CLASSICAL RIBBON LOUDSPEAKERS

by RICHARD SEQUERRA

Dick Sequerra has been in the forefront of producing loudspeakers ever since the introduction, in 1976, of the first T-1 ribbon speaker. The T-1 has become the reference standard for ribbon speakers. MIT, Bell Labs, Bruel & Kjoer are among the prestigious institutions now using the T-1.

HISTORY AND DEVELOPMENT

The ribbon loudspeaker is quite old as a concept, and was developed by European Acoustic Laboratories after the First World War. The ribbon microphone is essentially the same device used in reverse. The microphone has, of course, been used for decades in extremely high quality recording and broadcasting. Unfortunately, the principal realization of the ribbon loudspeaker was limited by the 1920's magnetic technology ( It would have required an enormous field coil and an equally large dc power supply, which in total would have equaled the size of the largest bookshelf speakers, to have generated a sufficiently strong magnetic field.) Ribbon loudspeakers were, therefore, shelved in the late 1930's when the introduction of high flux permanent magnets presented the possibilities for their practical development.

RIBBON DESCRIPTION AND THEORY

Fundamental physics support the ribbon as the most perfect type of transducer. A thin ribbon of aluminum is suspended in a powerful transverse magnetic field. The signal current passes through the current and magnetic field. The electromagnetic circuit has complete control over ribbon motion. The sound is projected by the ribbon itself. The ribbon serves both as voice coil and diaphragm. Every part of the ribbon is driven directly and simultaneously without any energy storage.

The ribbon loudspeaker improves the performance of speaker systems for the following reasons:

(1) Having no resonant color of its own when it is added to any loudspeaker system, it functions only as a high speed range extender.
(2) Its speed of response, and almost infinite acceleration and deceleration, permit it to maintain the leading edge or ictus of musical sounds and their harmonics.
(3) Its extremely low intermodulation and FM distortion permits it to maintain mid-range musical tone and harmonic structure without corruption.
(4) Being non-dispersive in the time domain, no addition or subtraction to the harmonics of music are produced.

It might further be useful to point out the advantages of a ribbon loudspeaker from a theoretical point of view:

(1) It is a true FORCE OVER AREA DEVICE. (It is electrically and acoustically equivalent to an IDEAL transformer. No other transducer is so simply represented.)
(2) It is an extremely low mass device. (Equivalent to 1 wave length of air at 14 kHz.)
(3) It is capable of the highest efficiency known in direct radiator transducers. (It's losses are purely resistive.)
(4) It has a high rate of Young's modulus.
(5) It has an extremely wide, flat frequency response (2 kHz through 100 kHz.)
(6) It has extremely low distortion. (Particularly: Intermodulation and Frequency Modulation.)
(7) Its reliability is equal to the cone transducer or better.

DEFINING SPATIAL DIMENSIONS

One characteristic of the ribbon speaker is its ability to faithfully reproduce supersonic signals extending to 100 kHz. What is the importance of these frequencies when the maximum upper range of human hearing does not extend beyond 20 kHz in the healthiest of circumstances?

One of the ear's most important characteristics is termed intermodulation detection response. This is human's ability to detect and resolve "difference tones" produced by frequencies above the range of hearing. In music when a note is produced, it is never a single pure tone but a harmonically related series of tones called forments. These forments do not arbitrarily stop at 20 kHz but continue into the supersonics at decreasing intensity. The ear demodulates these supersonics as interleaving difference forments which belong to normally perceived tones. These existing difference forments help us to define spatial dimensions, image and air; the perceived reality and ambiance of the world around us.

COMPARISON TO OTHER PLANAR SPEAKERS

Favorable comparisons of ribbon speakers extend beyond that with the ubiquitous cone speakers. A fundamental difference contributing to the improved performance of a ribbon speaker over planar speakers is that other planar speakers use plastic as the medium the voice coil is attached to. JVC, EMIT, Apature, Panasonic and Magnepans fall into this category. The innate qualities of stretched plastic result in energy storage, beaming and distortion. A good comparison is a drum, in which an elastic material is stretched over a surface and clamped at the perimeters, much like a non-ribbon planar. The sound produced from the center of the speaker or drum precedes that from the edge, resulting in beaming.

The tendency of elastic material is to continue to move, thus storing energy, resulting in smeared transients. Because the voice coil in non-ribbon planar tweeters is not suspended in a uniform magnetic field, but is adjacent to the magnetic field, non-linear motion of the voice coil results.

SUMMARY

The ribbon loudspeakers are superior to other transducers because all design factors are correct. Other high range extenders do not blend easily when added to loudspeaker systems because they add their own distortion by transient smearing, energy and non-linear response. The quality of materials used in ribbon speakers has the ability to "clean up" and "open up" not only the extreme highs, but also the middle tones without adding any distortion. The addition of a pair of ribbon loudspeakers to any quality speaker system is the most cost effective way to improve the performance of a loudspeaker system.

A simple experiment will demonstrate this performance improvement to anyone. Take a stereo pair of high quality loudspeakers, install the ribbon tweeeter on one and not the other, set the mode switch on and the preamp to mono and A-B the performance of the systems with and without the ribbon tweeter.

You will be amazed!





Copyright 1998 R. Sequerra Associates