for slow orchestral music). Below the echo threshold, the reflections are perceived as reverberation .
- The echo threshold is also dependent on the "type of reflection pattern". Echoes are particularly easy to perceive when there are only one or only a few reflective surfaces (bridges, rock walls, etc.) with a large number of reflective surfaces (e.g. in churches), the time density of the reflections tends to give the impression of reverberation.
- The echo threshold is also dependent on the "level" (ie the "volume") of the reflections as a level difference to the direct sound. The higher the level of discards compared to direct sound, the lower the echo threshold.
The statement that reflections that arrive later than 30 to 50 ms are perceived as disturbing echoes lacks the important addition that the level of the rejection must be exactly the same as the level of the direct signal (primary signal). This information does not apply at a lower reflection level, because different sound impressions arise depending on the type of signal, level difference and time difference. They range from comb filter-like discolouration to spatial impressions, residual pitch levels and the perception of separate auditory events, which we call echo, but only above the echo threshold.