Directional Sound - Speaker Arrays

Speaker Arrays

While a large loudspeaker is naturally more directional because of its large size, a source with equivalent directivity can be made by utilizing an array of traditional small loudspeakers, all driven together in-phase. Acoustically equal to a large speaker, this creates a larger source size compared to wavelength, and the resulting sound field is narrowed compared to a single small speaker. Large speaker arrays have been used in hundreds of arena sound systems to mitigate noise that would ordinarily travel to adjoining neighborhoods, as well as limited applications in other applications where some degree of directivity is helpful, such as museums or similar display applications that can tolerate large speaker dimensions.

Traditional speaker arrays can be fabricated in any shape or size, but a reduced physical dimension (relative to wavelength) will inherently sacrifice directivity in that dimension. The larger the speaker array, the more directional, and the smaller the size of the speaker array, the less directional it is. This is fundamental physics, and cannot be bypassed, even by using phased arrays or other signal processing methods. This is because the directivity pattern of any wave source is the Fourier Transform of the source function. Phased array design is, however, sometimes useful for beamsteering, or for sidelobe mitigation, but making these compromises necessarily reduces directivity.

Acoustically, speaker arrays are essentially the same as sound domes, which have also been available for decades; the size of the dome opening mimics the acoustic properties of a large speaker of the same diameter (or, equivalently, a large speaker array of the same diameter). Domes, however, tend to weigh much less than the weight of comparable speaker arrays (15 lbs vs. 37 lbs, per the manufacturer's websites), and are far less expensive.

Other types of large speaker panels, such as electrostatic loudspeakers, tend to be more directional than small speakers, for the same reasons as above; they are somewhat more directional only because they tend to be physically larger than most common loudspeakers. Correspondingly, an electrostatic loudspeaker the size of a small traditional speaker would be non-directional.

The directivity for various source sizes and shapes is given in. The directivity is shown to be a function only of the source size and shape, not of the specific type of transducer used.