|The popularization of line array loudspeakers brought with it an inherent physical problem: the line array effect depends on wavelength-dependent spacing of drivers, which was possible to achieve for low frequencies, but required spacing that was impossible in a practical sense for the high-frequency drivers. A true high-frequency line source, such as a ribbon, could achieve the desired behavior, but ribbon drivers could not output sufficient SPL for high-level applications. Also, since the line array effect is the result of interactions between drivers, the dispersion characteristics of the loudspeakers are crucial. In other words, while broad horizontal coverage is desirable for line arrays, tight control of vertical coverage characteristics is critical.
In most cases, the dispersion of a high-frequency section in a sound reinforcement loudspeaker is determined by the length and shape of the horn. Narrower coverage is possible with longer horns, but that is logistically difficult and degrades the sound with additional distortion, since distortion is proportional to the number of wavelengths sound travels inside the horn. A long horn entails making considerable tradeoffs to gain vertical dispersion control.
The REM manifold was aimed at obtaining the best of both worlds — the high output and maturity of compression driver technology and the line array behavior of a ribbon — by introducing a multipath manifold in between the driver and the horn that it fed. The manifold splits the input into several short but identical length waveguides that all fed the horn input, thus mimicking the behavior of multiple, closely spaced sources, rather than acting like a single compression driver. Keeping the path length short (relative to the wavelengths passing through it) avoids length-induced distortion, while making the path lengths identical ensures that sound arrives at the horn throat in phase. Distortion is further reduced by having the cross-sectional area of the waveguides increase from input to output. Thus, the REM achieves the desired directional control without adding distortion.
The REM was first introduced in the M3D line array loudspeaker, which used an REM that accepted two compression drivers, each feeding four waveguides.