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Home : Avoiding Feedback : Avoiding Feedback, Loudspeakers

Avoiding Feedback, Loudspeakers

Loudspeakers, along with microphones and other pickups, are the transducers in this audio chain. They convert energy from one form to another. In the case of loudspeakers, it's from electric to acoustic and they are not very efficient in doing so. Nevertheless that small percentage of electric energy which becomes acoustic energy can be managed.

Directivity is an important factor in the selection and placement of loudspeakers. A consistent pattern throughout a large bandwidth is desirable. Most PA loudspeakers are designed with some degree of pattern control in mind, intending to direct sound to where it is wanted. This is commonly achieved using wave guide (horn) or line array (column) systems.

Directivity Q is the relationship of sound level at differing angles, comparing on axis sound level to the level at various degrees off axis, measured in horizontal and vertical planes.

Directivity is usually wider in the lows and narrower in the highs of whatever device is being considered. Some high frequency horns are especially designed to become narrower in the vertical plane as frequency rises (above about 8 KHz.). This is done to increase energy in the horizontal plane, compensating for narrowing of pattern with frequency rise and also for HF roll off in compression drivers.

Midrange directivity can be achieved in several ways. Larger waveguiedes can be used incorporating specially designed cone drivers or large-format (2") compression drivers. Line array configurations also work well. These consist of vertical columns of loudspeakers, each speaker being reinforced by its neighbor above or below, the pattern resembling a pie-piece shaped section of a cylinder. Bandwidth becomes a function of the loudspeaker's diameters and configuration (how many, how far apart), and whatever wave guide system is used (if any) in addition to the loudspeakers themselves. An optimum match between driver, horn and crossover will ensure a smooth extended response with a consistent pattern. Acheiving this is no easy task.

Single direct-radiating loudspeakers have a conical directivity pattern, not discriminating between vertical and horizontal planes. This also becomes narrower as frequency rises. It is a function of cone diameter, the larger the cone, the sooner it becomes directional as frequency rises. Some cones are designed with mass de-coupling rings (circular ribs), effectively making the radiating surface smaller as frequency rises. This somewhat extends HF response and widens directivity.

In multi way systems there will be variances of directivity within the frequency spectrum. At the crossover points, the lower band will be at its narrowest and the higher band at its widest. Overlap at the crossover frequency can blend the two but problems such as phasing come into play. For me, best results have been achieved with co-axially mounted constant directivity horns, a horn within a horn (these are large). Others successfuly use multiple drivers operating into one wave guide system. Well refined d'Appollito configurations produce good results also.

With low frequencies controlling directivity is more of a problem. Due to the long wavelengths involved, horn systems become very large, even when folded. A horn responding well to 40 Hz has a mouth area of more than 300 square feet. There are other ways though in achieving low frequency directivity. These involve phase cancellations in the directions where sound is to be diminished. The process is quite complex, using dedicated signal processing and multiple large diameter drivers. With common bass bins and subs, low frequency output is omni-directional, its character being influenced mostly by room placement.

Knowing where your loudspeakers project their sound is key to optimal placement. In most cases the idea is to reach only listeners, not room walls or pickup sources.


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