Horns have been used to amplify the sound generated by a membrane since the days of mechanical gramophones.
The small membrane, excited by record player stylus, was not able to provide the sound suitable for a home environment of normal size, therefore the engineers chose the use of horns to amplify this faint source.
Looking at the picture below, you can see that the system was already using the compression chamber to increase the transduction efficiency.
Below is a diagram of a dome loudspeaker for mid-high frequencies, suitable for midrange and tweeter.
These loudspeakers have good properties, such as frequency response, angular dispersion and power handling; however, they have not good transduction efficiency and are therefore not suitable for constructing speaker systems with high efficiency.
Focus now your attention on the basic diagram of a horn loaded dome speaker:
Like the gramophone, also a dome electrodynamic transducer can be loaded with a horn to increase its efficiency.
Efficiency may further be increased thanks to a compression chamber placed between membrane and horn throat; in this way, however, there would be a significant phase cancellation with relative worsening of frequency response, because of different sound paths coming from different distances between each emission point of the membrane and the horn mouth.
Then a device came out, the “Phase plug”, which was just invented and developed in order to solve the problem of cancellation due to the phase displacement of sound waves within the compression chamber.
Here's an example:
This device became an integral part of the compression chamber, forcing the sound waves emitted from different parts of the membrane to run through a precise path, such as to minimize the effects of phase cancellation.
Even if the Phase Plug had solved some of the problems of interfacing between membrane and horn, the performance of this device was not yet satisfying; then, in the '30s, the Western Electric Company, at the forefront of transducer technology, managed to produce the famous driver WE 594A:
The technicians first took advantage of the well known technique of placing the dome convexity on the opposite side, and on this basis they enhanced the system ability to minimize the phase cancellation with the introduction of a “fragmented” Phase Plug, that would have made the paths of pressure waves even more similar among them.
This device can be considered the first modern driver, and nowadays, new drivers are still designed on the basis of the model WE 594A.
The presence of the compression chamber, however, causes distortion phenomena that are considered detrimental to the sound desired in Hi-End world.
Furthermore, today's drivers are designed to withstand the enormous power of live concerts and also to have the maximum possible efficiency at expense of response regularity, distortion absence and sound coloration.
Their sensitivity, ranging from 105 dB/W/m to 115 dB/W/m, is really too much high for Hi-Fi use, because driver should be coupled to direct-radiating woofer with a sensitivity of no more than 98-100 dB/W/m; therefore it is necessary to use a resistive attenuator in order to make the output level suitable for the level of woofer.
However, we think that this attenuator is responsible on one hand for a compression of dynamics that causes a flattening of sound and on the other hand for the covering of the micro-contrast that would otherwise be present under high-level sounds.
Because of these characteristics, these drivers are not suitable for Hi-Fi systems, where regularity of frequency response and low distortion are the main aspects to take in consideration.
Even in recording studios, where the use of compression driver was a rule, in recent years these devices have been neglected in favor of horn-loaded systems without compression chamber and with good linearity.
Here is a simple scheme:
We can note that membrane is directly coupled to the horn throat, so it is very important to study the coupling between these elements which interact so closely.
This system has sensitivity up to 98-100 dB/W/m, clearly insufficient for a professional application but very good for Hi-Fi use, since it allows to avoid the presence of attenuation resistive networks which are bad for micro and macro sound dynamics.
Furthermore, horn speakers are not only the best solution for high efficient transduction, but they are also the strictest way to obtain a constant directivity all over the audio frequency range.
In a multi-way system, in fact, directivity should not vary, neither by steps nor in other manner, when going from one driver to another; in a three-way speaker, for example, woofer and midrange should have the same directivity at their crossover frequency and so should be for midrange and tweeter. In all the cases, this result can only be obtained by using an acoustic impedance adapter, that is to say a horn, since the size of membranes are obviously very different, so that midrange can be adapted to woofer and also tweeter can be adapted to midrange.
Focus now your attention on the operative range of a single speaker. In traditional horn (exponential or similar profiles), the directivity varies with frequency, so that you have, on one hand, an almost perfectly omnidirectional sound emission at low frequency, and, on the other hand, a very directive sound emission at high frequency.
Constant directivity horn, instead, allows you to have a frequency response that is almost constant in all directions covered by the features of the horn itself.
A device with a constant directivity feature has a tonally balanced emission across the whole angle of coverage, so that the sound coming to you from the reflecting walls is almost the same of the sound heard in front direction.
Ultimately, then, the “Zero Compression Direct Front Loading” technology of Casta speakers provides high efficiency, good linearity of frequency response, constant directivity, very low distortion and no loss of micro-information.