Early studies reported that, when two identical signals are simultaneously reproduced from a pair of loudspeakers that are placed at ear level and arranged symmetrically from the listener position, the resulting phantom centre image would be perceived to be elevated in the median plane. It was also confirmed in the studies that the degree of perceived elevation would increase as the loudspeaker base angle increased from 0° to 180°; the image would be perceived almost right above the listener’s head when the base angle is 180°.

This project investigates into this psychoacoustic effect further, providing more systematic subjective data and theoretical explanations, and also develops a new virtual 3D panning method called VHAP (virtual hemispherical amplitude panning) based on the effect. The main findings and outcomes so far as follows:

• The strength of this effect significantly depends on the type of sound source; sound sources with a flatter frequency spectrum and more transient nature would be perceived to be more elevated (Lee 2018).
• A new theory has been proposed: in addition to conventional explanation regarding spectral energy balance at high frequencies, it has been proposed and verified that the phantom image elevation effect is associated with spectral notch below 1 kHz that is caused due to the combination of ipsilateral (direct) and contralateral (interaural crosstalk) signals from the loudspeakers; e.g. for the 180° base angle the spectral notch for a phantom centre image is around 640Hz, which matches the spectral notch frequency for a real source elevated at 90° in the median plane (Lee 2017).
• VHAP (virtual hemispherical amplitude panning) creates an elevated phantom source on a virtual upper-hemisphere with only four ear-height loudspeakers, based on the phantom image elevation effect. A set of constant power gain coefficients are applied to loudspeakers at ±90° and 0° for panning to a target azimuth and elevation in the front region, and to those at ±90° and 180° for panning in the back region (Lee et al 2018).
• Listening tests in loudspeaker reproduction show that VHAP can locate a phantom image at various spherical coordinates in the upper hemisphere with some limitations in accuracy and resolution (Lee et al 2018).
• Listening tests in binaural headphone reproduction indicate that the binaural rendering of VHAP is able to externalise elevated phantom images in various degrees of perceived distance (Lee et al 2019).

Researchers: Dr Hyunkook Lee, Dr Dale Johnson and Maksims Mironovs, Kacper Borzym

Publications: 

• Lee, H, Mironovs, M, & Johnson. D. (2019) Binaural Rendering of Virtual Elevation using the VHAP Plugin. In Proceedings of 146th AES International Convention
• Lee, H., Johnson, D., & Mironovs, M. (2018). Virtual Hemispherical Amplitude Panning (VHAP): A Method for 3D Panning without Elevated Loudspeakers. In Proceedings of 144th AES International Convention [Convention Paper 9965]
• Lee, H. (2017). Sound Source and Loudspeaker Base Angle Dependency of Phantom Image Elevation Effect. AES: Journal of the Audio Engineering Society, 65(9), 733-748. https://doi.org/10.17743/jaes.2017.0028 
• Lee, H. (2016). Phantom image elevation explained. In 141st Audio Engineering Society International Convention 2016, AES 2016 [9664] Audio Engineering Society.