THE NEW WAVE
A unique project is under construction in Las Vegas to probe the limits of immersive sound, creating a ‘sweet spot’ for every audience member. But how will it work?
Directional Holosonics speaker arrays may be able to deliver a new breed of audio both live and in the home. Derek Powell investigates.
Since Alan Blumlein invented the concept of stereophonic sound in the 1930s, audio designers and engineers have been striving to create the perfect ‘immersive’ sound experience. Stereo provides a soundstage, but only in one dimension, laying out the instruments and singers in a line between the two speakers (give or take a little soundstage depth). Quadraphonic sound and later Dolby Surround and its successors took a step forward and created what was effectively a twodimensional soundfield, with the sound sources localized both left to right and front to back. More recently ‘immersive’ surround formats like Dolby Atmos have been trying to add the third dimension of height (though only upwards from the listener, not yet downwards).
Keeping it sweet
But all these formats have suffered from one limitation — the ‘sweet spot’, the limited zone where the signals from the left and right speakers (and the rear or surround speakers in quad or surround sound) are perfectly balanced. Only there is the soundstage reproduced exactly as it was intended. Move to the left or right of the sweet spot and the spatial localisation of the individual sound sources is progressively distorted, giving undue emphasis to sounds from the nearer speaker.
So conventional stereo speakers provide a satisfying illusion for a single listener who is stationary, but there are many situations where that simply isn’t good enough. The most obvious cases are large-scale amplified concerts, or cinema sound in large theatres. In these situations, while the sweet spot can be fairly large, the majority of the audience members are going to be positioned where a conventional stereo (or surround) image will be skewed because they are too close to one or another of the speakers.
For some, the Holy Grail of audio reproduction would be to create a soundstage in the listening room which would be truly stable and three-dimensional, such that a listener could get up, walk up to, say, the cello player and walk right around them while the sound of the cello remains exactly in place. This would be truly immersive audio. It would be the audio equivalent of the holographic projection of Princess Leia in Star Wars where you could move all around the image and see (or in our case hear) it from all sides. This kind of sound reproduction is sometimes called ‘holosonic’.
Holosonics
There are a number of reasons why orthodox two, four or more channels of audio are unable to give a convincing reproduction of a holosonic soundstage. But techniques do exist that make this kind of sound imaging possible. We first reported on holosonic technology in this column way back in 2005, when the scientists at the Fraunhofer Institute (where the technology for MP3 was invented) developed the IOSONO system of wave field synthesis.
Conventional speakers act as a ‘point source’ of sound, creating sound waves that move out in a circular pattern, like ripples on a pond. Curved sound waves make it easy for our ears to localise the actual sound source, just by turning our head or moving slightly, which can quickly destroy the illusion of a soundstage. But if sound waves can be made that are straight or even convex, very convincing sound fields can be created that do have holosonic properties. The IOSONO system created planar waves by using literally hundreds of closely spaced speakers, with each speaker driven by a separate
channel of amplification from its own digital signal processing (DSP) source. At the time it was thought that the generation of planar sound waves would be simplified into something we could use domestically, but it didn’t turn out quite that way. IOSONO was bought by Barco and some of the tech was incorporated into its ‘Auro’ cinema sound system (a competitor for Dolby Atmos) that could create 3D sound effects.
In a stereo system, we can make an instrument appear to move on the soundstage by altering the volume of a sound source that is sent to each speaker. More volume to the left speaker makes the instrument appear further to the left. But it only works if the listener is in the sweet spot, equidistant between the two speakers. Auro, like Atmos takes the dry sound of an instrument (or sound effect such as a moving vehicle) and adds fake echoes in different channels to create a sound ‘object’. Our ears interpret the time differences and directional cues from these fake ‘first reflections’ to locate the location of the sound object quite precisely, without having to be in a narrow sweet spot between the speakers.
Clever processing and sound objects can produce great effects in a cinema with only a relatively few channels, but if you go back to the idea of creating straight or focused beams of sound by using closely spaced speakers, it is possible to do much, much more. When multiple speakers are close enough, the individual circular sound waves from each driver couple together to form straighter, planar sound waves. Most of the largescale concert PAs from manufacturers like Bose, JBL, L-Acoustics, Meyer and others feature a single line of closely spaced boxes whose individual outputs couple together. These line arrays allow concert speaker systems to aim sound accurately and produce very even volume levels over throw distances of a hundred metres and more. Combining line arrays with the techniques used to create sound objects is currently a major trend in the Concert Sound industry and these systems can create immersive sound — particularly in outdoor venues.
Don’t lose the Holoplot
But now, one company has gone further. A German company called Holoplot has developed special speaker modules that use a twodimensional array of very closely spaced drivers. Rather than a line array, these blocks are assembled to create a literal wall of sound (a 3×3 array is pictured below), with each of the hundreds of drivers fed by an individual channel of DSP. Using wave-field synthesis algorithms, it is possible to recreate any acoustic environment in any given space, claims Holoplot. Moreover, multiple beams of sound can be formed simultaneously targeting different parts of the space with “laser-like” precision (main image, left). Each audience member experiences accurate source localisation, with no sweet spot, no identification of speakers and audio objects that for the first time can come close to the listener.
The first large-scale implementation of the Holoplot system has been announced and it is truly mind boggling. The Madison Square Garden Company (MSG) has broken ground on a new project in Las Vegas which takes full advantage of the Holoplot beam-forming audio technology. Dubbed the ‘MSG Sphere’ it is touted to be an entertainment venue “unlike anything else on planet Earth”. The 150-metre diameter 18,000-seat venue will feature a 250-million-pixel 15,800m2 LED screen arching across the interior of the dome that will be, according to MSG Ventures CEO David Dibble, “the largest display ever imagined on Earth — like “VR without the goggles”.
But behind the LED elements, covering the inside of the dome, are hundreds of the Holoplot Io 64-speaker modules with a total of 157,000 drivers. This “dome of sound” should be able to re-create real or imaginable acoustic environments for live concerts, to handle any surround sound video soundtrack played on the IMAX-like dome screen, or even to provide multiple simultaneous translations of spoken word presentations by beaming different languages to precise locations in the audience.
Whole lotta shakin’ goin’ on
The Io modules each contain 16 three-inch drivers and 48 two-inch drivers, so low frequencies in the Sphere will be handled by a separate ‘haptic’ flooring system. There’s not much information on that, and an article in Rolling Stone quotes Dibble as saying: “It’s still a bit of a work in progress, but the lowest bass response, instead of being transmitted through the air, is literally transmitted through the floor, directly into your feet or onto the chair in which you sit, and it’s a remarkable experience.”
The company is planning to open a second MSG Sphere in London by 2022 and is looking to diversify the concept, ultimately planning a consumer version. It isn’t clear what sort of content might be created for a residential Sphere and I am guessing that could be a fair way into the future. But if you do ever wind up owning what MSG has already dubbed a ‘Home Dome’, remember you saw it here first! Derek Powell
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