Generating spatial audio from portable products - Part 1: Spatial audio basics

The term "acoustic beamforming," when applied to loudspeaker audio reproduction, refers to the ability to direct audio waves in a particular direction other than the loudspeakers typical radiation pattern.

An ideal loudspeaker acts as a piston-type source set in an infinite baffle and has a radiation pattern that is dependent upon the frequency being reproduced relative to the speaker disk radius. The degree of acoustic sound beaming is related to the ratio of the radius of the loudspeaker piston to the wavelength of the sound.

At low frequencies, the loudspeaker's sound spreads out evenly in all directions in front of the speaker. This is the reason you can set a subwoofer at almost any location in the front of a room and hear it equally well from anywhere else in the room.

As the frequency is increased, the radiation pattern becomes more focused in front of the speaker, increasingly becoming a narrowing cone shaped pattern around an axis perpendicular to the loudspeaker face. The sound pressure level is strongest within the cone pattern and drops off rapidly outside the pattern. You can test this by listening to high frequencies from your hi-fi loudspeakers while moving from side-to-side.

Also, real loudspeakers are mounted in relatively small finite boxes, not infinite baffles. The edges of the box will cause diffraction of the sound waves resulting in a more complex radiation pattern. If the loudspeaker was placed in an open air setting, you would correctly hear all of the frequencies reproduced only if you listened at a position that was on the axis perpendicular to the front face of the speaker.

If the loudspeaker is placed in a room, the listening position is less critical since the sound is also being reflected from the walls and other furnishings at various angles. While reflections make it easier to hear from any position, the reflections arrive at different times and intensities than the original signal and result in sound that lacks clarity.

Acoustic beamforming in the extreme case attempts to direct the sound energy emanating from the speaker to a specific angular position within the room environment. Acoustic beamforming with one speaker is very difficult, so typical applications use two or more speakers. Using multiple speakers also allows the use of constructive and destructive combinations of sound wave energy to create certain directional patterns.

Multiple speakers, as seen in Figure 3, are placed in an array where the array size and shape also make certain directional patterns possible. The array is usually physically linear, but may also be built as two dimensional arrays such as curvilinear, planar, circular, or combinations thereof.

As a general rule, spatial effects are more easily accomplished and discerned at higher frequencies. However, a large array along with speakers with very good low frequency response will allow better directional control over low frequencies.

The size and shape of the array partially determines the technique(s) to use to achieve the desired spatial effect. The other factor is the purpose for which the array is being built. For example, if the array is constructed for the picture-in-picture feature, allowing two viewers to watch two different TV channels (low use case), another technique would be needed if the purpose was to simulate a 5.1-channel surround sound environment (high use case) for several people seated in front of the TV. As a practical matter, the spatial audio system may have to support both methods (and possibly several others) in order for the consumer to have pleasurable listening experiences regardless of the TV program-mode.

Go to Part 2 of this article.

About the author:
Kenneth Boyce is Audio Technologist for Texas Instruments Silicon Valley Labs. He previously served as Technology Director for National Semiconductor's Audio Products Group, and before joining National, Boyce served as director of the Audio and Communications Division at Oak Technology, which developed initial implementations of AC-97 Codecs and Digital Audio Controllers. He holds a bachelor of science degree in electronics from West Virginia University.

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