Evaluation of the Phantom Image Effect for Phantom Images

This paper introduces the author’s recent research on the elevation effect perceived with horizontal phantom images. Early research in stereophony suggests that a phantom centre image produced by two loudspeakers symmetrically placed from the listener position would be perceived in an elevated position, with its elevation angle increases as the loudspeaker base angle increases. In particular, an image presented from loudspeakers placed around the listener’s sides would be perceived overhead. With 3D audio formats employing height and overhead channels in mind, the aforementioned elevation effect is considered to be useful for creating a virtual overhead loudspeaker image, especially for sound effects using just ear-level loudspeakers (e.g. in downmix scenarios). Another important psychoacoustic principle relevant to 3D audio formats is the so- called ‘pitch-height’ effect, which suggests that the higher the frequency of a sound is the higher its image will be perceived. However, past research in this topic only considered loudspeakers placed in the median plane. From the above background, several subjective experiments have been conducted on the elevation of horizontally oriented phantom image. This paper first presents a vertical localisation test conducted with frontal stereo loudspeakers using octave-band noise stimuli. The results not only confirm the elevation effect for broadband noise, but also show the existence of an elevation effect for middle frequency bands. The second experiment introduced in this paper verifies the existence of the virtual overhead perception depending on loudspeaker base angle but also shows the effect heavily depends on the type of sound source.