The Ribbon Loudspeaker Versus Other Technologies
Myths, Facts, and Some Considerations
Music is communication, and loudspeakers are the physical tools we have for transmitting the musical emotions from the program source to our ears and soul. In its role as transducer (i.e. converting electrical energy into mechanical), a speaker’s task mimics that of a microphone and is difficult to achieve with a high degree of accuracy. In search of the “Holy Grail”, scientists and engineers have tried almost every possible technology there is.
In theory, a designer creating a driver with a working range from 400 Hz and upwards, has the following to choose from:
1 The dome-shaped electrodynamic driver.
Materials for dome drivers include:
A. Treated fabric. Often silk, but other materials are also common. Hand-treated fabric is used as the material of choice. Fabric domes have high internal damping and suffer from poor rise times, i.e. lack of transparency.
B. Aluminum. The majority of metal domes use aluminum diaphragms. Aluminum has an extremely high stiffness-to-weight ratio but poor damping characteristics. This at worst results in severe ringing.
C. Titanium is slightly heavier than aluminum. It is much stronger than aluminum and has slightly better internal damping.
D. Beryllium may be the ultimate metal for use in dome-shaped diaphragms. Its "stiffness-to-weight" ratio is the highest of any known metal. Although its self-damping properties are not inherently superior, its stiffness usually raises its resonance to well over 50 kHz, obviating the requirement for special surface treatments. Beryllium is one of the most expensive materials from which to fabricate diaphragms since the manufacturing process has to be rigidly controlled to contain the material’s highly carcinogenic toxicity.
E. Kevlar. Although Kevlar is most typically used in woven form for larger coned drivers, one company still fabricates two dome tweeters from this material.
F. Ceramics. A European company produces dome-shaped drivers using thin aluminum oxide ceramic diaphragms. Used correctly, ceramic can be an almost perfect material. Although it’s not as fragile as many assume, fabricating ceramic diaphragms suitable for use in tweeters is quite challenging. It is an interesting fact that despite all these different materials, one company has gone so far as to produce a dome-tweeter were the dome is made from industrial diamonds.
2 The Planar Magnetic Transducer.
A long strip of thin material is bonded to several conductors to act like the "voice coil" on a regular type drive unit. This assembly, typically a Kapton or polyester film with aluminum strips, is suspended on all sides in front of and/or between continuous rows of bar magnets. When current passes through the "voice-coil" sandwich, part of it it is repelled and attracted by the surrounding magnets. The voice coil has typically no severe reactive component, which makes it a basically resistive load.
However, the fact that the membrane, (as is the case with dome tweeters) isn’t driven over its entire surface, cancellation of certain frequencies as well as resonance nodes occurs. Lack of transparency is one of the consequences. The tendency of elastic material is to continue to move, thus storing energy. This causes smeared transients. Because the voice coil in "ribbon-type" planar tweeters is not suspended in a uniform magnetic field but is adjacent to the magnetic field, non-linear motions of the voice coil results.
3 The Electrostatic Loudspeaker.
It consists of a thin sheet of plastic film, suspended in between two conducting grids. When fed a sufficiently high voltage, the sheet will push and pull between the grids because of electrostatic forces. Think of it as a large capacitor. The ESL requires a high voltage DC power supply to charge the grids. The high voltage to drive the sheet is typically generated by a step-up transformer. This results in a highly reactive (difficult) load to the amplifier. The ESL does have one real advantage – it’s driven over its entire surface. A liability is the step-up transformer. It degrades the sound quality. Also, the larger the radiating surface, the more the sound starts to beam as a function of narrowed dispersion. To avoid this, some manufacturers have bent the panel to act like a part of a cylinder. This in turn creates new problems. The membrane now has to move asymmetrically.
In the case of the electrostatic speaker, the grids on both sides of the membrane are a critical factor. Imagine a violinist playing on the opposite side of a thin wall with a multitude of holes in it. The dilemma is obvious - the more holes, the more of the music reaches the listener. But the more holes, the weaker the mechanical assembly becomes, and the more uneven the critical electrostatic force factor. A vibrating grid will modulate the music and cause IM distortion.
4 The Ionic or Plasma Speaker.
This technology avoids moving parts altogether. A corona is created, utilizing a 27MHz oscillator, modulated by the incoming music signal. Warming up the air (rather than pushing it with a membrane), creates sound pressure.
In actual practice, this theoretically good idea has a number of drawbacks:
A. The creation of more ozone than is healthy. (Catalyzers are a must).
B. The 27MHz oscillator produces a lot of heat and can interfere with surrounding electronics.
C. The corona tip, made from Tungsten, has a short life span due to high operational temperatures.
D. The low efficiency of the system makes horn loading a necessity.
5 The true Ribbon Transducer.
A true ribbon driver has a membrane made of a very thin, corrugated aluminum strip or strip assembly suspended between continuous rows of magnets at its narrow ends only. True ribbon drivers boast an extended performance in the high end of the audio spectrum, (as opposed to planar magnetic devices that exhibit a HF roll off caused by the film’s mechanical properties and horizontal tensioning forces).
Similar to a planar magnetic driver, the impedance of a ribbon transducer is mainly resistive. The true ribbon drivers biggest advantage is the absence of an extra membrane or film to which it is attached and the fact that it is a singular strip through wich the current floats. This results in superior acceleration capacities and close to zero energy storage.
