Mar 1, 2004 12:00 PM, By Mike Klasco
Ribbon transducer manufacturing has traditionally been a specialized craft, even when part of a larger speaker manufacturing operation. The wisdom and perception for permanent installations was that the expense and temperamental aspects of ribbon diaphragm assemblies, their limited dynamic range, and the larger surface radiating area required for adequate low-end response limited their market acceptance. The smooth, transparent, and crisp sound quality and very low distortion has made the development of wider dynamic range ribbons a seductive goal.
In the 1990s, several firms carried the torch for ribbon speakers as they strove to achieve the elusive targets of wide dynamic range, high power handling, extended frequency response, and compact size. Now there are ribbons that have finally achieved their performance goals and are being installed successfully in more and more venues. Multiple ribbon elements may be arrayed for additional output and control over directivity, and the technique of line arrays is now in fashion because their excellent pattern control is a great asset in many difficult installations.
The general concept of using an array of transducers goes back to the column speakers of the 1950s and 1960s, when columns from Electro-Voice, JBL, and Bozak in the United States and Philips in Europe were commonly used. The approach is making a comeback in the guise of line arrays, and ribbons represent the closest approach to the ideal.
The planar ribbon speaker is, like the typical cone speaker, an electrodynamic driver. While there has been no shortage of product announcements and mock-ups of planar transducers at trade shows, actual market share of flat-panel speaker technologies (such as NXT) have not yet measured up to the hype of their proponents. On the other hand, the number of leading brands that have licensed the various flat-panel speaker technologies is impressive. The appeal for less intrusive speakers that will complement flat-screen video displays as well as offer a shallow profile for all the speakers needed for intelligent signage in commercial applications, such as at airports, is compelling. With all the raunchy-sounding flat speakers making the news, it is easy to forget that the finest audiophile speakers have been and remain ribbons and electrostatics, both charter members of the planar clan.
To start at the beginning, flat-panel speakers are not new. Quad, the brand name of the United Kingdom's Acoustical Manufacturing Company, demonstrated its full-range electrostatic planar speaker back in 1954. There have been dozens of (at least sonically) successful electrostatic products over the years, including the legendary KLH 9 and offerings from Acoustech 10, Janzen, Dayton Wright, and the Pickering models of the 1960s, followed by a rash of offerings in the 1970s from ESS (best known for its Heil Air-motion designs, a variant of the ribbon family), Infinity, SAE, Crown, and Soundcraftsman.
Most of this batch used the RTR mid/high-frequency electrostatic panels and a conventional woofer. Several high-end audiophile speakers that are essentially handmade have continued to appear during the past two decades. Although many concede that ribbons and electrostatic speakers frequently sound better than their more conventional competition, they do not account for significant business, nor do most have the dynamic range or the high output needed for public spaces. Yet the typical large vertical size of planars fits into the line array frenzy that has gripped the pro-audio industry and has brought planars into the limelight.
Ribbon magnetic structures are mechanically close to conventional cone speakers. They both share magnetic structures and voice coils. The original scheme for a ribbon transducer was a corrugated aluminum foil diaphragm/conductor. This approach tended to have very low resistance and required a transformer to bring the ribbon diaphragm's load impedance up so that the amplifier could drive it. To keep the mass low, thin aluminum foil was used and functioned as both the diaphragm and voice coil, but this resulted in a fragile driver. Today the most common ribbon arrangement is for a nonelectrically conductive film substrate to which an aluminum or copper conductor pattern is laminated. Traditionally, Mylar was used, and recently, Kapton and Kaladex (renamed Teonex by its manufacturer, DuPont). The polymer/conductor composite diaphragm is the same construction as a flexible circuit board commonly used in laptops, cell phones, and PDAs. The impedance of this diaphragm can be readily designed to be in a range that the amplifier can happily drive; not just a reasonable impedance, but also essentially a resistive load.
MADE FOR LINE ARRAYS
When the audio signal is run through the conductors, which are immersed in the magnetic field, the diaphragm will move in a mechanical analogy to the electrical signal, more or less like any cone speaker, but with a significant subtle differentiation that makes ribbons superior for line array configurations. The ribbon has the advantage that the diaphragm is directly driven uniformly by the conductors, which are in intimate contact with the film diaphragm. Ribbons have been called regular phase (by Foster) or isoplanar because the entire diaphragm is being driven, and the radiating surface is moving as one. With cone speakers, the vibrational energy from the voice coil has to pass by transconductance up through the voice coil bobbin and into the cone apex, where the sound will then move from the center of the cone outward and radiate into the air. The effective radiating area versus frequency is never uniform on a cone speaker, and the phase and group delay is not consistent over the operating frequency range. But with ribbons, the entire diaphragm moves simultaneously at all frequencies. Incidentally, this is antithesis of the concept of distributed mode transducers such as NXT, and the cone speaker falls somewhere in between. But for line arrays, ribbons are ideal.
