WVL SERENDIPITY – Sound Pressure Level

Amazing efficiency from a speaker of this size!

Intuition plays tricks on us.

Let's take a simple example: How does a speaker cone move when we try to create an "impulse"? Wanna bet that some would guess that the diaphragm moves forward quickly and then returns to its rest position just as quickly? From this, we might conclude that a heavy diaphragm would perform quite poorly. Very reasonable, but unfortunately also very wrong.

After all, acoustics don't work that way. If it did, you could create a constant pressure by pushing a diaphragm forward and holding it there. What really happens is that the air just gets out of the way of the diaphragm, and the earth keeps spinning.

You couldn't even create a constant pressure by moving a membrane at a constant speed. It would create what we call wind, but as soon as the wind blows, the pressure settles back down. In short, a moving diaphragm can only create pressure by pushing on the air and accelerating it. The generation of a short pressure impulse is then nothing else than a short pressure shock (acceleration).

This just shows how fallible our intuition is.

The sound pressure does not correspond to the membrane position. Not even the membrane velocity. It corresponds to the acceleration. So now we can answer the question: What motion does a membrane make when asked to emit a sound pulse? It accelerates during the impulse and then oscillates out again. Now isn't the amplifier supposed to stop the motion of the diaphragm? Eventually it does, but not nearly as fast as the myth of the damping factor would have it. 

Basically, all this means is that moving mass only affects sensitivity, while what people call "fast" or "slow" boils down to bandwidth. If a speaker's frequency response is smooth, and continues for a long period of time, it is "fast" and "low distortion".

Only the combination of driving force, moving mass and stiffness of the diaphragm construction allows statements about the characteristic sound pressure. 

WVL SERENDIPITY. Amazing efficiency from a speaker of this size!

WVL SERENDIPITY. Amazing efficiency from a speaker of this size!