La Rosita 5 avenue Jean XXIII 06130 Grasse - France

Gravit Feet

 

Man has the gift of speech and is therefore capable of communicating knowledge, and to look back at what others have imagined in response to problems they have been confronted with. For quite some time, in the field of electro-acoustics, certains principes with regard to speakers have been viewed as a necessity, such as designing them to be heavy and inert.

 

It was soon realized that this was a poor apprehension of the energy generated by the acceleration of acoustical membranes. In order to assess what acceleration generates in terms of energy, one must look back to when we used to listen to vinyl records. Years ago, we experimented many trials and configurations: At one point, having at our disposal an inertial platform (essentially a big -1m3- cube of concrete) dedicated to the measurement of extremely fine posology's, we had the idea to place a turntable on this platform.

 

The turntable had no vibration absorption apparatus. There was no felt under the record but a ceramic disc, nor was suspended but a direct contact (glued with non-film glue) between the bearings of the plate of the turntable and the inertial platform. Eight high definition accelerometer sensors were placed around the base of the inertial platform which was insulated from the floor. The sensors were connected to Audiomatica (specialized software) which would analyze the energy, if any, produced by the tone arm's needle applying a force of 1.5g on the groove of an engraved plastic disk spinning at 33 revolutions/minute !!!).

 

Imagine our surprise when we discovered that the simple reading of the record's grooves caused the inertial platform to vibrate !! The eight sensors had detected energy. In fact the quantity of energy generated by the needle on the vinyl disk was enormous.

 

Years later, when we embarked on the design of the Ipanema and the Babell, we wondered how a speaker cabinet would react to the (often rapid) movements of the speaker's drivers when these reproduce music.

 

The standard solution was to create a VERY heavy speaker cabinet which will be connected to the floor with spikes which are supposed to act as a mechanical diode and as a jack. « Diode » because the energy generated by the speakers only goes to the spikes, and « jack » because the quantity of energy which accumulates at the base will be proportionately concentrated on the spikes.

Ultimately, this would work if the speaker cabinet had infinite mass and was placed flat on the ground (for example) in a field, i.e. directly in contact with the Earth (even though the Earth has a finite amount of mass). The point is: how do you cancel the effects the vibrations created by the back and forth movements of the speaker's drivers? Placed in a living room, speaker cabinets thus produce parasite vibrations which the most sturdy cabinets sitting on spikes cannot cancel completely.

 

 

As music is a superposition of sinusoidal waves, this means that energy is spread iso-dynamically (i.e. with equal strength and intensity); because the force which pushes a speaker's driver is equivalent to the force which pulls the speaker's membrane. Control must therefore be sought in the horizontal rather than the vertical plane.

 

At the beginning of the 20th Century, war ships experienced a similar problem caused by the recoil of canons which could rip apart a ship's deck. The solution was a "reactive base" constituted by an iron ball lodged between two spherical caps.

 

Based on measures of the canon's mass and the force of the canon's recoil, it was possible to calculate the arc of the spherical caps which would serve to create an inversely proportional force to that of the canon's recoil. This forgotten device is the most effective solution to control the micro-horizontal displacements of a speaker cabinet while maintaining its mechanical integrity and without changing its resonance frequency.

 

We adapted this system to the Ipanema and the Babell speakers, and called it Gravit Feet.

 

 

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