Some History

In order to appreciate the existence of horns today it is important to look at the history of electroacoustical reproduction of sound and music. I will try to give a brief summary of the most basic things, in case you are just getting into horns. (I am a relative newcomer myself. By the way, I'm 23 years old and my name is Thomas Dunker.)
Okay, yeah...why horns? Most people KNOW, without knowing exactly why, that sound can be 'amplified' by anything that more or less looks like a horn. The concept has been used for musical instruments since the dawn of time, though exactly how a horn works belongs to relatively modern science.
What is interesting to the horn LOUDSPEAKER people how horns for use with electrodynamic/electromagnetic transducers have evolved. This has a lot to do with the development of radio and electronics in general. When the triode tube was invented by Lee DeForest, it was suddenly possible to AMPLIFY an electric signal to levels capable of driving 'receivers' (the headphones of that era, with a fixed coil on an iron anchor producing vibration in a metal diaphragm suspended close to the coil) to louder outputs than previously possible. The prospect of listening to radio without wearing headphones prompted some new developments. Enter the horn! The first horn loudspeakers were basically headphone 'receivers' with a horn attatched. The earliest triodes produced very small power outputs, practically in the milliwatt range. The 'receivers' used on the early horns had poor sound quality (though probably thought to be amazing at that time) and the moderate efficiency did not help get the most from the precious little power one had at ones disposal.
Needless to say, when power was so limited, one had to make up for it by high efficiency speakers. An important breakthrough happened about 1927, when two gentlemen named Wente and Thuras, engineers at Bell Laboratories came up with a design that would prove to be a huge milestone in the history of high quality audio reproduction: The compression horn driver. (In horn terminology, a DRIVER is the actual transducer attached at the throat of the horn.) What they tried to do with this design was to get the highest possible efficiency as well as an extended frequency range. In most ways, compression driver design was pretty much DEFINED by this early invention. Their driver used a field coil to magnetize the pole pieces (permanent magnets of sufficient strength were not practical or economical to use at that time). They incorporated an underhung aluminum ribbon edgewound voice coil (still the 'state of the art' with compression drivers in 1995, almost 70 years later).
The diaphragm was an inverted aluminum dome (still used in many compression drivers today) attached to the voice coil which was self-supporting (ie. not cemented to a coil former) to minimize the moving mass. The Wente and Thuras driver also had a phase plug - a device placed between the diaphragm and the horn throat, whose purpose is to ensure that the sound waves from the diaphragm merge into a coherent wave front in the horn throat. The design of the phase plug and the transition to permanent magnets are the only true differences between the Wente/Thuras driver and today's compression drivers.
The Wente and Thuras 'high efficiency receiver' - patented in 1928 - was put into manufacture by Western Electric (a subdivision of Bell/AT&T) and early WE models like the 555/555W horn drivers are very close to the patent. These early WE compression drivers are considered to be good sounding even by today's standards, but unfortunately 99% of them have found their ways into japanese audiophiles' homes. The Western Electric drivers were important components in the early Movietone/Vitaphone movie theater sound systems of the 1930's.
Compression type drivers will not handle reproduction of BASS, so compression driven horns will need some sort of bass system. These can be either horn loaded bass drivers or direct radiators (using loading concepts other than horn loading). The drivers used in bass horns are not conceptually different from bass drivers (woofers) as we know them today.
Important pioneer work on low frequency and full range horns and drivers in this period (20's-30's) was done by P.G.A.H Voigt (UK) who was an enthusiast for the tractrix type horns that have seen a reneissance the past decade. Voigt teamed up with another UK company, Lowther and formed Lowther- Voigt. Lowther still exists and still builds drive units according to Voigt's practices. These are all full range paper cone drivers designed for horn loading. (Rear loaded bass horns.) Lowther speakers have a cult status with some people, and most Lowther owners are convinced that there is nothing that beats the Lowthers.
To continue with the historical thread, what would happen as time progressed was that power amplifiers became more powerful, at lower cost, reducing the need of ultra-high efficiency loudspeakers. When Rice & Kellogg 'invented' the direct radiator speaker, horns soon disappeared from domestic sound reproduction, being big and unwieldy compared to the more compact direct radiators. Soon after WWII, push-pull pentode amps relieved the triode amps, and later yet, the transistor amps arrived - with the ultimate 'power per buck' rating. Complementing this trend on the amp scene, speakers generally became less and less efficient. 'Hi-Fi' speakers today have efficiencies 20-40 dB below the figures which were common for the horn speakers of 'the olden days'. Interestingly, power amps today are only 10-20 dB more powerful than a typical 1930's triode amplifier. Horns no longer have a major position with 'hi-fi' speakers. However, they have survived in some arenas where their paricular qualities (besides high efficiency) make them the first choice. (Movie theaters, Sound Reinforcement, PA applications.)