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Technology: Distributed Resonance

Laser-trimmed frame showing distributed resonance spacing Sound Lab products employ a revolutionary patented principle, called "Distributed Bass Resonance", that virtually eliminates the two major drawbacks of dipole speakers: the membrane "drum-head" resonance and dipole energy cancellation. These will now be explained and how their effects are eliminated by Sound Lab's technology.
     First, the resonance peak exhibited by all stretched membranes (the drum-head membrane is a good example) can be a serious drawback in the electrostatic speaker (or any type of membrane speaker) if it is not eliminated. It can cause a large acoustical energy peak at the resonant frequency that can be as high as 30 dB. This sonic peak not only "colors" the sound but it also limits the usable dynamic range of the speaker.
     The orthodox approach used by others to eliminate the reonance peak is to use acoustical damping, such as a fine mesh, similar to that used in the manufacture of sheer nylon stockings. Unfortunately, resistive damping does "surgery" over the entire pass band of the speaker, giving it a lack-luster quality. Furthermore, the resonant energy is dissipated and not put to good use.
     We considered the possibility of using the membrane's resonant energy constructively rather than throwing it away since one characteristic of an under-damped membrane at its resonant frequency is that it is very responsive at the resonant frequency. In fact, it can be so responsive that if it isn't controlled it can cause the diaphragm to slap the stator electrodes at relatively low signal input levels. The reason why this effect is so dramatic is because every unit of diaphragm area contributes its parcel of energy to the peak of energy at resonance.
     An ancient philosopher stated: "Asking the proper question leads to the answer". We asked: "instead of permitting the entire diaphragm to contribute to one resonant peak, why not set up a situation where different sections of the diaphragm resonate at different frequencies in a graded fashion". The resulting set of resonant frequencies distributes resonant energy over the pass band of the woofer. Two wonderful results are realized: the "drum-head" resonance is eliminated and the efficiency of the woofer is dramatically increased because it is functioning on very responsive resonant energy. The overall result is bass response that is devoid of single-peaked (juke-box) bass and, even more important, it is fast, dynamic and unrestrained.

Typical response of an unequalized dipole diaphram

Variable cell size to distribute resonances

Algebraic summation of distributed resonances

The distributed resonance principle also solves a nasty problem associated with dipole radiators: dipole cancellation. An acoustic dipole radiator is basically a vibrating membrane in which the acoustic energy emanating from both of its sides is permitted to propagate freely. Dipole radiation has the lowest acoustic "coloration" of any diaphragm loading scheme. However, since the two waves radiating from opposite sides of the diaphragm are mutually out of phase, they begin to cancel one another at lower frequencies, which results in degraded bass response. By judiciously selecting the "law" by which the resonant energy is distributed, the effects of dipole cancellation can be virtually eliminated. Figure 3 shows the typical response of an unequalized dipole radiator taken axial to one side. Figure 4 shows one of several methods of sectoring a diaphragm to distribute resonant energy. f₁, f₂, etc., represent the resonant frequencies of each of the sectors. Figure 5 shows the resulting flat acoustic response.
In the "near field" (that is, very close to the speaker) the bass response of the speaker has a rising effect as frequency is decreased. This occurs because up close to the speaker the effects of dipole cancellation are not as apparent. However, in the far-field (that is, normal listening distances and beyond) the rising characteristic seen at the "near-field" adds just enough energy at each frequency to make up for the energy lost due to dipole cancellation. The resulting frequency response is flat. In other words, the "law" of the distribution of frequencies is such that it is the complement of the speaker's frequency response curve when it is under the influence of dipole cancellation. Obviously, an electronic equalizer could be used to obtain the same effect, but it would reduce the effective low-frequency dynamic range of a given amplifier by 15 dB or more. By using "distributed-resonance equalization" the dynamic range of a given system is not compromised. This is about as close as one can come to "still having your cake while eating it".