Horn-loading is by far the most efficient technique. It is between ten and over a hundred times more efficient than any others. ‘Efficiency’ means it gives the most acoustic intensity for a given power input, from the amplifier. Only when a horn (or ‘flare’) is coupled to a transducer with a low output (e.g. a ribbon driver) is the overall efficiency not ‘streets ahead’ of all the other driver+enclosure combinations.

The most efficient drivers are the familiar electro-dynamically-driven cone, dome and compression types, particularly those with an optimum balance between the strength of magnetic and electric coupling, the levity of the moving parts and the compliance of the suspension.

In the midrange, some ESLs can be as efficient as the cone-driver, both in the context of a refined domestic speaker. The least efficient enclosures are:

(i) none (this holds true at low frequencies only),
(ii) the sealed box (SB) or ‘infinite baffle’ (IB), and
(iii) the transmission line (TL) – used to extend bass response.

Of these, the latter two are important, practical forms that have to be lived with. They can in any event be made relatively quite efficient by making the enclosure big. To some extent, Colloms’ law holds here: “Loudness (per watt or volt) is inversely proportional to bandwidth and smoothness”.

This is fine until we come to consider the refined horn-speakers which do not attempt 50% efficiency, and where a minimum of three types are needed to cover the audio band. While at least ten times more efficient, there is little or no bandwidth narrowing over ordinary speakers.

Compression- and Piezo-drivers are those usually coupled to horns (flares), and may need no other boxing or at least not any specific enclosure, as their rear chamber is usually already sealed. Sound radiation is then mainly defined by the horn, subject to mounting.

Ribbon drive units may be also horn mounted; or if ‘Planar’ types, then along with ESLs, they may be simply mounted in a frame that has little effect on the sound radiation which is dipolic, ie. two sided, like a harp’s.

The other two types of drive units – the cone and soft-dome are usually mounted in closed (‘infinite baffle’ or ‘sealed box’) enclosures, or in the case of cone drivers alone, in ported (‘Thiele-small’, ‘vented’ or ‘reflex’) enclosures.

Cone drivers are also used ‘coupled to’ horns – either midrange, or bass (‘bins’). In practice, as the rear of the cone’s basket mounting frame is open, and the fragile magnet is also unprotected, cone drivers in bins and horns are always mounted inside the overall enclosure.

Horns, transmission lines, sealed and vented boxes, and other loading types may form complete loudspeakers in free permutation. Of these, only combinations of horns or of sealed boxes can cover the full audio frequency and dynamic range within their own family, i.e. without involving each other or the other types.

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