The Analysis of Frequency Dependent Vertical Localisation Thresholds and the Perceptual Effects of Vertical Interchannel Crosstalk

In the context of microphone techniques for recording three-dimensional (3D) sound in an acoustic space, vertical interchannel crosstalk occurs when the height layer of microphones capture direct sound. This effect can cause sound images to be formed as vertically oriented phantom images, at positions intermediate between the main and height layer of loudspeakers, as opposed to at the desired position of the main layer. Additional spatial and timbral effects will also be perceived, although these have not yet been examined in the literature.

Previous research has examined the minimum amount of attenuation of direct sound in the height layer necessary to prevent vertical interchannel crosstalk from affecting the perceived location of the main channel signal, which has become known as the 'localisation threshold'. However, existing methods of applying this have not considered the frequency dependency of median plane localisation. The present thesis therefore examined if localisation thresholds could be applied through the frequency dependent manipulation of the direct sound in the height layer (band reduction), as well as the most salient perceptual effects of vertical interchannel crosstalk. The operation of the precedence effect in the median plane was also considered.

A review of human localisation mechanisms was first conducted, with a particular focus on how such characteristics might be able to be exploited for the development of a band reduction method. Additionally, consideration was also given to how secondary vertical sources might affect direct sounds, in order to gain further understanding of what the most salient effects of vertical interchannel crosstalk might be.

The frequency dependency of localisation thresholds was considered in anechoic conditions, with subsequent localisation experiments being conducted to assist in explaining the results. Following this, localisation thresholds using blanket reduction (attenuation of the direct sound in the height layer evenly across the spectrum) were analysed. The frequency dependency of localisation threshold was subsequently examined in a natural listening environment, with a series of band reduction methods being developed based on the results. The band and blanket reduction thresholds were then verified in localisation tests. The final experiment considered the most salient effects of vertical interchannel crosstalk, how these were affected when the different localisation threshold methods were applied and which was the most preferred method by subjects.

The results showed that localisation thresholds are frequency dependent in both anechoic and natural listening environments. In particular, more level reduction was necessary for the mid-high frequencies compared to low frequencies. Additionally, a series of different band reduction methods were found to be effective. Elicitation experiments showed that the most salient effects of vertical interchannel crosstalk were increases in vertical image spread, source elevation, loudness and fullness, with the perception of these when the localisation threshold was applied being dependent on the method being used. Moreover, although subjective preference could not discriminate between the methods tested, the presence of direct sound in the height layer was consistently preferred compared to situations where it was absent. Furthermore, no evidence was found to support the existence of either the precedence effect or localisation dominance in the median plane.