NEW LOUDSPEAKERS FOR VWF

Everyone has at least one set of loudspeakers. It would appear that to eliminate sweet spot listening, all that is needed is to place pairs of these one behind the other to form Vector Wave-Front (VWF) or Depth Render (DR) arrays. New signal sources and signal processing will be needed, but having the loudspeakers would potentially be a good start.

 

Can present-day loudspeakers be used to re-create VWF sound fields with accurately placed sources and no listener sweet spot restrictions?

 

First the good news. Your existing loudspeakers can be used as either the front or the rear loudspeakers of a VWF array and will provide surround-style reproduction. This approach will, however, not ensure that sound sources are correctly fixed in space.

 

Now the bad news with your existing loudspeakers.

1. Standard surround loudspeakers have poor off-axis response. For expanded listening areas, the listener is not constrained to the one sweet spot location as was the case with surround systems and formats. Thus good on-axis response is no longer good enough. The on-axis response of the loudspeaker has to be maintained well off axis. A well-maintained omni-directional output source loudspeaker will be required. VWF speaker amplitude and phase response both on and off-axis need to be exemplary. Surround sound loudspeakers are not. They are designed for on-axis listening – needing to be “aimed” at the intended sweet spot. Very few (if any) maintain their phase response or even their average spectral output off-axis – they do not have to with surround sound reproduction!
2. Most surround loudspeakers available today exhibit resonant modes for their cone structures. The most problematic of these are the resonances in the surround structures supporting the cone. When only the on-axis response is considered (as with surround sound systems), it is possible to adjust the response at manufacture to provide generally consistent on-axis results. But when output has to be maintained over a wide listening angle, this does not work. Resonances in a driver surround will create frequency dependent directivity anomalies that are very hard to eliminate.
3. Cost constraints for modern day surround sound speakers have resulted in rising distortion figures. Any distortion products in the loudspeaker will create spurious frequency harmonic components with inappropriate phase response. These will cause undesirable peaks and dips in the response in certain directions.
4. The size of the loudspeaker will often be a fundamental limitation to the behaviour of the created sound field. First, the loudspeakers will get in the road of themselves. They are not designed to be configured as array elements and are often just too big. Second, large diameter cones on loudspeakers have a directivity that beams with increasing frequency. (it may seem counter-intuitive but a larger speaker cone actually beams more). A simple single driver full range direct radiating loudspeaker source with reasonable omni-directional characteristics up to 20 kHz (to pick a popular figure) could be no more than 6 mm in diameter because of this limitation. This is totally impractical for low frequency reproduction. Third, the use of multiple loudspeakers covering the same or overlapping bands will not solve the problem as it is the effective driver size that controls the directivity of the created waves. The use of multiple small drivers to cover a common frequency band again increases the apparent cone size with attendant beaming.
5. It is extremely difficult to make a multi-way or multi-driver loudspeaker that behaves as a point source for all audible frequencies over a wide polar pattern in three or even two dimensions by mounting multiple drivers separately on a flat baffle, as is the common practice today. Co-axially mounted drivers are a better starting point.

 

So the short answer is no, present-day loudspeakers do not lend themselves to VWF formats eliminating sweet spots. A new category of High Fidelity loudspeaker is required that takes directivity into serious account.

 

The ideal VWF loudspeaker

The ideal loudspeaker source would provide:

·           Adequate sound pressure level at the listener (103 dB everywhere in the listening environment, not just the sweet spot, if reference level ITU 775 is used, or greater if all known real life sounds are to be faithfully reproduced!).

·           Distortion that is low – below human perceptual masking levels and so is hidden.

·           A response that is maintained over a wide (or the desired) listening area in two or three dimensions as required.

·           A physically small sized unit for shadowing, aperture, directivity and cosmetic issues.

 

Such loudspeakers would effectively behave as spherical wave generators (point sources) for all frequencies of interest and so be suited to controlled divergence arrays.

 

The design of suitable loudspeakers has required the development of new design and construction techniques utilising Controlled Diffraction plate (CDP) structures. CDP structures enable control of the directional phase and frequency response of loudspeakers. These have previously been used in other fields such as electron microscopy and particle physics. Their use in the design of loudspeakers is new and has opened a new realm of loudspeaker performance possibilities.

 

For reproduction where the listeners walk completely around the reproduced sounds in the room and still maintain clearly defined acoustic images, the loudspeakers must be truly omni-directional. Where the format requires that the sound field perspective change as the listeners move around the created sound field region, loudspeakers that behave as both a point source and create sound field components that also change with direction will be necessary. This could be achieved by having separate line array sets radiating out from a point or by having one sectored array such that the sum of the directional components add at a point. Each sector provides the necessary directional wavefront divergence coverage [[1]].

 

These techniques and apparatus are new and the term Vector Wave-Front or VWF format has been coined to describe them.

Are there any VWF capable loudspeakers out there?

Figure 1 shows a 32 element omni-directional array loudspeaker. This unit is intended for test purposes. It includes CDP treatment of the individual drivers and is sectored to enable both distance and direction reproduction.

 

Figure 2 shows a domestic VWF capable 180 mm diameter two way loudspeaker. This unit features CDP filters for the two drivers, providing a sound source extending upwards from 100 Hz with response 3 dB down at 20 kHz at 80 degrees off-axis. As the unit does not need to be aimed, it is suited to flush in-wall mounting.

Any VWF capable loudspeaker array would be fully compatible with all other equidistant formats such as stereo and surround. You just turn off the distance render.

An added bonus of the VWF format is that you have the capability to render equi-distant surround formats at the distance of your choice without moving the loudspeakers. You could place the loudspeakers in a convenient location and then electronically move the rendered surround field to the desired location. Many other effects are also possible.

 

The use of omni-directional VWF capable sources has further benefits. VWF loudspeakers are capable of radiating full acoustic energy at all frequencies consistently in all directions. Sweet spot aimed systems sound very dull by comparison when listened to off-axis. Acoustic reflections from room boundaries can become an issue because overall the VWF systems sound brighter and are more responsive to listening room acoustic treatment. This is not strictly a detractor because the acoustics of the room would be exactly as for the original instrument played in that room (instruments do not aim at the sweet spot!).

 

Where the audience is known to be located in a particular restricted location, directional coverage can be used with VWF capable loudspeakers. This is the case particularly with cinema and in-vehicle sound systems where seating is well defined and listener movement is restricted.

 

The CDP acoustic correction approach used with VWF has the added advantages that the equalisation tracks with changes in the properties of the medium (air), and that the resulting filter structures assist in reducing distortion, and that the design is readily mass-produced. The only disadvantage is that it is hard to design in the first place.

 

We now have the well-behaved omni-directional loudspeakers we need for placing sound sources at points fixed in space so that listener movement and turning has no effect and listening experiences can be shared. This means sweet spot free reproduction in rooms can now begin.

 

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