WHITE PAPER –
AIRSOUND TECHNOLOGY AND ITS PRACTICAL IMPLICATIONS.
SYSTEM DESIGN AND PLACEMENT
The basic form of Airsound involves conversion of a conventional stereo audio signal into a ‘main’ mono signal (by simple mixing of left and right stereo channels) and a ‘spatial’ signal (derived by analogue processing and filtering). The three reproducing loudspeakers are normally mounted in an enclosure where the rears of the speakers occupy a common space. The principal forward facing loudspeaker is driven from the ‘main’ audio signal, and two smaller loudspeakers, either side of the ‘main’ speaker, are driven from the ‘spatial’ signal in reverse polarity to one another. Physical design of the enclosure, choice of loudspeaker type and electronic design, are all critical to the production of a satisfactory Airsound sound field.
Airsound operates optimally within rooms and enclosed spaces; the field of sound created by Airsound is aided by, but not dependent on, reflections from walls and objects within an enclosed space.
One of the factors enabling the reproduction of a satisfactory stereo sound field is the existence of a flat surface close to the spatial loudspeakers. In many of the designs (available from Airsound LLP) a form of flat surface is a part of the design, but it can be an advantage to operate the Airsound enclosure either standing on a flat surface (as with DAB radios) or against a wall (as in flat-screen televisions). Where this is not possible, it is preferable for there to be a small flat surface integrated into the enclosure, adjacent to the spatial loudspeakers; for example, with the ‘Futuros’ table lamp, and types of suspended loudspeakers.
POWER CONSUMPTION
Conventional left and right channel stereo requires two identical power amplifier channels. In most Airsound systems the main information radiates from a single loudspeaker which requires very much less amplification power than the combined power of the conventional stereo amplifier channels; not only is a single loudspeaker considerably more energy efficient than a pair of loudspeakers, there is also a psychoacoustic perception of greater clarity and definition from a single transducer, as well as the elimination of the acoustic cancellation of certain frequencies with signal propagation from two separated sources.
Within the Airsound system, the stereo image effects are created by the ‘spatial’ loudspeakers where the amount of acoustic energy needed, and the power to drive them, is a fraction of that requirement by the ‘main’ loudspeaker.
EXPERIMENTAL PROOF
As a part of the ongoing development of Airsound, we set up practical and repeatable experiments to establish the relationship between power consumption and the introduction of Airsound.
The experiment comprised a conventional stereo pair of loudspeakers and a comparable Airsound system, all driven by 20 watt power amplifiers. Measurement of the incoming power to both systems was recorded (standing current was deducted and eliminated from all calculations). The experiment used pulsed tones of 1Khz and 600Hz (mixed together and asymmetrical in the sound field) . This test signal is easy to ,measure and provides realistic sound behaviour in the room.
The stereo system was set up to provide a loud but comfortable listening level at the ‘sweet spot’ (equidistant from the loudspeakers at a distance of 2 metres). The Airsound system was adjusted to give the same level at a measuring microphone. The power required to produce the same measured acoustic volume was measured. The experiment was repeated at various microphone placements in the room.
RESULTS
A power level of 1.2 watts per loudspeaker (for the conventional stereo system) was used to provide the ‘loud but realistic’ volume for measurement. When the measuring microphone was positioned at the ‘sweet spot’, there was little or no difference in the power requirements of the two systems. Moving the measuring microphone increasingly beyond the conventional stereo sweet spot, an increasing difference in the power consumption for the two systems was recorded.
At a distance of 3 metres on-axis with either of the stereo loudspeakers, the power requirement was 15% less for the Airsound system. When the microphone was placed off-axis of the loudspeakers, the power difference rose to a maximum of 32%. This value proved to be reasonably constant throughout the room, At distances greater than 4 metres from the two systems, and off-axis of the loudspeakers, the audio became increasingly more difficult to measure accurately, but a power differential of 32% or greater was demonstrable.
During the experiment, the measurements were taken from the calibration microphone. Subjectively, the Airsound system appeared to be louder than the conventional stereo system in all respects and at all times. It should be noted that the ‘clarity’ of the test tones was noted to be subjectively better on the Airsound system at all times.
PSYCHOACOUSTIC EFFECT
Experiments attempting to quantify the psychoacoustic improvement in sound quality and clarity, while not capable of providing precise and verifiable data, subjectively indicate that to achieve an equal level of stereo effect with conventional stereo systems, Airsound systems require 40% less power. That is, the drive current to the loudspeakers is 40% less with Airsound.
PERCEIVED AUDIO QUALITY
One of the most readily perceived attributes of the Airsound system is the audible improvement in signal clarity, compared with a conventional stereo replay system. The reasons for this are complex, but were forecast by Alan Blumlein, back in 1931 in his treatise on stereophonic recording and reproduction. Blumlein recognised that while it was possible to reproduce spatial information from a pair of loudspeakers in what we now recognise as ‘stereo’, the quality of the reproduction will inevitably suffer because of the ‘wave’ nature of sound, and the cancellation and distortion effects caused by the generation of sound from two sources separated by a distance.
In simple terms, at low frequencies (below around 200Hz) sound is non-directional, and may be generated from more than one source with no detrimental effects. At mid frequencies (approximately 200Hz to 4KHz) sound becomes increasingly directional, and our sensitivity to its direction increases.
At frequencies above 4KHz the audio wavelengths are short (in the order of 10cms), and so when such sounds are generated from more than one source the separate wave fronts interact with one another, creating distortion artefacts. These are perceived by the human ear as as ‘muddled’ sound, with many false clues as to the direction of source.
