Distributed Resonance

What is Distributed Resonance and Why Use It?

Why Distributed Resonance? Sound Lab products employ a revolutionary patented principle referred to as “Distributed Resonance”, This principle 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.

Membrane resonance: The acoustical resonance peak exhibited by taut membranes (a 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 some designers to eliminate the resonance peak is to use acoustical damping such as a fine mesh stretched over the radiating panel. Some use a mesh similar to that of sheer nylon stockings. Unfortunately, since mesh acts as acoustic resistance, resistive damping does “surgery” over the entire pass band of the speaker, giving it a lack-luster quality. Furthermore, the resonant energy is dissipated or “thrown away” and not put to good use.

Dividing the single resonance peak into a series of resonance peaks: 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 posed the question:

“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, thus spectrally spreading out the resonant energy which eliminates the undesirable single peak response”

Membrane Sectors

We tried this and found that the resulting set of resonant membrane sectors nicely distributed energy over the pass band of lower frequencies, thus eliminating the single resonant peak. Why is this good?, you may ask. For starters, the magnitude of the distributed peaks is lower than that of the single peak. Thus, at the least the speaker can play louder. This is just one advantage, there are more. Please read on.

Results of distributed resonance: Two wonderful results are realized: the single-peak “drum-head” resonance is eliminated and the efficiency of low frequencies is dramatically increased because it is functioning on very responsive resonant energy.

The overall result is bass response that is devoid of single-peaked (jukebox) bass and, even more important, it is fast, dynamic, efficient and unrestrained.

How is the distributed resonance principle implemented? Sound Lab electrostatic panels use a single membrane. By employing a movement-blocking technique the membrane is separated into a series of radiating sectors, each differing in size and, hence, also differing in frequency resonance. The resonance of each individual sector follows a “law” such that the collective effect of all resonant sectors complements, therefore offsets, the loss of low frequency energy caused by dipole cancellation.

What is a dipole radiator? This is an essential question. 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 into space. In other words, the radiator isn’t placed in a “box” such as is done with conventional “box” speakers. The purpose of the box is to isolate the energy from one side of the membrane (or cone for dynamic speakers) from the energy emanating from the other side so that they can’t interfere with each other.

What is dipole cancellation? A natural characteristic of a vibrating membrane is that the energy radiating from the membrane does so in a well-defined dispersion angle. The magnitude of the dispersion angle is a function of the ratio of the membrane dimension, width or height, to the wavelength of the frequency it is radiating in the direction of the width or height, respectively.

For instance, consider a square membrane being one foot long per side. Consider a frequency of 20,000 Hz. The wavelength is smaller than one-half inch. The ratio of width (or height) to the wavelength (directivity ratio) is small, being about 0.042″. This suggests a very directive, narrow, small-angle dispersion of energy, analogous to a laser beam. That’s why most tweeters are small in order to increase the angle of energy dispersion.

In comparison, the wavelength of 20Hz is about 55 feet, giving a large directive ratio of about 55, indicating a very wide dispersion, greater than 180 degrees.

Energy radiating from the two sides of a dipole radiator are 180 degrees out of phase. Since the movement of a membrane “pushes” air (pressure) on one side and simultaneously “pulls” air (rarefaction) on the other side, the sound emanating from the two sides of a vibrating membrane are 180 degrees out of phase and would cancel one another if they occupied the same space. As mentioned above, the “box” approach prevents this from happening but we wish to show that this isn’t the best approach for optimum sound quality.

Is a dipole speaker better than a “box” speaker? Both dipole speakers and “boxed” speakers have their own unique benefits and problems. The “box” eliminates dipole cancellation but it adds “box coloration”, which is another word for chamber resonance caused by the interaction of the moving mass of the speaker and the acoustic “spring” of the confined air. The main advantage of the “box” is that low-frequency energy is preserved and good bass response can be obtained using a relatively small box, not withstanding the box effects.

On the other hand, an audio purist cannot tolerate “box colorations”. It’s a true quandary since the “box coloration” issue has been solved but the loss of bass energy due to dipole cancellation may be just as unacceptable. Thus, comes our hero that solves the problem, as described above, Distributed Resonance!

Congratulations! You’ve made it to the end. Hopefully, this discourse has not been confusing. Dipole Cancellation is an important part of Sound Lab speakers and it is our desire that our customers understand the technology behind their speakers.