No other technology possesses a better mass/force ratio. If engineered in the form of an open dipole, true High End audio performance becomes possible. An open dipole version of the true ribbon suffers no cavity-related resonance at all!
As mentioned above, currently only two technologies exist whereby the input signal force is evenly supported over the whole surface of the membrane: 1: the electrostatic speaker and 2: the true ribbon element. In case of the latter, the ribbon is suspended between two rows of elongated magnets - hence no obstacles between you and the membrane.
Some myths and facts:
Myth #1: "True ribbons can’t take high input power."
Fact: If constructed properly, they can take hundreds of Watts of input power, more so than any other technology within the same frequency range. One of the reasons is the membrane itself. It acts as an efficient double-sided heat sink.
Myth #2: "Ribbons are excellent but suffer from bad efficiency."
Fact: Ribbons can be made to have a sensitivity that equals or surpasses that of many conventional dome tweeters.
Myth #3: "Ribbons can be a tough load for an amplifier."
Fact: Ribbons are the closest thing to a resistive load. Even a traditional dome tweeter represents a tough load by comparison.
Myth #4: "Ribbons have to be dampened by utilizing a plastic film onto which the aluminum is applied, in order to avoid coloration."
Fact: In fact the opposite is true. Any damping layer will add mass and cause lower efficiency. The damping layer will store energy and thereby destroy one of the ribbon’s most obvious advantages - its low mass. As a true ribbon is supported at its ends only - furthermore corrugated in its elongated direction - it is less prone to inherent resonances than any other driver technology.
Myth #5: "By encapsulating the back side of the ribbon, you can make it suitable for acoustic suspension speakers."
Fact: Even if that were possible, it is strictly not recommended, unless the rear cavity is formed to act like a dampened quarter wave transmission line enclosure. This is mandatory for MF and HF transducers in general. The thin film in the ribbon is very transparent. Any reflections will bounce back through the membrane and cause time-domain perturbations and distortion.
It is worth mentioning that acknowledged designers such as B&W are recognizing this problem. In their Nautilus series, the dome-tweeter’s rear wave passes through a hole in the magnet structure and then is progressively damped in a long tapered tube.
Myth #6:"Cavity resonance problem can be easily dealt with by implementation of notch filters.”
Fact:To deal with a problem by implementing its reverse doesn’t mean you solve it. You merely walk around it.
Summary of the pros of the true open dipole ribbon:
Fundamental physics support the ribbon as the transducer closest-to-perfect. A thin ribbon of corrugated aluminum is suspended in a powerful transverse magnetic field. The signal current passes through the current and magnetic field. The electromagnetic circuit has control over ribbon motion. The ribbon itself projects the sound, thus serving both as voice coil and diaphragm. Every part of the ribbon is driven directly and simultaneously without any energy storage. The true ribbon driver improves the performance of speaker systems for the following reasons:
1. Its quick response and fast acceleration/deceleration potential permit it to maintain the leading edge of musical sounds and their harmonics.
2. Its low inter-modulation and FM distortion permit it to maintain midrange musical tone and harmonic structure without corruption.
3. Being non-dispersive in the time domain, it produces close to zero harmonic addition or subtraction.
4. It is a true force-over-area unit. (It is electrically and acoustically equivalent to an almost ideal transformer. No other transducer is so simply represented.)
5. It is (depending on the magnet strength) capable of an efficiency of 96 dB or higher. (Its losses are mainly resistive.)
6. It has a high rate of Young’s modulus.
7. It has an extremely wide, flat frequency response (0,5 kHz - 40kHz or higher.)
8. It has extremely low distortion. A line source true ribbon is capable of a THD less than 0,03% at 90dB.
So, are there any cons?
The ribbon is acoustically transparent and as such prone to let sound pressures behind it pass through. Most ribbon manufacturers prefer to encapsulate the ribbons at the back. Unless the cavity is constructed like an asymmetrical transmission line, (progressively filled with wool and thus pretty deep), stored energy inside the cavity will reflect back into the listening room, heavily distorting and causing time smear. Furthermore, random cancellations occur.
One way to avoid these problems is to design the ribbon system as an open dipole. Very few manufacturers utilize this technology.
Admittedly the transformer can be a problem, as the signal tends to get distorted at both ends of the frequency spectrum due to leak inductance and stray capacitance; especially so if you want to benefit from the ribbon speakers advantages in the lower parts of the frequency spectrum. (Phase errors).
This leads us to the following conclusion:
The best ribbon would be a transformerless, purely resistive, (coil-less and carrier-less), pure metal film ribbon of open dipole-type, suspended at its ends only, wide enough to give a good coupling down to the lower frequencies. This is exactly what the Transmission Audio Ultra Propulsion© technology is all about.
Manufactured in widths of up to 3 inches (still connected at their short ends only), available in heights of up to 4 meters, the Ultra Propulsion technology sets new standards for linear response, freedom of phase errors, low distortion and SPL capacity. The increased radiating surface increases the coupling to the air, making it possible to load the ribbon all the way down to 200Hz, (-3dB point).
A tall ribbon (5 feet or taller) furthermore has the benefit of being a true line source, hence vastly reducing the risk of tinnitus caused by high sound pressure levels in near-field, as the sound output as a function of distance is so different from a conventional point source or a horn construction.