Take a Walk in Loudspeaker Park with me – Part 2 – Loudspeaker Impedance
Campbell’s Corner – By Dick Campbell
Ever wonder why loudspeaker impedance is so complicated? Dick Campbell’s shows what’s going on in the loudspeaker-to-amplifier interface.
Getting ready for the box requires some understanding of the goals: good time accuracy, uniform response, adequate low frequency performance, correct integration with the rest of the loudspeaker system.
Assuming the box is sealed, it becomes an air spring loading a back of the driver. In addition we find that the air spring of the box is not the same as the air spring in, say a bicycle pump, which heats up because the pumping action is so slow that the heat of compression can move to the sidewalls of the pump and warm your hand.
But at audio frequencies, the wave cycle is so fast that heat does not move appreciably. The thermal diffusion speed in air is ~ 1.5m/sec (physicists will jump all over me for saying that!). If the heat does not transfer between air particles, then that retained energy simple adds to the spring constant. An ‘acoustic’ air spring is 1.4 times greater than the bicycle pump spring, a factor called γ in acoustics. See this link regarding isentropic expansion for a more complete explanation.