Blog 11. The gearbox

What can be the connection between a loudspeaker and a boat engine with an under-dimensioned propeller?

I have always been very interested in audio equipment, especially loudspeakers. Not only to play rock’n roll on terrible high volume, but also because a loudspeaker is very complex. The quality of an audio chain, from the initial recording in the music studio to the reproduction in your living room, the loudspeaker is the unit that distorts the music at most.

The loudspeaker mechanics is very simple to understand; in a magnetic field, a coil is placed, and when it gets current from the amplifier, it moves according to the music played. The coil is glued to a membrane, and it causes the air to move, to finally be perceived by your ear. But as always, there is a catch.

The membrane oscillates in the air, but the air is hardly not loading the membrane. What I mean is a bit hard to understand, and here comes the outboard boat engine into the explanation. Imagine a huge engine with a tiny propeller, like the figure above. Imagine what would happen when you put the propeller into the water. Not so much of the huge engine power will drive the boat through the water. So one can say the water doesn’t load the propeller enough.

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The same happens with the membrane when putting it into the air. The air will not load the membrane enough, and a big portion of the huge power from the amplifier will not transform to air sound pressure. Of course, you will hear some music playing, but you can compare it with the boat engine; most of the power will not come into use.

So the question is, how to let the engine and membrane be adequately loaded. In the case of the boat motor, it is simple; you enlarge the propeller. But also observe, if you enlarge the propeller too much, say to the same size as the engine, the water load on the propeller would be far too high, and the engine could not spin the propeller fast enough, and the boat would fail to be driven. Now assume that the propeller size function like a “gearbox” between the engine and the water.

Now over the loudspeaker again, you may begin to see my metaphor. A satisfactory gearbox must be arranged between the membrane and the air, which is a “horn”; see my own constructed mid-range loudspeaker above. Also, the tweeter is supplied with a horn. A “gearbox horn” is also very useful for a woofer, but the size will be in meters rather than decimeters and centimeters.

With a rightly constructed horn, the efficiency of the transformation from the amplifier into air sound pressure rises from a few percent up to tenths of a percent. Guess how loud you can play rock’n roll with full-horned loudspeaker equipment!