The planar high frequency transducer concept.

Planar loudspeakers or ribbons (as they are known in the Hi Fi world) are not a new idea but some of the materials that make their construction practical and their performance reliable are. As of now there are very few reasons why planars should not become the high frequency component of choice in the vast majority of modern loudspeaker systems. Conventional magnetic gap and circular coil designs only dominate because this has long been the accepted way of doing things.

Basic principles:

Planars produce a cylindrical flat wave-front that is phase coherent in the vertical axis. Conventional high frequency transducers produce a spherical wave-front that has the same dispersion pattern in both the horizontal and vertical axis. In it self this may not be an issue but when used in conjunction with other identical components serious problems occur where the sound overlaps. This results in additions and cancelations that cause peaks and dips in the frequency response at the overlap. This aberration occurs whether they are placed side by side or one above the other. When they used in groups it is impossible to place them in such a way that cancelations will not occur. The result is that the listener is subjected to what are often called “dead spots” in a theatre or an arena. Dead spots often mean an audio system design has in fact been a failure and is labeled so accordingly. Planars put an end to this problem permanently!

In the case of the planar design there will be no phase aberrations when they are stacked in a vertical line making them the perfect device for line array speaker systems.

The planar produces a cylindrical output. This means that they can also reduce ceiling and floor reflections considerably. They also have good horizontal dispersion so they tend to sound much the same when the listener is located off axis. All in all they are set to make a large contribution to the audio sound field in most listening environments.

Robustness. 

Modern planars are made from very high tech materials that are able to remain reliable at very high temperatures. Up to 400 degrees centigrade. As the voice coil is printed onto the diaphragm surface itself it is unlikely to fracture or breakup as in the case of conventional coil former and magnetic gap combinations. As a planars coil does not sit in a narrow magnetic gap it will never be jammed up by dirt, corrosion or dust as in the case of conventional construction. Power handling is also considerably improved and in the order of several times higher than conventional loudspeakers. Excessive low frequency input does not appear to cause damage whereas with conventional designs it often destroys them.

Applications.

As planars are very difficult to obtain they have not been fully subjected to the reactions of the mass market and therefore there is still a lot to learn about how they will be accepted and evaluated by the general public. Planars would appear to be beneficial in nearly all audio applications of both the Hi Fi and professional markets. Some manufacturers have even developed horn flares to be used in conjunction with planar drivers. There does not appear to be any area other than perhaps long throw horn applications that are unsuitable for planar speakers. As time goes by more and more manufacturers will begin to adopt planars and include them in their speaker enclosure designs.

Constructional advantages and disadvantages.

Several construction methods where investigated and it was eventually decided that apart from doing something different an extrusion section  would make an excellent clam shell type main housing and also permit the manufacture of planars of any length using the same tooling and material stock. It was also very cost effective both in setup  and component costs. Physical strength and accuracy where other considerations as was the final appearance and finishing along with cooling, machining and weight. Aluminum panned out well in all of these areas.

The clam shell design allows for on site servicing which has not been possible before this design was conceived. Also the clamp up self locating sections make assembly relatively simple and can be done by non specialist individuals with only limited abilities in the technical sense.

In the prototype the diaphragm frame is also aluminum as it is easy to cut out, light and here again transfers heat well. The Toenex diaphragm material is glued onto the support frame using RTV. The frame is clamped in place between the clamshell sections without the need for any fasteners. Various tapes and felt pads are attached to the metalwork prior to final assembly to aid diaphragm stiffness and eliminate any buzzing. With the existing DuPont diaphragm material this may be unnecessary but with the much thinner material used in the Hi Fi version these tricks may well be needed.

The side panels design has yet to be settled on but these could also be an extrusion or a casting and may end up being the result of a think tank. Collective contributions mean all good ideas and advantages are weighed up and tossed around before a final design is frozen and the often expensive tooling costs are finally committed to.

Aluminum can be anodized, sand blasted, etched, powder coated, mock chromed or industrial coated so a wide range of finishes are possible and available.

An allowance for a rebated anodized rear label and drill holes for the lead in wires will need to be made.

If planars present any disadvantages it is purely and simply due to the massive strengths of their associated magnetic field. When the two halves of the clamshell are brought together they attempt to force each other apart again and this makes assembly difficult. A pair of removable guiding pins are needed to line up the screw holes until the fasteners grab and can be pulled down. This means considerable care needs to be exercised during assembly or diaphragm replacement. However with some training and practice this is not a major issue.

Another down side is the cost of the highly specialised neodymium magnets. The price of these items does reduce considerably in quantity but nevertheless they do represent the main cost component of planar speakers and there is no real substitute for them yet. 

The line array speaker system. 

Line arrays are very vogue in the pro audio scene at present. While our audio industry does tend to go through various phases there are good arguments that the line array concept is here to stay. At least until someone comes up with a way of bringing massive acoustic power levels of full range sound from a single point with no phase issues.

Line arrays use a long string of loudspeaker enclosures hung from an elevated position in an arc shaped array. The enclosures are designed in such a way as to ensure the high frequency components are set in a nose to tail configuration almost touching each other. The closer the better. Because their output is cylindrical they are able to link up or couple acoustically while causing no interacting phase shift between them. The result is a wave pattern that looks like a single long cylinder from the top to the bottom of the array or string.

Because planars have a narrow vertical dispersion pattern and as they overlap while staying in phase they have the unique ability to project sound a long way. This characteristic when used in conjunction with the curvature of the arc means it is possible to set up a speaker system that has very even sound pressure levels all the way from the front of the seated area to the back. This means that the audience at the front of the venue do not get blasted yet those at the very back can still hear clearly. This has made a huge contribution to the audiences listening pleasure in many venues. Thus the advent of the planar has been very welcome. Because planars are still quite rare most line array manufacturers are not yet using them. Instead they use a device known as a phase coherent lens that they mount on a conventional horn driver, These devices are large and clunky and have some acoustic properties not considered very desirable. They also struggle to produce a good frequency response at the extreme left / right ends of their coverage patterns. 

The current situation.

In the world today there are only three companies making professional planars that have any real credibility. These are SLS in the USA, Alcons in the Netherlands and Stage Accompany in Germany.

Stage Accompany are the only organisation selling these components loose while the other two only sell them in their own in house manufactured speaker enclosures. Bohlander Graebener in the USA and Infinity are the two main manufacturers making ribbon Hi Fi tweeters but there are a number of other lesser known companies making them as well. Across the world there may even be a hundred or so small companies manufacturing ribbons that do not have international market penetration.

Our prime reason for developing a planar was the realisation that if we were going to own a line array system we would need to gain access to a suitable high frequency transducer. Upon learning the true situation regarding the availability of planars we decided that it may be worthwhile and possibly even profitable if we could develop our own planar with the view to using them in our own enclosures and possibly selling them loose.

We were very fortunate in finding a company who were willing to supply us with a good quantity of suitable diaphragm trial material and also in locating a Chinese magnet manufacturer who was willing to custom manufacture a magnet to our own spec in a small quantity. These two achievements made the whole project possible. Further to this the ongoing email assistance we received from an American engineer employed by Bohlander Graebener cannot be underestimated.

Further developments include trialing a lighter weight diaphragm material, testing even stronger magnets and experimenting with damping techniques rear cavity and exit vent shapes.

In reality there is no end to the evolutionary process of any speaker component.

We are at present giving some thought to the development of a planar device capable of handling frequencies in the 100 Hz to 2 kilo Hz range. This would probably be the worlds first professional mid range planar component.

For any further information on this or any other blog topic please call Paul Johansen on Ph: 444 8776