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Ribbons, Line Arrays, and Hi-Fi

Ribbons – Have you ever noticed how ribbons seem to add that special sparkle to packages? Maybe that is why wives save every ribbon that they receive; memories of the beauty of the package. For years audiophiles have been experiencing a special sparkle that ribbons add to their home theater listening experience. I remember my first experience with ribbon loudspeakers. Another (equally picky) audiophile in my apartment complex had a set of Magnepan loudspeakers. I was hooked; I had to have ribbons. No, not the package type, those wonderful planar ribbon loudspeakers.

As I enjoyed my new loudspeakers, I immediately noticed one glaring deficiency; lack of substantial Sound Pressure Level (SPL). In fact, in a very short time I managed to “burn up” my loudspeakers, trying to play them at too high a volume.

Subsequent to that experience, I have followed the development of ribbon loudspeakers with great interest. I learned that as early as 1925, patents were issued for ribbon loudspeakers. These early devices were quickly recognized as a tremendously effective way to convert acoustical energy into electrical energy and vice versa. Scientists learned quickly that ribbon transducers were especially efficient at reproducing the crispness and expressive tonal quality of higher frequencies. However, as with my experience, they were continually hampered and limited by thermal problems. Therefore, the use of ribbon transducers for anything other than near-field, close environment listening was very difficult at best; and considered by some as impossible.

To be sure, there were attempts by various manufacturers to build ribbon devices that could be used in larger listening environments. However, as with their predecessors, heat was continually a problem. The first company that developed ribbon planar transducer that overcame this thermal problem and became applicable to the contemporary sound reinforcement market was SLS Loudspeakers in Springfield, MO.

What makes a planar ribbon driver so unique? The ribbon driver has less moving parts (see Figure 1), greater surface area, and less distortion due to break-up resonances, transmission delay or wave cancellation.

Compare the simplicity of construction of the ribbon planar device with the mechanical aspects of a standard dome transducer shown below.

In a standard dome transducer, the energy is transmitted through the voice coil, adhesive joint, bobbin, and then through another glue joint to produce the sound. The driving force is distributed along the circular joint of the voice coil and the dome, not directly across the vibrating diaphragm. In standard low frequency drivers, the dome is connected through a glue joint to another larger cone constructed of paper or other light-weight material.

In the case of compression drivers, the mechanical requirements of forcing energy through the throat and around the phase plugs in the compression chamber generates turbulence and distortion that results in a harshness that is often controlled using electronic equalization.

This distortion can be easily observed in the cumulative decay spectrum (or waterfall) plot obtained with a MLSSA analyzer. As shown below, there is a significant amount of high frequency interference visible above 1700 Hz for an extended period of time.

The diaphragm of the ribbon transducer is a flat radiating surface that does not change its character of vibration with frequency. The air mass vibrating along with the diaphragm creates an ideal coupling condition for sound energy transfer of the ribbon, shown below.

A planar ribbon transducer has a less reactive impedance, virtually free of problems associated with voice coil inductance, diaphragm resonance, or phase shifts. This results in a coherent wavefront. Very importantly, with a mass weight of approximately 50 to 100 times less than conventional dome or cone transducers, planar ribbon drivers have excellent sensitivity and high frequency extension. Because of the light diaphragm weight, the ribbon’s decay time is greatly reduced as can be observed in the waterfall plot below.

Simply speaking, the planar ribbon transducer is a more accurate transducer with less distortion than either a dome radiator or compression driver. The broad frequency response of the SLS “PRD” drivers make them especially suitable for use in a line array.

Line arrays are unique. They are not new, but they are relatively new to the contemporary music market. They provide pattern control and coverage (distance or throw) capabilities for a given amount of power that is not possible with conventional loudspeakers. Sound quality, if originated from a properly designed line array, can be more natural, rich, and full than ever thought possible before. However, there are some basic principles that must be understood if line arrays are to be used successfully.

Most loudspeakers can be considered as point source devices. As such, their energy is distributed in a spherical wave front. The total energy available from the loudspeaker is spreading spherically (forward, up, down, etc.) as sound moves away from the loudspeaker. The radiation of sound from a point source provides a steady decline in energy based on the distance traveled. Specifically, the energy from a point source loudspeaker or cluster of loudspeakers will lose power (6dB in sound level) for every doubling of the distance the sound travels. Technically, that is – the sound pressure level of a spherical wave is inversely proportionate to the square of the distance from the source.

That is not the case with line arrays. Due to continuous and extended dimensions of the vibrating element (i.e. the number of transducers), a line source radiates sound in the form of a cylindrical wave. This pattern difference is depicted in the following drawing, the sphere representing the point source loudspeaker and the cylinder representing a line array.

