Immersive Audio Series: Pt 2

In Part 1, I covered some of the Psychoacoustic aspects of Immersive Audio. This included discussing terms like Interaural Time Differences, Interaural Frequency Differences, and HRTFs. The Physical Acoustic contributions are still related to these terms, which I’ll cover this week.

Sound localization is greatly impacted by the physical acoustic aspects of a listening environment. When a sound directly hits the listener’s ear, this sound is least interfered with and most accurately represents the sound (direct sounds). On the other hand, this sound is also affected by room reverberation, room textures, objects in the environment, and the body of the listener. This sound is now compromised by those interferences, called indirect sounds [1]. Sound sources are therefore not so easily identified as coming from a single point.

Example of direct and indirect sound paths in a room [2].

The distance from the sound source to the listener also impacts localization. Distance affects the amplitude of a sound as stated by the Inverse Square Law. “As a sound wave propagates spherically, the sound energy is distributed over the ever-increasing surface diameter of the wave front surface. The Inverse Square Law teaches us that for every doubling of the distance from the sound source in a free field situation, the sound intensity will diminish by 6 decibels” [3]. This important to keep in mind when mixing for immersive formats.

Illustration of the Inverse Square Law [4].

The coordinates used to map a sound source are Azimuth and Elevation. “Azimuth is defined by the angle (θ) between the source location and the median plane at 0° azimuth (projected onto the horizontal plane) and elevation is the angle (δ) between the source location and the horizontal plane at 0° elevation (projected onto the median plane)” [5].

Why is all this relevant? I know, it sounds like a lot of technical terms and definitions, but it’s actually really cool how all this plays into how we perceive sounds around us. If sounds are familiar, then the listener can more accurately pinpoint the sound source [6]. But when the sound is unfamiliar, more information is needed to better localize the sound. As the engineer, whether you are working on sound design or music mixing, sound source placement needs to make sense to the listener. Depending on where in the 3D space you are placing sounds, the sound can be affected by artificial room filtering and it can be attenuated, for example, if placed far in the virtual space.

A good tool that can be used to start mixing in a 3D space is the Gaudio Works plugin. It provides a visual map view that shows you the azimuth and elevation coordinates of the sound sources you are placing in your sound field. I’d also recommend exploring similar plug-ins so that you can compare and hear the differences between each.

[1] Kendall, Gary S. A 3D Sound Primer: Directional Hearing and Stereo Reproduction. 1995.
[2] ROOM ACOUSTICS, THE BASIS FOR HIFI AUDIO. (2016, March 28). Keep Coding. https://blog.keepcoding.ch/?p=715

[3] Acoustics 101 - learn about acoustics. Acoustical Surfaces. (n.d.). Retrieved October 8, 2021, from https://www.acousticalsurfaces.com/acoustic_IOI/101home.htm#:~:text=The%20Inverse%20Square%20Law%20teaches,will%20diminish%20by%206%20decibels.&text=The%20intensity%20of%20the%20sound,wavefront%20from%20the%20signal%20source.

[4] Audio Calculators. (2021). Extron. https://www.extron.com/calculators/inverse-square-law/?tab=tools

[5] Roginska, Agnieszka Paul Geluso. Immersive Sound: The Art and Science of Binaural and Multi-Channel Audio (Audio Engineering Society Presents). 2018.