Ribbon microphones have been around for about 70 years, and ribbon speakers first appeared later in the form of tweeters, such as the British Kelly Ribbon. The first ribbon tweeters used a corrugated diaphragm rather than a stretched tensioned film, but both are still commonly used. Corrugated ribbon tweeters are still used for studio monitors and audiophile tweeters but tend to be less robust.
Panasonic introduced the first film ribbon, which it named the Leaf Tweeter, essentially using a flex circuit board tensioned diaphragm. This is the construction used today for high-power ribbons. Panasonic affiliate JVC and also Foster (Japan) supplied ribbon leaf tweeters and midranges on an OEM basis for many years. Magnaplanar introduced full-range speaker units in the 1970s using the conductor on substrate approach for the lows and mids and the corrugated diaphragm for the tweeter. Infinity has featured its EMIT (tweeter) and EMIM (midrange) ribbons since the 1970s.
There have been several popular ribbon tweeters used in many speaker systems, but full-range ribbon speaker systems have previously been limited to small-batch high-end audiophile production. A few years ago, Sonigistix of Canada introduced its Monsoon ribbon multimedia system. Eastech now makes these ribbons in Malaysia. Using ribbon satellites and a conventional cone subwoofer, the Sonigistix Monsoon MM-700 was the first multimedia mainstream ribbon product that has achieved mass-market success.
So what about professional application for ribbons? Aside from studio monitor tweeters used in Genelec and others, the first really ambitious high-output ribbon was from Stage Accompany in the late 1980s and was based on a scale of a Philips home speaker design. Stage Accompany pursued touring sound applications and used the ribbon in place of a compression driver.
One of the first to match the form factor of a ribbon to a theoretically ideal line source line array was the Radia Pro Speakers group at Bohlender-Graebener. Aside from large vertical ribbons, it offers the Radia Series of ceiling and in-wall speakers using ribbon tweeters. In addition, the firm is an OEM supplier to Genesis, Eclipse, SLS, Martin Logan, and others.
SLS Loudspeakers took high power/high sound pressure level (SPL) ribbon driver technology further in terms of power handling and developed its own range of high-performance ribbon drivers based on high-energy neodymium magnets and Kapton/aluminum diaphragms. These drivers are incorporated in several of its line array systems for various professional and commercial applications.
A BETTER APPROACH TO PLANARS
Electrostatics and planar magnetics (ribbons) are driven uniformly over their entire diaphragms. This is the exact opposite to the distributed mode ideal of flat NXT panels, which operate in random phase across their surface. Cones, on the other hand, are uniform (pistonlike) up to the midrange. It is generally conceded that ribbons and electrostatic panels tend to sound better than cones, while the distributed mode panels sound worse. However, in certain pro applications in which a uniform phase line array's excellent pattern control is not needed, the nature of NXT panel radiation (the “antiribbon”) is an advantage. The wide coverage and random phase of a well-executed NXT design, such as the Armstrong i-Ceilings panel, is an advantage in the struggle to achieve wide uniform coverage with low ceilings as well as high acoustic levels before feedback. The random phase nature of the panel helps prevent feedback, effectively randomizing the phase of the signal. The Armstrong panels actually sound quite decent, to boot.
In the past, ribbons came up short on power handling, sensitivity, and low-end response. These limitations were due to the low energy of the ferrite magnets and the low temperature tolerance of the Mylar film diaphragms. With the falling price of neodymium magnets, along with their higher magnetic energy density, the sensitivity issues and limitations on magnetic gap width were lessened. Kaladex/Teonex and similar superior mechanical and thermal films allowed for higher temperature operation and further increased sensitivity owing to the possibility of thinner diaphragms.
Designers of ribbons using ferrite magnets have always been confronted with response anomalies. That is in part because the sound radiation coming from the diaphragm must pass between the bar magnets, and the depth required for the ferrite magnets results in resonant cavities with tuned peaks, typically between 5 and 10 kHz. Neodymium has high magnetic density, so the magnets are not as thick and the air column in front of the diaphragm is shallower, and any resonance is minimized and is typically above the audio band.
Assuming a generous gap, Neo structures yield 0.6 to 0.8 tesla (6,000 to 8,000 gauss), whereas a ferrite magnet would have provided less than half of that. The higher electrical damping that results eliminates the usual “camel hump” response of an underdamped speaker. Neodymium's higher energy enables wider gaps without the flux dropping below useable intensities, so higher diaphragm excursions are possible without hitting the magnets. As the excursion is increased, the diaphragm surface area can be reduced to a more reasonable size, which also improves dispersion.
DuPont Mylar has traditionally been used for both ribbons and electrostatic planar diaphragms. A single-sided conductor layer is usually laminated to the film, but some applications use conductive layers on both sides of the film. The conductor side is then silk-screened using a chemical resist coating that prints the pattern, with the uncoated paths of the surface being the conductor paths that will be etched away. This roll or sheets of silk-screened laminate is then chemically etched and die-cut into individual diaphragms. The lead-out wire is attached to the completed diaphragm, and the diaphragm is then assembled and tensioned into the magnetic structure frame. All that is far easier said than done!
