As has been seen, in a reverberant environment, sounds reach the ears via several different paths. Although the direct sound is followed by multiple reflections which would be audible in isolation, the first-arriving wavefront dominates many aspects of perception, including source localization [Litovsky et al, 1999]. This theory, known as the precedence effect, or the law of the first wavefront, states that when multiple coherent signals are presented at the ears, localization is primarily based on the earliest arriving signal. One study [Begault, 1994] showed that for delays up to approximately 0.6ms the source moved laterally toward the non-delayed side of the listener’s head. For delays greater than approximately 0.6ms and less than 35ms the source position remains unchanged but some timbral coloration may occur. Even greater delay times result in two distinct sources as the delayed signal is
perceived as a distinct echo. It should be noted that the precise time values at which these perceptual effects occur is highly dependent on the nature of the source signal [Litovsky et al, 1999]. Other studies have also shown that the presence of significant early reflections can have an effect on localization accuracy [Hartman et al, 1985]. Lateral reflections from side walls were found to be particularly detrimental for azimuthal localization while the effect of early reflections from the floor and ceiling was less conclusive. Hartmann speculates that as the angular direction of these reflections matched that of the source signal, reflections from the floor and ceiling may reinforce horizontal localization [Hartmann et al, 1985]. In general, it has been found that signals with strong transient characteristics are localized independently of
the room reverberation time, but may depend on the specific room geometry which can result in significant early reflections [Hartmann, 1983].
Early reflections and reverberation also has a significant effect on both the perceived size of the source, and the spatial impression of the acoustic environment. The term spaciousness has been used to refer to both these effects, and other terms such as spatial impression, ambiance, apparent source width, immersion and envelopment are also frequently used, sometimes interchangeably, and sometimes with more specific definitions. A strong correlation has been found between the degree of coherence between the two ear signals and the lateral component of the ear signals, and this is though to influence the spatial impression of both the source and environment [Blauert, 1997; Plenge et al, 1975]. Early studies of spaciousness described this characteristic in terms of the lateral and frontal components of the ear signals [Barron et al, 1981]. However, what is meant by spaciousness is different depending on whether this lateral component is derived from early reflections alone, or from both early reflections and later reverberation. The addition of lateral early reflections results in a change in the perceived size of the source and the apparent source width (ASW) is generally used to refer to this source-specific measure [Beranek, 1996]. Early reflected energy arriving within approximately 80 ms of the direct sound results in an increased ASW and the extent of this broadening effect depends upon the ratio of the total energy to the energy of the lateral component [Barron et al, 1981]. Blauert introduced the term “locatedness” as a measure of the degree to which an auditory event can be said to be clearly in a particular location [Blauert, 1997] and this is clearly related to ASW. Localization refers only to the perceived direction and does not therefore directly relate to the locatedness or ASW, although clearly the source direction will be difficult to precisely determine in the case of a very large ASW. The important distinction between these two measures will be discussed in more detail in Chapter Six.
Later arriving reverberation alters the spatial impression in a different way which is primarily related to the acoustic environment. The term spaciousness is often used in this case, as opposed to ASW which is primarily related to the spatial impression of the source. The terms spaciousness and envelopment are often
considered equivalent, although occasionally slightly different definitions are given. When distinguished, spaciousness is described as the sense of open space in which the
source is located while envelopment refers to the sense of immersion and involvement in a reverberant sound field which fully surrounds the listener [Rumsey, 2001].
Numerous studies have shown that envelopment and spaciousness are generally desirable qualities in a concert hall [Schroeder et al, 1974; Barron et al, 1981; Beranek, 1996] and many concert hall designs have attempted to increase the amount of lateral reflected energy directed to the seating area for this reason. The acoustician David Griesinger has suggested that the importance of the distinction between early arriving lateral reflections and later reverberation is often overlooked in this context [Griesinger, 2009]. It has already been shown that early arriving
reflections reduce localization accuracy and Griesinger suggests that an increase in lateral energy of this sort will negatively impact clarity. This clearly contradicts previous studies which have stressed the importance of ASW. Griesinger suggests that the direct sound and later lateral reverberation should be emphasized to improve clarity (which corresponds to improved localization) and also spaciousness (meaning the desirable spatial characteristics of the hall).
Rumsey has similarly pointed out the difference between acoustic research, which suggests that large values of ASW are preferable, and studies of spatialization techniques which emphasize localization accuracy [Rumsey, 1998]. Griesinger has noted a similar contradiction between the differing levels of reverberation typically found in performances and recordings of classical music [Griesinger, 2009]. A clear distinction can be made between the three-dimensional spatial experience of a live instrumental performance in a concert hall and a two-channel stereo reproduction which must reproduce both the direct and reflected sound from the front. Improved localization accuracy is perhaps desirable in the latter case in order to distinguish sources in the foreground from background reverberation [Rumsey, 1998]. However, the situation is much more complicated in the case of electronic spatial music
performances which often utilize multi-channel loudspeaker arrays and electronic spatialization techniques within a concert hall. The distinction between ASW, spaciousness and envelopment introduced earlier may also be hard to maintain in this situation, as a multichannel reproduction of a single source from multiple positions around the audience will provide some sense of envelopment, but by the direct sound and not the reverberant field. In this situation, the sense of spaciousness may be associated with the source rather than the acoustic environment, and the distinction
between these terms becomes harder to define. In addition, it is hard to predict the effect of a lateral source on perceptual aspects such as ASW or spaciousness.
The preceding discussion indicates the significant effect of acoustic reflections on the perception of an auditory source. In addition, reverberation provides a
significant amount of information regarding the size and composition of the environment within which the source is situated. The degree of attenuation of acoustic reflections provides an indication of the nature of the reflecting surfaces while the time and duration of the diffuse late reverberation can indicate the dimensions of the space. Room reflections and reverberation can also provide information on another highly important aspect of spatial hearing, namely, the
distance of the source from the listener and this will be examined in more detail in the next section