3A patent filed November 30, 1972 n° :
• 72 42 542 for France
• 209 390 for Canada
• 468 868 for USA
• 53 787/74 for Japan
Synthesis
The acoustic pressure feedback (APF)
The speaker is a mechanical system which generates a sound while moving. This displacement is nonlinear (speaker's shape, suspension, non-uniform magnetic field) and the sound is distorted. So a speaker can reproduce low notes only if it's shut in a large enough « box »... In 1950, C.A. Briggs advised a brick enclosure of one cubic meter to reproduce a 16 feet organ pedal (32 Hz) !
To reduce these fantastic dimensions, engineers only found palliative (cavities, resonators, closed depreciation), but with these systems the mechanical movement of the boomer is not controlled, it is corrected. There are manufacturing of basses and not reproduction.
M. Brette and Perrin, french acousticians from the Ecole Supérieure d'Electricité (E.S.E.), were the first in 1961 to propose an acoustic pressure feedback loudspeaker, technique that was then aerospace technology. Later the company PHILIPS proposed the « Motional Feedback » (or M.F.B.) loudspeaker, based on a piezoelectric accelerometer. In 1971, Mister Daniel Dehay, Chairman of the company 3A, engineer E.S.E., succeeded to the industrial development of the acoustic pressure feedback loudspeakers.
This discovery not only removes the speaker deformations but also allow to reduces the size of the cabinets to the minimum. All mechanical movements of the driver are electronically controlled (slaved), as in the car with power brakes, power steering, in industry with machine tools programmed by computer or in aeronautics where rudders and flaps of the aircrafts are assisted.
So the basic idea was to remove the sound boxes and replace them with an amplifier that controls the movement of the driver. For that an electronic sensor records the movements of the membrane and instructs to the amplifier in the speaker to correct deformations related to the movement of the speaker. This process achieves incredible performance in miniature boxes : Linear bandwidth from 30 to 40 000 Hz in 20 dm³.
The amplifiers powering the slaved 3A loudspeakers are industrial design. All components used meet professional military standards (electrolytic or tantalum capacitors, layer resistances, integrated circuits). The circuit boards are epoxy glass, the oversized transformers are impregnated.
Each amplifier is tested at the vibrating table to test welds and avoid manufacturing defects. Finally, the electronics are made reliable by 100 hours of high power operation (60 watts), allowing debugging of components according to semiconductor technology for the space industry.
The electronics failure probability is then almost zero.
The APF system
1) - Speed bridge
The speed is detected by a Maxwell bridge system, known method for measuring the speaker characteristics.
2) - Analog calculator of slaving
This amplifier at transfer function Zm (speaker radiation impedance) was obtained experimentally by comparing the actual sound pressure in front of the speaker to the coil speed.
The vectorial composition of these two quantities allows to determine the module and phase function. A feedback electronic circuit on an amplifier with very high gain gives a very accurate picture of Zm.
The speed voltage is applied to this circuit and the output voltage is a perfect simulation of the sound pressure.
3) - The results
The amplifier necessary for the linear reproduction of 30 Hz is 125 watts. The speaker's membrane at this frequency moves about 2 cm for an electric attack power of 5 watts, corresponding nevertheless to a musical program of 100 watts.
Thus, the APF system provides to obtain from a miniature loudspeaker the same return in the bass and sub-bass than with a 15 to 20 times larger loudspeaker. Distortion is extremely low, since difficult to measure from 60 Hz. At 40 Hz, it remains below 2%. In the same volume without feedback control, a speaker would give 8% distortion.
Art & Acoustique Appliquée
3A patent filed November 30, 1972 n° :