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Is there such a thing as a loudspeaker that doesn't sound like a loudspeaker?

Although modern loudspeakers are capable of reproducing the sound of virtually any instrument, somehow they seem to sound more like a loudspeaker than the instrument itself. Why is this so? Is there such a thing as a loudspeaker that actually doesn't sound like a speaker?

Of all the components of a recording studio, PA system or even home hi-fi setup, the worst performing by far is the loudspeaker. Yes, modern loudspeakers can produce a wide range of frequencies; yes, they can go pretty loud. But you could never be fooled into thinking you were hearing a real instrument playing, or a real person singing. The characteristic coloration of the loudspeaker gives the game away every time.

The problem is that nearly all loudspeakers work to the same basic design - the moving coil. Briefly, this means that the cone (or diaphragm) is attached to a coil of wire suspended within the field of a permanent magnet. The input signal creates a magnetic field around the coil that interacts with the field of the permanent magnet, resulting in motion of the diaphragm.

The problem is that the 'motor' (which the above combination is called) is at the center of the diaphragm, and the vibration needs to travel all the way to the edge without the cone bending.

This takes time. In the worst case, an outward impulse from the motor might reach the edge of the cone just as the center was starting to move back in again. This causes bending of the cone known as 'breakup'. Breakup causes distortion, usually not as extreme as its name suggests, which is present in virtually every loudspeaker made.

This distortion, particularly in the low-frequency drive unit - the woofer - is primarily what gives the moving coil loudspeaker its characteristic sound that colors everything that is heard.

There is one type of loudspeaker however that does not suffer from breakup. That is the electrostatic loudspeaker.

Here, the diaphragm is driven over its entire surface by electrostatic attraction and repulsion rather than magnetism. There is now no need for the diaphragm to be cone shaped (which is necessary in the moving coil unit for stiffness) and it can be flat. Since it is driven over its entire surface, it does not bend, and the distortion produced is negligible.

Since the overall area of the radiating surface can be large, an enclosure is not necessary to isolate the out-of-phase rear radiation.

A good electrostatic loudspeaker can fool even the most sensitive ear into thinking that a real instrument is playing.

So why don't we see more of them?

  • They are expensive to produce
  • The maximum sound output level is low - if the unit receives a signal that is too high

    in level, a spark can be created than punches its way clean through the diaphragm

  • Cancelation of bass frequencies due to not having an enclosure (although not having an enclosure provides benefits in other ways)
  • They don't sound like loudspeakers ;-)

Otherwise, there really should be more electrostatic loudspeakers around.

They would have a role as studio monitors too because they really would tell you very accurately the sound you are recording, rather than the rough impression that moving coil loudspeakers give, particularly in the bass end where there is often a considerable resonance.

However, since the rest of the world, apart from a few electrostatic enthusiasts, listens on moving coil loudspeakers, it makes sense for studio monitors to be moving coil too.

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By David Mellor Wednesday March 9, 2011
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