Practical Systems

In order to achieve the best result for a particular product type and price engineers must adopt a pragmatic approach using the most appropriate mix of techniques. We will consider four such realworld applications each one using some of the methods discussed.

A System Creating ‘Virtual Loudspeakers’.

One way of creating the surround effect is to use the HRTF derived filters described above to create the sound pressure at the listener’s ears that would be produced by correctly positioned surround speakers.

The left and right (L&R) signals are fed to the appropriate speakers unaltered. The centre channel signal is fed equally to both left and right speakers producing a phantom image of the centre channel.

The surround signals are first modified to compensate for the crosstalk and then are filtered with the appropriate HTRF for a rear speaker position. Room reflections may also be simulated by adding delayed surround signals with some low pass filtering to give the effect of high frequency absorption by the walls. These modified surround signals are added to front signals on either side. [9]

A system Using Beam Steering to Create Wall Reflections

A system using beam steering to create a surround sound from just one enclosure containing an array of drivers is disclosed in [10]. Beam steering requires the signal for each driveunit to be individually filtered. A number of beams are produced from the same loudspeaker array by summing the individual signals required to produce the beams.

To recreate the correct effect the center channel is radiated directly. The left and right are aimed so they impinge on the listener after one reflection whereas the surround impinges on the side-wall at a shallower angle being reflected a second time off the rear wall. Achieving this requires significant DSP processing power and 40 separate amplifiers & DA converters.

A System Using a Dipole to Create Wall Reflections

An innovative system in which a pair of loudspeakers each housing a pair of full-range cone driveunits positioned side by side using a specialised processor is disclosed in [11]. Each loudspeaker radiates as a monopole for the left centre and right signals. For the surround signals the processor inverts the phase of the signal to one driveunit producing dipole radiation.

The center channel signal is routed equally to both loudspeakers giving a central image.

Where the loudspeakers are directed at the listener the listener will be positioned in the null where the direct signal is level low. In this position the listener will consequently hear mainly reflected sound.

A System Using Diffuse Dipole and Monopole Loudspeakers

A more recent approach disclosed by Dodd [12] uses a novel combination of diffuse dipole and coherent monopole. The diffuse dipole source is orientated with the null directed at the listener.

The diffuse dipole source may be most simply achieved by means of a Distributed Mode Loudspeaker (DML). The complex modal vibrations of a DML produce a strongly diffuse field since the radiation occurs from numerous regions of the panel with various phase and time delays. Opposite sides of the panel radiate with opposite volume velocity giving a dipole-like radiation pattern with nulls in the plane of the diaphragm [13].

The coherent monopole is provided by a coincident source loudspeaker directed at the listener in the usual manner.

In this type of system the surround and front signals are applied to these different types of loudspeakers. No special signal processing is required. The left and right signals are applied to the appropriate coincident source loudspeaker. The centre channel signal is routed equally to both coincident source loudspeakers in the usual manner. Surround signals are suitably delayed by 10-20ms and then applied to the appropriate DML.

To achieve optimum envelopment by diffuse source the DML is orientated edge on to the listener. This results in the listener being positioned in the null of the diffuse dipole radiation produced by the DML [13]. This null is not as deep as the null in a conventional dipole, however the IACC is lowest in the plane of the DML [13]. It is believed that it is the low IACC at the listener position that allows the DML to produce a sensation of spaciousness without room reflections. Some informal tests carried out by the authors have shown that a spacious sensation may be reproduced even in an anechoic chamber.

Another characteristic of the DML is that both the HF radiation and the IACC are greatest in the direction normal to the DML. This characteristic means that where a wall is suitably positioned specular reflections may produce virtual images to the side of the listener. In the ideal rectangular room these reflected sources may even surround the listener.

This approach results in unique combination of robust ambience, giving the sensation of being in the recorded environment, together with some reflected images where the domestic enviroment allows.

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