Mylar yields good sound quality but cannot tolerate conductor over 80 to 90 degrees Celsius because that is Mylar's glass transition point, and at that temperature, it becomes soft. Because most ribbon diaphragms are tensioned, under high-power input the conductive voice coil heats up the substrate, the diaphragm tension drops because of the softening of the Mylar, and the film becomes rippled, leading to distortion or buzzing.
What about operating temperature that drivers function in and power compression effects? Compression drivers and woofers have the voice coil enclosed in a tight magnetic structure. Even with vented magnetic structures and ferrofluid to transfer the heat, operating temperatures can reach 400 degrees Fahrenheit. As the voice coil temperature rises, so does the impedance. The speaker draws less power, and the output creeps downward. This is known as power compression.
Compared with cone and compression drivers, the structure is open on a ribbon diaphragm, and though air is not a good conductor of heat, the surface area of the conductors lets good heat transfer off the diaphragm so operating temperatures tend to be lower for a given sound level. Temperature tolerance is another story and is dependent on the materials and fabrication of the diaphragm. Older hi-fi ribbon designs used Mylar polyester that was typically laminated with an adhesive layer to an aluminum foil. Especially for smaller drivers, Mylar ribbons have limited power handling and get soft above 150 degrees Fahrenheit. DuPont's Kaladex and Teonex high-performance polyesters are now commonly used in ribbons and can handle almost double that temperature before softening.
For the highest power handling, DuPont's Kapton, a polyimide is the ultimate solution. Kapton can tolerate temperature excursions to 800 degrees Fahrenheit, but it is likely that the adhesives layer will handle only half of that. Some of the more advanced ribbons use a cast polyimide over the aluminum conductor, avoiding the failure of the adhesives layer. Noted ribbon designer Igor Levitsky pointed out that even with adhesivesless polyimide diaphragms, at elevated power levels, the aluminum work hardens and can crack.
How are ribbon speakers increasing power handling? Stage Accompany has offered forced-air cooling as an option on its high-power ribbons, SLS has integrated a heat sink to pull heat off the aluminum conductors on the diaphragm, and DuPont has introduced thermally conductive black heat emissive Kapton to improve the ability of the film substrate to dump the heat.
Bohlender-Graebener is one of the first ribbon manufacturers to offer Kaladex/Teonex film diaphragms. The temperature capacity is 130 degrees Celsius before softening, and considering the open structure of ribbon speakers, this temperature limit appears to be satisfactory for most applications. The higher tensile strength (which increases the tension you can use) and higher Young's modulus (correlates to the speed of sound in the material) of Teonex is better than in Mylar, yet the specific gravity is lower than Mylar. The cost is just above that of Mylar. Sound quality is excellent, as reflected in both frequency response extension, as well as lower linear and nonlinear distortion.
A ribbon's surface is immersed in a uniform magnetic field and moves in a uniform manner across its surface in response to electrical input. So by their very nature, properly designed ribbons are able to maintain uniform phase response across their surface, unlike traditional transducers. This results in a more uniform cylindrical wave regardless of frequency, while an array of point source radiators will produce an interference pattern with extreme comb filtering. The radiation pattern of any transducer is heavily influenced by the relationship between the wavelengths of the sound produced in relationship to the size of the active portion of the transducer.
Conventional, nonarrayed or nonline source speakers approximate a point source. A point source generates a spherical sound field in which sound pressure drops by 6 dB as a listener's distance from the source doubles. However, a line source, such as an array of long ribbon drivers, generates a cylindrical wave in its near field. This near field can extend large distances, depending on frequency and line source length, to hundreds of feet. A distinct characteristic of this near field used in line array applications is that its sound pressure drops only by 3 dB as a listener's distance from the source doubles. Although an in-depth discussion of point source versus the planar line source radiation pattern is beyond the scope of this article, suffice it to say that the ribbon works more like a line source than of the cruder approximation resulting from cone speakers mounted in a row.
The additional control over directivity is quite useful, both for where the sound does go (to the audience) and where it does not go (for example, a room's acoustically reflective floor and ceiling). That allows for much smoother and more consistent coverage and a superior ability to deliver direct sound to the audience even in reverberant environments. One of the many advantages of the control offered by a cylindrical wavefront is that patrons near the speakers do not have to suffer such high SPLs in order to ensure that audience members further back perceive adequate SPLs. Thus a line source system is a superior tool where intelligibility is important or the venue is acoustically difficult (for example, where there are glass surfaces everywhere, a longer throw is required, there are reflective or domed ceilings, or in corridor applications such as in museums).
Ribbons will continue their advance into mainstream commercial and pro-sound applications as their clean sound quality, ideal radiation characteristics for columns and line arrays, low weight, and increasing robustness, sensitivity, and acoustic output capacity attract more sound system designers and spec writers.
Mike Klasco is president of Menlo Scientific.
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