Depth – Some Thoughts On Spatial Sound

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Hugo Zucarelli invented a new way of looking at the audio world; he took the idea of echolocation from the bats, and with typical vivid imagination, he moved the concept sideways applying it to humans so that they have spatial awareness aided by holographic (holophonic?) sensing.

I do not subscribe to his ideas. The notion of the human ear creating some sort of supersonic signal and then registering that signal and processing it to provide exact positioning ability goes far beyond my powers of faith, particularly as we have no evidence at all for any such ‘transmission’ in the form of physical ‘transmitter’ or any organ to decode the ‘signal’ even if it exists.

While I am certainly open to arguments involving things unseen, in this instance I’m happy that while the truths of hearing are mightily complex and inter-related, they do not include echolocation.
My own work on spatial hearing certainly shows up some extraordinary abilities; how, for example, can a person with hearing in only one ear, hear direction and positioning? How do we discern height? How do we perceive front from back? It’s fashionable to wave ones arms in the air and mutter ‘pinna effect’, interesting, but not the core of research.

But more interesting for me is the search for ever better ways to reproduce sound in the most natural way possible; that is, including the dynamics and the ‘quality’, but most importantly, the space.

Studies in Spatial Recognition

Ignoring loudspeakers and recorded sound for a moment, the way we recognise the spaces that we occupy is with a mass of inter-related cues and clues.
It’s my opinion that visual information is very much LESS important in this respect than has been thought. Eyesight may seem to be wide and sensitive, but in reality, the field of view is extremely narrow, and so it contributes little to our constantly changing mind-picture of the form of our environment.
I believe that this spatial awareness is a constant in our minds, and is being continuously updated and modified by all our senses, but of course, most importantly by hearing.
Yet this updating is mostly from what we would describe as unimportant parts of audio input; from reflections of course, and from the relationships between direct and reflected sounds. And not only from sounds sensed by the ears, but also from the action of sound on all parts of the body. (Infra-sound via the diaphragm, LF and even some mid frequencies modified by facial bone structure, and wide-band frequencies directly through the skull.)
So we are an organism living and reacting to the space in which we exist, the knowledge and appreciation of which is being modified by all our senses.

Accurate Loudspeaker Listening— Possible?

If we have this built-in, constantly updated model of our surroundings inside our head, how then is it possible to implant a totally new artificially created model of another environment with some sort of audio playback?
I have to say that the short answer is that we can’t. But we can be ingenious, and we can create systems that accentuate those factors that are dominant in our spatial recognition, while ignoring those over which we can have little control.

Suddenly we are faced with practical difficulties, ingenuity is all very well but we live in a commercial world and practical systems need to be affordable.
The conventional and simple answer has been to listen to recorded sound via a pair of loudspeakers. The inference being that a ‘stereo’ signal will be reproduced via the loudspeakers such that a centrally placed performer will appear to be between the loudspeakers.

Over the 60 years of its existence, 2-speaker stereo has become totally accepted, and has even been developed further to give a semblance of spatial effect, but as anyone can easily hear, the most elementary form of spatial listening via two loudspeakers, can only be achieved in a carefully controlled environment and with the listener positioned exactly at the ‘sweet spot’.

Airsound— Partial Answers to Difficult Questions

A fundamental problem of any attempt to reproduce spatial sound is an obvious one….. The listener is in a real environment, and practically cannot be removed from it; the artificial environment is going to be modified, and will inter-act with the real one, affecting the perception.
I believe that Alan Blumlein understood and appreciated this during his original work on spatial sound recording in 1934. Interestingly, it’s not known exactly how he listened to his own pioneering recordings except that he used multiple loudspeakers.

Had he lived, I like to think that Blumlein would have developed a system not unlike Airsound for EMI in the years following the second world war. But sadly he was killed in an air crash, and his work was shelved, only to rediscovered very much later. 2-loudspeaker systems became the norm, and the word ‘stereo’, (meaning ‘solid’) came to take on other meanings.

By understanding the over-elementary thinking and arithmetic of Blumlein, it is relatively easy to manufacture a loudspeaker that will produce spatial effects on a par with 2-speaker systems, after all, with two loudspeakers the listener is trying to hear a performance from somewhere in between the loudspeakers; it is a virtual image. With a single-point system, the main information is coming from the loudspeaker directly; it is the spatial information that is virtual, as it should be!

But there is a lot more to this than simply trying to replace the 2-speaker system, admirable as that may be, there is scope for going very much further with the original aim, to reproduce spatial sound properly.
With Airsound, the audio signal has been converted from ‘left/right’ to ‘main’ and ‘spatial’. This offers a major advantage over other systems, it gives separate control over the spatial information, so we can enhance it to give greater input ‘clues’ to our personal inbuilt model of space.
I’m certain that Blumlein had no detail understanding of the importance of the accuracy of the generation of spatial information. Experiments show that extremely (even vanishingly) small errors in timing degrade this information causing our brains to reject the input. (This effect can be heard easily by listening to a conventional 2-speaker system and moving out of the sweet-spot…. The path-length from the two loudspeakers is varied and the spatial picture degrades. Note, the degradation is entirely inside your head!)
Absolute accurate timing, (as good as could be achieved in the original recording,) when properly reproduced, can produce a spatial environment that is ‘in focus’ and solid.
The very best 2-speaker systems are capable of excellent directional performance, but the limitations of the ‘sweet-spot’ are severe, and to hear the best possible ‘space’ it would be necessary to bolt ones head stationary.
This is another of the marks of superiority of Airsound over 2-speaker systems, there is no need to keep still! The spatial information is always there; much like a hologram where the original picture can be cut in half…. And it simply produces two complete pictures.


There are yet another set of parameters interfering with clarity, and this is to do with the way loudspeakers operate. A loudspeaker responds to the current flowing in a coil in a magnetic field. A power amplifier produces an output voltage, and the current is a function of the loudspeaker impedance.
This means that while the amplifier might perform very well and the loudspeaker may be very good, they are two entirely separate entities.

Latest Airsound experiments use a new technology such that the sound pressures produced in the air are from a loudspeaker/amplifier combination that is both predictable and accurate, this produces an even more sharply Airsound spatial image.

An even more exciting result of the use of ‘controlled current’ transducers is the unexpected appearance of height information on some classical recordings.

Presently, this is a ‘hit and miss’ event that occurs unexpectedly during some classical recordings, but it does seem to follow the use of these very high resolution replay systems, and it is a certain fact that this height information is delicate and highly sensitive, disappearing as soon as any distorting anomalies appear.


I suppose we have been working now for over four years on this path to audible space. The very first experiments were startling and exciting, and parallel research in other countries has spawned commercial product, but with inbuilt faults and failures.

But Airsound is a successful collection of ideas, a dynamic resource of research and achievement.
From the miniature T3 with its extraordinary aura of space around the head, to the T12, a middle-of-the-road hifi system that makes TV movies come to life, and on to new developments with these high resolution systems in the T21 and T1 project. Spatial sound has developed from a simple but superior alternative to 2-speaker systems, to the most exciting and beautiful audio there has ever been.

©Ted Fletcher 2008

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