In recent years, flat panel TVs have overtaken the boxy designs that for so long dominated the Australian lounge room. The boxy loudspeakers that are such a fixture in homes, however, aren’t likely to disappear any time soon, despite a flat panel loudspeaker technology that has been around for more than half a century.
Those panels are called electrostatic loudspeakers.
Regular loudspeakers work by having an electrical coil of wire glued to the back of a rigid (hopefully!) cone or dome. The coil rests in a strong magnetic field. When the output from an amplifier is fed into the coil, it becomes an electromagnet and moves. Just like an electric motor, it drives the cone or dome forwards and backwards to move air and make sound waves.
Electrostatic speakers don’t use a magnetic field. Instead they work in the same way that a plastic comb, rubbed vigorously with a cloth, attracts a small piece of paper.
A very thin membrane is suspended between two perforated metal sheets (called ‘stators’). Those sheets have your amplifier’s output applied to them, after it has been stepped up in voltage with a transformer and had a DC bias applied.
The membrane itself is coated in conductive material, and has the opposite DC bias applied. It is then free to move between the two metal sheets, pushed to and fro by the varying electrostatic field, in the same way that the piece of paper jumps up to your plastic comb’s electrostatically charged surface.
Commercial electrostatic speakers first achieved acceptance when introduced way back in 1957 by the British company now known as Quad Electroacoustics, but then as Acoustical Manufacturing. Within a decade these came to be revered as the among the finest loudspeakers made, and the original ESL57 models remained in production for nearly 40 years, disappearing from the market in 1996.
Electrostatic speakers sidestep a whole lot of problems associated with conventional loudspeakers, and consequently can deliver the lowest distortion of any design.
For example, the part that moves weighs almost nothing. When I said ‘very thin membrane’, I meant it. The norm is 3.5 to 6 micrometres (one thousandths of a millimetre) thick. A regular piece of 80gsm paper is about 100 micrometres thick. It weighs almost nothing.
Compared to the cone of a regular speaker driver (a substantial fraction of a millimetre thick) and the cylinder of wire glued to its back, the low weight of the electrostatic membrane makes it incredibly responsive.
So why haven’t they taken over the world?
Well, they are bipolar – producing as much sound from their rears as from their fronts – so they need to be very carefully placed. Some models tend to ‘beam’ sound rather than disperse it evenly around the room, resulting in a tiny ‘sweet spot’ for a good stereo effect.
The conductive coating on the membrane can be affected by atmospheric conditions, with some not working well in dry climates. In high humidity, there can even be sparking between the membrane and the stators.
And they are limited in their bass performance.