The cylindrical propagation of the energy from a line source results in a much more efficient projection of the sound wave. As the following drawing depicts, the sound pressure level (SPL) from the line source reduces only 3 dB with each doubling of the distance traveled. Specifically, in a cylindrical wave with the doubling of the distance sound pressure level decreases by only -3 dB.

It is understood that the inverse square law is valid for any loudspeaker in the far field. However, wave propagation of a line source provides more of a near field listening experience in the far field. Generally speaking, for a conventional single loudspeaker in a typical space, the near field is usually around 1 to 3 meters. This listening distance is quite common in a studio, but not the case in an auditorium or concert situation. Therefore, the added SPL available from only a -3 dB reduction in energy from line arrays can be very advantageous in those environments. Hence the popularity of line arrays for contemporary music concerts.

However, loudspeakers are often stacked on a stage, although some space is often provided between the audience and the performer. This space is usually no more than 6 to 10 feet from the stage. That means to project an SPL of 126 dB over a distance of 200 feet (not unusual at all), listeners near the stage are subjected to a SPL of up to 140 dB. Anything over 106 dB has a serious impact on listener fatigue. To circumvent this, many designers install additional (delayed) clusters to cover distant areas without overpowering the listener at the front.

Additionally one must consider the sensitivity and power handling capacity of the driver in order to project the desired SPL over the coverage distance. A line array has a major advantage with its unique – 3 dB SPL reduction rate. That’s one-half the power to produce the same levels with a line array.

Also, the greater the distance, the more effect the natural room acoustics will have on the sound being heard. The energy ratio for the arrival of the early and late reflections (C50 & C80 parameters) has a direct impact on the intelligibility of the sound. The further a listener is located from the source, the more indirect energy will be heard due to the influence of reflections and reverberation. This can create a sever imbalance to the delivered bandwidth because, if the distance is very great at all, air absorption may be noticeable at very high frequencies. The higher frequencies (shorter wave lengths) do not project as far and are masked by the abundance of low frequency sounds in the rear of the listening area. Systems comprised of conventional voice coil devices, either dome tweeters or compression drivers, suffer from this limitation.

However, ribbon planar line arrays do not have this limitation. Fortunately, a well coupled line array using ribbon planar transducers possesses an almost magical ability to project high frequency sound over very large distances, thus compensating for air absorption and reverberation effects and delivering high quality sound over the large areas.

Keep in mind that with any line source, there is some distance at which it is no longer a line source; wave propagation takes on the characteristics of a conventional (spherical) form. Therefore it is important to know at what frequencies the line array does not propagate as a cylinder. To know that, one must know the type and size of drivers being used to form the line array and the length of the array.

High frequencies from a cone are radiated from only the center of the cone. Depending upon the size of the cone, one frequency might be radiated from the outer edge of the cone, another (higher) frequency is radiated from the mid-point of the cone, and other (even higher) frequencies are being radiated from the inner part of the cone. A ribbon transducer is isophasic. All of the frequencies being produced by the diaphragm are in phase across the entire surface of the diaphragm and throughout the complete path of travel. To the listener, that means they hear all those frequencies simultaneously, as they were meant to be heard.

Lastly, when installing a line array, an installer must keep in mind that a line array should be considered as one loudspeaker with a cumulative coverage pattern resulting from the physical interaction of all the devices in the array. A major mistake made by ill informed designers or installers is in the aiming of the line array. While the array contains “lined up loudspeakers,” it is NOT a simple group of individual loudspeakers. Too often, ill informed installers will line up loudspeakers, “aiming” each device according to an individual coverage pattern. The result is not a line array, it is only a vertically clustered group of loudspeakers.

This is complex for the user and is helpful using a software program specifically developed for the design, aiming, and alignment of line arrays. In the case of SLS Loudspeakers, this software package is called Line Array Simulator Software (LASS) and is available from SLS to its dealers. The picture below depicts a line array design and identifies the aiming lines of individual loudspeakers while providing frequency specific information about the cumulative affect of the entire array. Properly utilizing this software, contractors can be confident that the SLS system they are installing will perform as predicted.

It is hoped that from the above information, the reader has begun to develop a familiarization of line array basics, and more specifically, planar ribbon arrays. There are additional aspects to line arrays that should be pursued for a more complete understanding of how they work. Using this knowledge, we would like to encourage the reader to investigate the use of line arrays in specific designs. Likewise, we would encourage you to compare the sound of a true planar ribbon device, such as the SLS PRD Series of ribbons. Enjoy true high-fidelity listening at concert sound pressure levels.

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