Speakers & Headphones

There once was a "Loudspeaker" made by the Leak company that was "electrostatic" in its operation. They were used for Hi-Fi in the early 1960's - so I suppose they were good. Very slim-line, they were expensive and looked like a radiator! We don't see them anymore.

I think all modern speaker "transducers" operate on an electro-magnetic principle. We want to move the air at various frequencies to hear our voices and our music. Long slow strokes of the diaphragm for the bass, very short fast strokes for the treble. That's one reason for the many different sizes and, indeed for the development of the "CROSSOVER" circuit used in Hi-Fi to drive them. We send the higher frequencies to the small one and the lower freqs to the larger one. Sometimes there's a "Midrange" unit (speaker).

All these units are constructed on the magnet & coil principle. They are the opposite of a generator. Here we have a varying applied (ac) voltage that generates a changing current in a coil. The ensuing magnetic field alters in sympathy and reacts with it's core magnet thus causing the coil assembly to move. A thin, (was paper - now often plastic), cone is attached to move the air. Too simple? Go and have a look at one!

We need these units to be very "responsive" & that means following the applied current in an exact copy. It ain't gonna happen folks! There's a raft of problems. Not least of these is the inertia, the impetus and the IMPEDANCE. {Got there!}.

The fact that the impedance varies with frequency is a big problem. If we know the impedance at a certain spot fequency, it still only gives us a basic idea of how far the impedance will range.

CROSS-OVER DRIVE
We can use different size units, coupled with a frequency steering & impedance matching network, but we still get an awful problem of "LINEARITY" in the drive current between frequencies. A novel, much used way around that is by a device within the amplifier known as NEGATIVE FEEDBACK. Were it not for this I doubt that true High Fidelity would be possible. Even then - but wait, enough unto the day etc. MORE SOON.

NEGATIVE FEEDBACK
The effective gain (amplification) of the amplifier is altered by taking a sample of the actual output and reducing gain when it is too much and increasing it when too little. We are able to do this via frequency filters if we want to, it's just the principle we need to grasp. If we take a sample of what is delivered into the speaker (load) we can level things out. There!