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The FrontRo Compact Electrostatic Loudspeaker
Rear view of a FrontRow loudspeaker Picture of FrontRow loudspeakers in a living room A cartoon similar to the previous one except that the guest is listening to Mellow speakesr while the guest is saying 'You shouldn't have gone to the expense of hiring a live band'. Cartoon of guests being greeted by their hostess at the front door while the host listens to conventional speakers. One of the guests is saying 'I have some downloads you must hear'. Exploded view of the electrostatic driver Picture of the pattern of holes in the stator
For sound that goes beyond hi-fi

The FrontRo is a hybrid electrostatic loudspeaker that is designed and hand-built in England, using locally sourced materials and joinery, and embodies fresh design theory. It has a 12Ē electrostatic unit that handles the mid-range and treble frequencies (600 Hz to 20 kHz), while the conventional 5.25Ē dynamic woofer handles the lower midrange and bass down to the lowest note on a double bass or bass guitar (40 Hz). The lower limit is constrained by the size of the enclosure, which is designed to be as unobtrusive as possible in the home, and anything below this limit is deliberately filtered to prevent distortion. Deeper bass may be obtained using a sub-woofer that can be tucked away discretely in a corner or behind a sofa. A major advantage of a hybrid electrostatic loudspeaker is that it is less sensitive to room placement than a full-range version and can be placed closer to rear walls, although we recommend a clearance distance of 40 cm or more.

The FrontRo is a passive loudspeaker, which means that you can choose whatever amplifier and partnering equipment, such as a turntable or streaming device, that you wish to use. The standard wood finish is light oak, but for a small premium you may order a pair in a veneer of your own choice such as American walnut. Grille cloths are available in gunmetal, navy or burgundy. Underneath the cloth of the electrostatic unit is a special screen that excludes dust and moisture to ensure long term reliability.

Why electrostatic?

Nearly all loudspeakers currently use the age old 'dynamic' principle of a coil in a magnetic field pushing a cone or dome back and forth to produce sound. The moving parts are slow, heavy and very prone to producing unwanted vibrations of their own so that they 'sing along' to the music. The recoil from the moving parts and pressure fluctuations within the box also induce unwanted vibrations in the walls of the box. Try tapping a loudspeaker box and you will probably hear a hollow ring.

By contrast, with an electrostatic loudspeaker:

Some common misconceptions

'My ears are so poor that I canít tell the difference.' In fact, even partially deaf people can hear the extra musical details reproduced by electrostatics.

'My room is completely unsuitable for hi-fi listening.' As we shall explain below, a good loudspeaker in a poor room will always sound much better than a poor loudspeaker in a good room.

'Loudspeaker design is done, isnít it?' Sadly, most loudspeaker designs ignore the effects of the listening room as we shall see.

Have you ever wondered why you can tell if music is live even before you enter the room?

The shocking truth is that most loudspeakers are designed to produce a flat response in an anechoic test chamber (a room with virtually no reflected sound) with the microphone pointing straight at them. Little attention is paid to the sound emanating from any other direction apart than that firing straight at the listener. Yet most of what we hear in a real room is reflected sound coming indirectly from those 'other' directions, just like with real musical instruments. If the reflected sound doesnít have the correct tonal balance, it canít possibly sound natural.

One solution might be to treat your listening room acoustically to make it more like an anechoic chamber, but that wouldnít make for a pleasant listening experience. The reflected sound envelops us and creates a beguiling sense of space. One might expect all that reflected sound to blur the stereo image but, if it has the correct tonal balance, then the opposite is true because our ears combine early reflections with the direct sound to create a single sound that appears to come from the direction intended within the stereo mix. This is known as the Haas effect. However, this only happens if the reflections correlate with the direct sound so that the ear recognizes them as true reflections. Otherwise, uncorrelated reflections (as produced by normal loudspeakers) just become a distracting noise.

How the FrontRo works
Designed for real rooms, not anechoic test chambers

The indirect sound is at least as important as the direct sound. In other words, the loudspeaker should radiate sound as uniformly as possible in all directions, like dropping a pebble in a pond. A problem with electrostatics is that, unless corrected, they tend to do the opposite with the high notes, which shoot out like a laser beam.

Creating the ideal sound source

One solution is to curve the diaphragm, but this adds distortion to an otherwise distortion-free loudspeaker and curtails the lower notes due to increased diaphragm stiffness. If you curve a piece of paper, you can support small objects on it, which is why such designs only use shallow curves that spread the sound out over a relatively narrow angle.

Another approach is to progressively delay the sound from the center towards the outer edge, so that by the time the sound comes from the edge, the sound from the center has had a head start and is already some distance from the diaphragm. Hence a curved wave-front is produced. This was pioneered by Peter Walker in the Quad ESL63 to imitate a point source behind the diaphragm. Because the diaphragm could not be infinitely large, the sound towards the edge had to be attenuated to achieve a smooth response. Such attenuation is too expensive a luxury in a compact design where we need every decibel we can get.

It occurred to Tim Mellow that, in theory, the perfect dipole sound source is a rigid sphere oscillating back and forth in free space because it spreads the sound out in the same wide pattern across the entire musical spectrum and has a smooth response. If the delay could be arranged to imitate such a source, using a flat diaphragm, no attenuation would be needed. The paper entitled 'How do we make an electrostatic loudspeaker with constant directivity?', which describes this idea, is available for download from this website.

Inspired by nature

The electrodes (4) that create the electrostatic field to drive the charged diaphragm (1) are partitioned into rings and each ring is fed from a tapping on a delay line to produce the hemispherical wave front. The pattern of holes is the same as that of the seeds on a sunflower head. It turns out that nature has found the most effective way of packing circles within a circle!

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