Sound diffusion systems - Efficiency

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This is the effective measurement for the acoustic power of a loudspeaker. In other words: its ability to transform electric energy into acoustic energy. Clearly the greater the efficiency of the loudspeaker, the greater the quantity of electric energy which gets transformed into acoustic energy. The part of electric energy which doesn't get converted into acoustic energy is dissipated by the loudspeaker as heat. This is one of the reasons why the coil within the air gap is often kept air tight: the presence of air would consent a greater temperature increase as a result of the dissipated energy, with the risk of damaging the coil.

Efficiency varies depending on frequency, therefore a loudspeaker will be used in the frequency band where efficiency is at its highest and almost always constant. The efficiency of a loudspeaker is usually very low, in the order of 1-2% to a maximum of 8%.

To increase its efficiency, various different methods are used depending on the reproduced frequency band. At the lower frequencies, cone shaped membranes are made that are better than flat membranes at gathering the air before it and moving it. The following diagram illustrates this situation:

Sound diffusion systems - Comparison between a cone shaped and flat membrane

Comparison between a cone shaped and flat membrane

9.4.1. Pneumatic suspension Loudspeakers

With loudspeakers for low frequencies, efficiency turns out to be particularly low due to the fact that the elastic suspension considerably slows down the oscillations to prevent the production of undesired sounds. Pneumatic suspension loudspeakers are employed to increase efficiency. In this case, the loudspeaker is fixed to an air tight container and the material which joins the membrane to the loudspeaker is deprived of its "slowing-down" characteristics, which now occurs as a result of the restoring of the pressure variations generated by the membrane's oscillations. In other words, seeing that the posterior area of the membrane is air tight, a movement of the latter provokes a variation of the internal pressure which gets restored by the air pocket. This allows for a much greater excursion of the membrane, and therefore a considerable increase in efficiency.

9.4.2. Acoustic horn loudspeakers

To increase the efficiency of the loudspeakers reproducing high frequencies, they are fixed to the base of a horn-shaped conduit, as illustrated in the following figure:

Sound diffusion systems - Horn loudspeaker

Horn loudspeaker

This way, the so-called acoustic impedance matching takes place. Without the horn, the membrane is in contact with an air surface which is theoretically far greater than that of the membrane itself, and this generates a dispersion of the acoustic energy in all directions. With the horn, on the other hand, the membrane is in contact with an air surface similar to its own surface. The first air layer (which has a surface which is slightly greater than that of the membrane) is in turn in contact with the next air layer, which, due to the shape of the horn, shall be a little larger than the previous one, and so on and so forth. This way the air movement is progressively passed on from one air layer to the next, with surfaces increasing layer by layer. This allows the acoustic energy to be well channeled, avoiding dispersions. Naturally there are different kinds of horn shapes, each with its own characteristics, even if the functioning principles remain the same. With these systems, efficiency increases by up to 30%.

As well as improving this system's efficiency, this system is also used to direct the high-frequencies which, as we know strongly depend on the direction of diffusion.


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