Envelopment in Immersive Sound Reinforcement

Immersion denotes the perceived dive into a fictional world. Surround audio system is an essential contribution and has therefore been employed , e.g., in cinemas, for a long time. However, the sense of auditory envelopment only works convincingly at a few centred seats of the hall. The EnImSo project will advance the fundamental psychological-acoustical and technical understanding of the matter and investigate design criteria and technical solutions that are urgently needed. In the future these shall enable to enlarge the high-quality zone in the audience substantially, and it shall provide useful tools for sound engineers and sound artists. In the close proximity of a loudspeaker, the sound is much louder than further away. This holds for every single loudspeaker of a surround audio system. On a seat at the boundaries of the audience area, nearby loudspeakers will therefore be louder than the ones from the opposite side. This causes a loss of the balance in the auditory image and in part, sounds will be directionally drawn to the closest loudspeakers, or such from more distant directions might not be heard anymore and get lost, inadvertently altering the mix. Similarly, while reproduced reverberation may be heard as enveloping at the central seat, it sticks to the closest side at outer seats, which is not enveloping. Large-scale sound reinforcement employs optimally-curved line-source arrays for the frontal sound. These arrays are suspended from the top of the stage, can be nearly ten meters tall, and they are configured such that the whole audience area, from the stage to the most rear seats, is covered with a barely noticeable loss in sound level. This technology requires a lot of hardware, but serves as a starting point for new, more compact solutions. EnImSo investigates how surround systems need to be designed in order to provide all seats with an enveloping reverberation, but also how the balance between instruments and voices from different directions is maintained across the whole, extended audience area. Therefore, an extensive series of listening experiments will be conducted in the acoustics laboratory, with participating listeners and surrounding loudspeakers. The goal is to find out how specific conditions isolated from each other and combined affect what we hear. What are the effects on the perceived envelopment of changing levels, arrival times, and directional parallax, at which the surround loudspeakers appear for different seats? Which technical optimizations to the loudspeakers achieve successful improvements? Knowledge from the laboratory shall be tested in a listening experiment employing a large sound reinforcement system. Newly developed acoustic virtualization technology allows to digitally archive the extensive experimental setup for future use in research, accessible per virtual reality, also to other laboratories. Moreover, loudspeaker prototypes shall be built that advance rendering of envelopment for mid-sized audiences.

EnImSo (Envelopment in Immersive Sound Reinforcement) investigated how the auditory sensation of envelopment changes over listening positions in surround sound reinforcement, why its magnitude changes, and which improvements could be possible. Laboratory experiments with loudspeakers in the anechoic room revealed that simple point sources arranged for surround sound from the horizon are insufficient, despite they are common in cinemas or concert venues for electro-acoustic music, nowadays. Along the direct path, the sound of point sources get softer with 1 over the distance r, or audio-technically speaking: with a distance attenuation of -6dB/dod (decibels per doubling of the distance). As first central finding, contrary to point sources, our laboratory experiments showed surround reinforcement for diffuse sound (e.g. reverberation) to be optimal with -3dB/dod distance attenuation, whereas for surround reinforcement of different direct voices/instruments (without reverberation) requires -0dB/dod. As second key result, since distance attenuation that is only simulated for one dedicated listening position in the lab is not yet a loudspeaker type applicable in an event venue, the EnImSo project devised a design theory for progressively curved and phased line-source arrays. This loudspeaker type and the design theory enable to freely configure a -6 beta dB/dod distance attenuation via a design parameter beta. In the project, 3D printing technology was used to design and print small loudspeaker housings, out of which 8 such line arrays were built with 8 elements each, and therefore a total length of 64cm each. As third key result, listening experiments were carried out at the IEM CUBE, a venue for electroacoustic music, using the mini line arrays. Despite additional room acoustic influences, experiments clearly validated the laboratory experiments. Moreover, the benefit of a dual-target design was indeed confirmed, which meets both the -3dB/dod for diffuse, enveloping components in a mix, while direct single voice/instrument components use 0dB/dod. In another experiment, listeners found -1.5dB/dod an acceptable compromise with high quality. Within a master's thesis, the results and knowledge underwent further validation on professional, high-quality line-source and point-source loudspeakers in comparison in the L-ISA laboratory of the company L-Acoustics in London. The project moreover investigated the virtualisation of the loudspeaker types and auditory impressions regarded, and validated them in virtual acoustic reality, came up with a more detailed description of localisation properties, and investigated envelopment and engulfment regarding the required temporal and spatial event densities. And a mathematical theory on diffuse field synthesis could be published. The EnImSo team reached a true improvement in psychoacoustic and technical understanding of professional immersive sound reinforcement. It is going to improve technology in the upcoming decade and provides the relevant design criteria. Electroacoustic music and cinema sound will benefit from the improved means of artistic expression.