What is reverberation?
Reverberation is intuitively described by the concept of reflections. The desired source produces wavefronts, which propagate outward from the source. The wavefronts reflect off the walls of the room and superimpose at the microphone. In Fig. 1 this is illustrated with an example of a direct path and a single reflection. Due to differences in the lengths of the propagation paths to the microphone and in the amount of sound energy absorbed by the walls, each wavefront arrives at the microphone with a different amplitude and phase. The term reverberation designates the presence of delayed and attenuated copies of the source signal in the received signal.
Reverberation is the process of multi-path propagation of an acoustic signal from its source to the microphone. The received signal generally consists of a direct sound, reflections that arrive shortly after the direct sound (commonly called early reverberation), and reflections that arrive after the early reverberation (commonly called late reverberation). The different sound components will now be discussed in more detail.
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Direct Sound: |
The first sound that is received through free-field, i.e., without reflection, is the direct sound. In case the source is not in line of sight of the observer there is no direct sound. The delay between the initial excitation of the source and its observation is dependent on the distance and the velocity of the sound. |
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Early Reverberation: |
A little time later the sounds which were reflected off one or more surfaces (walls, floor, furniture, etc.) will be received. These reflected sounds are separated in both time and direction from the direct sound. The reflected sounds form a sound component which is usually called early reverberation. Early reverberation will vary as the source or the microphone moves within the space, and gives us information about the size of the space and the position of the source in the space. Early reverberation is not perceived as a separate sound to the direct sound so long as the delay of the reflections does not exceed a limit of approximately 80-100 ms with respect to the arrival time of the direct sound. Early reverberation is actually perceived to reinforce the direct sound and is therefore considered useful with regard to speech intelligibility. This is often referred to as the precedence effect. This reinforcement is what makes it easier to hold conversations in closed rooms compared with outdoors. Early reverberation is mainly important in so-called small-room acoustics since the walls, ceiling and floor are really close. Early reverberation also causes a spectral distortion called colouration. |
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Late Reverberation: |
Late reverberation results from reflections which arrive with larger delays after the arrival of the direct sound. They are perceived either as separate echoes, or as reverberation, and impair speech intelligibility. |
Source: E.A.P. Habets, `Single- and Multi-Microphone Speech Dereverberation using Spectral Enhancement', Ph.D. Thesis, Technische Universiteit Eindhoven, The Netherlands, June, 2007.