It is difficult to know where this idea originally came from, as it is easily and simply denied by the physics of sound reproduction. There are forms of distortion which are not often discussed as part of the standard how loud, (amplitude) how often, (frequency) questions, such as how fast, or how large. In the how large question, if we begin with a large original source, such as a symphony orchestra, which has perhaps two and one half hundred or more square feet of radiating surface, and if we then measure some total delivered radiated sound power by that symphony orchestra, and then divide that total radiated sound power by the total radiating square footage of all of the instruments of the orchestra, we derive an average energy output per square foot of radiating surface. We are essentially measuring the overall, average intensity of the sound per square foot of energy source area. Now, in playing this total radiated sound power back, in the reproduction cycle, we are typically dealing with a few square feet, two or more square feet of radiating energy surface, there has been an enormous increase of intensity per unit area of radiating surface. A simple example will illustrate the idea.

We can imagine a square surface of one hundred square inches moving one quarter inch peak to peak in reproducing a 100 Hertz wave at 100 decibels. Now let us suppose that we halve the square footage of the radiating diaphragm. If we still wish to create 100 decibels at 100 Hertz, we will now need to double the excursion of that same diaphragm to hold total radiated sound pressure as a constant. Our one hundred square inch diaphragm needed to travel 25 inches each second to produce 100 decibels at 100 Hertz, our 50 square inch diaphragm will need to travel 50 inches each second to produce the same total radiated sound pressure level. It is interesting to consider what this extra distance over time will mean when expressed as a peak velocity, and what the magnitude of the increased back electromotive force will be from the amplifier point of view, expressed as a changing mix of current and voltage in phase net resistance, as a percentage of increasing current and voltage out of phase net reactance.

It is interesting now to consider how our point source loudspeaker would behave: with essentially no radiating surface area, the active excursion of the device, and thus the peak velocity, and increased back electromotive force would be immense. Accurately controlling a high velocity piston is increasingly difficult in an intrinsically low internal impedance medium, the air. A good source load match would be with a driven diaphragm which essentially did not weigh any more than the air it displaced.

SM