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Monitor loudspeakers that *always* tell the truth. Yes, it's possible!

Without accurate monitoring, how do you know for sure what your recording sounds like? Well did you know that it's possible for monitors to *check themselves* for accuracy, and always give you pure, perfect sound?


Let me ask you a question...

Do you think I use a PC or Macintosh for audio? You think I use a PC? Well, you're wrong. You think I use a Macintosh? Yes, you're right.

Let me ask you a question... Do you think I use a PC or Macintosh for audio?

Hmm, you might have got the answer right first time, but if you didn't, there's no chance you'll get it wrong second time round.

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The reason why? I have given you feedback, that's why. I commented on the accuracy of your answer, and now you are sure next time the question comes along.

The same thing happens in audio amplifiers. All of them from mic preamps to hulking great PA power amplifiers. They all compare the output signal to the input signal. If the output signal isn't identical to the input, only bigger, then corrective action is taken. In practice it all happens so fast that the amplifier doesn't get chance to make a mistake. It is always accurate within infinitesimally small margins of error.

In electronics, this is known as negative feedback. 'Negative' in this context is a good thing. Positive feedback would cause oscillation.

Not all of the equipment we use employs negative feedback to make sure that the output corresponds exactly to the input.

Loudspeakers, for instance, do not use negative feedback. They rely on very clever design and construction to accurately convert the incoming electrical signal to sound.

And because there is no 'quality check' mechanism, the output is usually distorted, to a lesser or greater extent. The distortion produced by a standard moving coil loudspeaker is always audible. I can easily confirm that statement for you by asking whether you have ever been fooled by a loudspeaker into thinking it was a natural acoustic sound source. Never, I am absolutely sure.

But loudspeakers can be given negative feedback. It is sometimes called 'motional feedback', and the technique is to fix an accelerometer onto the cone of the woofer, which is the worst offender in terms of distortion.

So the accelerometer can sense the motion of the cone, and pass this back to the amplifier. If the cone is moving in way that isn't exactly what the amplifier told it to do, corrective action can be taken so that it does.

Unfortunately, in the early days of motional feedback, back in the 1970's, the application of the technique was too primitive to get it to work well. It wasn't particularly effective, and the few motional feedback loudspeakers that were released onto the market didn't sound a lot better for it.

The problem was that the accelerometer could not sense the movement of the cone as a whole. Some sections could be going seriously astray while the accelerometer knew nothing of it.

It might be thought that a more advanced way of applying motional feedback would be to place a very accurate instrumentation microphone in front of the speaker and use this to generate the feedback signal. Unfortunately, it would take sound too long to reach the microphone for the instant response that is necessary.

But how about scanning the diaphragm with a laser? This is already done in loudspeaker design. Surely modern technology has the ability to, in real time, scan the movements of the full area of the cone and apply a feedback signal instantaneously to the amplifier so that corrective action can be applied.

The result could be a loudspeaker that produces sound that very accurately follows the input signal. There would be far less distortion. There is no reason why this could not be applied to hifi, studio and PA loudspeakers. Everyone would benefit.

You know... the moving coil loudspeaker is the worst performing piece of sound equipment in the domestic environment, in the studio and in the large-scale sound system.

What do you think? Could the principle of motional feedback be made to work? Is there any other way loudspeakers could be improved? They certainly need it!

By David Mellor Monday January 30, 2006