This system combines the results got with a closed enclosure and those obtained with a labyrinth and folded pavilion.
It is indeed recognized that a closed chamber has many advantages for the damping of the speaker, so its transient response, provided you have a very flexible speaker and strong magnetic field. However, the resonance frequency of the speaker in an enclosed cabinet (of reasonable size) significantly increases and the acoustic performance below that frequency decreases very rapidly to 6 dB per octave. On the other hand, the back wave is lost. The efficiency of such a speaker is in any case 6 dB lower than a system that recovers the rear wave.
The Infinite acoustic line system is to use the rear wave. It is "forced" in a labyrinth tapered whose length is at least equal to 1.50 m
The labyrinth is folded so as to collect the back wave in phase* with the front wave to enhance performance in the bass and sub-bass...
Then The originality of the system resides in an internal acoustic filter which acts as a "low-pass" for the rear wave and "high-pass" for the front wave, the assembly acting as two loudspeakers coupled without delay (unlike the passive-active system).
The speaker A (front wave) sees its sound pressure decreases by 6 dB per octave from the resonance frequency of the speaker in the enclosure (70 Hz to 100 Hz depending on the speakers) and the speaker B (rear wave) sees his sound pressure almost constant of 30 Hz to 70 Hz or 100 Hz to decrease above 100 Hz.
The combination of the two waves in phase gives a perfectly linear reproduction from 30 Hz.
(*) Rigorous phasing is obtained by a progressive damping of the rear wave through increasing density absorbent.
The careful calculation of absorbent densities allows to artificially lengthen the path of the rear wave up to 3 times the geometrical length of a constant section pipe.
Art & Acoustique Appliquée
3A patent filed décembre 1, 1975 n° 75 36 678
