acoustic / WFS /
Air resistance adaption of the loudspeaker diaphragm
Dynamic loudspeakers mounted in compact boxes posses a common problem: Its efficiency is pitiful. In one meter distance hardly more as one percent of the injected electric power is converted in sound energy. The problem isn't mainly caused by the efficiency of the driving system itself, the main problem consist in the lousy adaption of the diaphragm regarding the load resistance of the moved air.
Air is a lightweight; put on the scales just around 1.3 milligrams per litre. Over and above its viscosity is low, that means the molecules possess loosely structure, air can easily evade in all directions. It is traceable, we cannot move noticeable amount of air in the direction of some meter distanced listener with a movement of our hand. We would perceive hardly resistance; no load would appear in front of the hand.
That's Maladjustment! Since in water, the conditions would better without all doubt. Water has more as eight hundred times of the weight from the same volume of air. Our hand or our loudspeaker diaphragm would be able for transmit power into the medium. But who wants to live in the fish tank only for hear the music louder? Better would be taking a piece of paperboard in our hand. Now we must spend a little force for move the air. If we flutter fast or the diaphragm radiates high frequency, the air hasn't time for move simply around the paper or the loudspeaker box. The resistance adaption increases. But for low frequencies we cannot avoid this cowardice of the air to countervail against the force.
If the loudspeaker mounted in the infinite baffle, the air cannot move behind. That increases the efficiency, but the best adaption can be reached if the wave front becomes squeeze in a conduct. In form of a horn such conduct avoids sudden change of diameter, what would be connected by resonance effects. But the air hasn't the chance for simply evade anymore. The air mass in the passage must become moved from the loudspeakers diaphragm. That improves the resistance adaption because the additionally mass posses really load for the loudspeaker system. The system resonance is decreasing, while the resilience of the air in compact boxes increases the resonance.
The efficiency of such guided system 20 to 40 times higher as the compact box solution, peerless dynamic of the sound is the effect. The only disadvantage of the solution is the size. Full range horns must have aperture of some meters, the given function for resonance free form causing some meters horn length for deep frequencies.
Shorter solution for the same principle would be the “acoustic curtain” from some hundreds loudspeakers, steered by the wave field synthesis principle. In range of long waves adjacent speakers work nearly synchronic, as visible in the animation:
It's comprehensible, the air cannot move besides because the same pressure is providing from the neighbouring speaker in that range. Some calculations promise, some micrometers movement of the diaphragm would provide sufficient sound pressure in main tone range of the home cinema. That would open less expensive solutions for the high amount of loudspeakers by the foil solution or digital loudspeakers in future.
Patent description DE 10 2005 001 395