N.B. It is true that if a ‘stereo’ system is correctly set up, and the listener is located precisely equidistant from the loudspeakers, then the stereo image and the sound quality will be good. Moving further away from this listening position, the accuracy of the stereo image and the level of sound quality are increasingly impaired.
In the basic Airsound system, the main information is generated from a single wide bandwidth transducer.. The sound, although modified by the environment, is not distorted by multiple signal sources; an unnatural effect. Modification to the sound by reflection off adjacent bodies and surfaces is a natural sounding effect that is ‘normal’ to our ears.
Spatial information is provided by smaller and less powerful spatial loudspeakers. These are directed at 90 degrees to the ‘main’ loudspeaker, and near 180 degrees from each other. Due to the increasingly directional nature of sound with frequency, interactions between the three loudspeakers are most significant in the mid frequency band, where they generate the spatial soundfield. Interactions at higher frequencies are reduced; here the spatial speakers act to produce further improvement in the ‘fidelity’ of the system, similarly to the high frequency drivers in a conventional hi-fi speaker.
Improved signal quality and clarity are particularly noticeable at listening positions away from the conventional stereo ‘sweet-spot’, as is the maintenance of a spatial soundfield; an effect that is completely lost with a conventional left and right channel stereo system.
EFFECTS AND LIMITATIONS
DAB RADIO
When applied to DAB radio, the spatial effect of Airsound works well unless the RF signal strength is low and error correction mechanisms within the DAB system become stressed. Under these conditions audio becomes fractured and time-displaced. The effect is that mid and high frequency artefacts are created and, because of the time displacements, they manifest as audio signals at the extreme left and right of the stereo signal. Because of the greater perceived signal clarity of the Airsound system, these artefacts are of greater annoyance than would be so with a conventional system. Presently, there appears to be no answer to this problem, until this acknowledged limitation of DAB technology is addressed by the system designers. In the short term, Airsound is suitable for DAB applications, but with the qualification that there are quality limitations in poor signal strength areas. Overall, however, the improvement in sound quality and spatial soundfield more than compensate for this occasional disadvantage.
OPERATIONAL PROTOTYPE AIRSOUND SYSTEMS
- HI-FI SYSTEMS
Integrated MP3 dock and CD player home entertainment systems: low to medium cost home Hi-Fi market.
These Airsound units provide high quality stereo sound, filling the normal living-room space with crisp, clearly defined spatial sound with little regard for the positioning of the Airsound loudspeaker. Given the many constraints on stereo speaker positioning in the domestic environment, Airsound provides an inherent solution and performance advantage for mass market home stereo.
2) TELEVISIONS
Flat-screen televisions are an obvious and very effective application for Airsound. We have demo systems for both stereo and surround sound televisions. Airsound has clear and important advantages over both conventional flat televisions and home cinema surround systems; the Airsound front loudspeakers produce the solid centre image necessary for comfortable television viewing, as well as a field of spatial sound, ideal for 5.1 channel listening. Airsound produces stereo and surround soundfields previously unattainable with integrated TV audio systems and low to mid budget home cinema surround sound systems.
- RADIOS AND OTHER PORTABLE SOUND SYSTEMS
Airsound LLP has working Airsound versions of six types of DAB radio. Airsound produces a level of sound quality not previously possible from portable radios. Airsound stereo radios possess added depth and height, while mono transmissions are completely unaffected. Similarly, Airsound enables the reproduction of real stereo from portable CD players (boom boxes) and MP3 docks of all sizes; where the small closely spaced stereo speakers currently integrated within these products cannot do so.
- MINIATURE SYSTEMS
Airsound has huge potential when applied to mobile telephones and personal media players: Airsound enables the reproduction of high quality stereo sound from smaller portable audio systems than ever previously imaginable. Recent developments include ‘Bodysound’, a personal stereo loudspeaker producing a close range stereo aura.
- PC AUDIO AND IT PERIPHERALS
Airsound offers significant potential for the development of both integrated high quality stereo audio systems for PCs, and one box peripheral stereo speaker systems. This offers improved audio quality and true stereo performance from miniaturised integrated audio components and compact external enclosures. The integration of Airsound systems in laptop computers also offers savings on power draw and hence battery life. Dias, a PC peripherals manufacturer is already one of the first Airsound licensees, producing an Airsound enclosure and sub system for PC, with iPod dock.
- COMMERCIAL INSTALLATIONS
Suspended ‘ball’ loudspeakers and ‘Tannoy’ type public announcement systems have always been mono devices. Three types of suspended units converted to Airsound stereo, plus wall and ceiling mounting types of enclosures demonstrate the effectiveness of Airsound in larger commercial and public environments, and show how fire and safety evacuation systems may integrate with good quality music playback systems. Airsound equipped PA systems provide far greater coverage and intelligibility than current systems, and offer significant improvements for public safety and information delivery.
Airsound stereo systems offer significant improvements in quality, cost and performance in commercial entertainment installations, providing stereo coverage in environments where the positioning of left and right channel stereo speaker systems is impractical, and providing higher quality audio with a lesser number of speaker enclosures, requiring considerable less amplification power.
Airsound can provide a unique solution for large area coverage. By using suspended Airsound units, the number of loudspeakers required to provide coverage for a given area can be reduced by 35%, at the same time reproducing high fidelity spatial stereo sound, rather than poor quality mono signals.
AIRSOUND TECHNICAL RESOURCE
The Airsound research laboratory is in Torquay and is staffed by audio engineers of considerable experience and flair. A significant part of the acceptance package for the use of Airsound is the offer of free technical assistance; not only in providing advice on realising the best results, but also in the preparation of practical models and prototypes to enable the implementation of Airsound technology with minimum delay and cost.
©2007 AirsoundLLP
Limited Liability Partnership No. OC325211
