About Stage Accompany

 Since it started at the beginning of the 20th century, the audio industry has always been searching for new methods of achieving the best possible reproduction of the audible frequency spectrum.

In the early years, the radio industry was the great driving force behind innovations. But it was not until the FM-band was introduced (not so very long ago) that "high fidelity" broadcasting could be considered. But for professional audio technology the emerging film-industry was much more important. After all, the sound system had to be suitable for large groups of people and, for those days, high demands were also made on the high fidelity of sound. So it is no surprise that it was precisely this industry which provided major progress.
In those days only tube amplifiers were used for signal amplification. The engineers of that day had to do without the transistorized amplifiers we have now, with their high outputs (more than 500W).
A top notch design tube amplifier then supplied some 50W. So this output had to be converted to audible sound as efficiently as possible. But a dynamic loudspeaker (as we still know them now) is an ineffective "converter". Only 0.1 - 3% of the electric energy is converted into acoustic energy (=audible sound). The reason for this is the very unequal proportion between the moving mass of the loudspeaker cone on the one hand and the air mass moved by that cone on the other. In the ideal case this proportion should be 1:1. For dynamic loudspeakers, however, this is usually 50 to 100:1. This factor is directly responsible for the very poor efficiency.

But the engineers of those days obviously did not leave it at that. A scientific article was published as early as 1920 in which this problem was solved. An "acoustic transformer" was invented which would "couple" the heavy cone mass via a so-called horn to a larger air-mass which improved the proportion mentioned above and considerably increased the efficiency. These horns are still used today.
The same article also reported on the principle which would give its name to generations of high-tone speakers. Thus the "Volume Compressor" was described. This technical invention raised the efficiency of loudspeakers even further. All professional medium- and high-frequency loudspeakers manufactured since then are therefore designated by the general English expression "Compression Driver".

The principle is simple. The loudspeaker is coupled to a chamber with a very narrow opening. As the cross-section of this opening is less than the cross-section of the loudspeaker, compression is induced in the chamber when the cone moves. This small opening in turn is coupled to the above mentioned horn but now at an even higher ratio and with larger final cross-sections. The result is even higher efficiency. In the years before the Second World War, this combined system also made it possible to generate an adequate acoustic output using a low amplifier output.

But then there is the old saying "Nothing ´s for free". Here too. The compression mechanism adds additional distortion and the mechanical construction clearly limits the maximum output. Compression drivers therefore tend to sound irritable at high outputs (professional loudspeakers are generally used at the limit of their maximum output in practice; Distortion figures of some 25% are then customary).
But greater efficiency was crucial in those days. There was no alternative to accepting the drawbacks. Since then there has been no change in the operating principle. All current compression drivers use the compression chamber to generate acoustic energy. For 75 years minor improvements were always being made, but the idea was never abandoned.