Dr.-Ing. Peter Strassacker

Email: peter@lautsprechershop.de

Email: peter@lautsprechershop.de

The drawing on the left shows the structure of a driver. It mainly consists of a diaphragm and a drive system. The drive system consists of a magnet with the purpose of generating a magnetic filed in the area that is marked green. The voice coil (red) - under current - is located in this green marked
area. A force F is generated in the voice coil that is vertical to the
magnetic field and vertical to the direction of the current in the conductor.
The picture shows that What are the variables determining the driver system's behaviour? The current running through the voice coil generates a force F, exactly where the voice coil is suspended in the magnetic field:
_{m} is the length of the conductor within the magnetic field (or
the number of windings x circumference of a winding), B is the strength
of the magnetic field and I is the current.
The expression in brackets is also called electromagnetic force factor F/I or B*L (where L indicates the length, however, we would like to use s for length since we need L for inductance). If current I is applied to a driver, the diaphragm moves with a certain
force F = (s However, what is the value of the current running through the voice coil? To answer this question we need to introduce further variables.
_{R} is the voltage applied to the
driver and R is the ohmic resistance. This is correct when DC is applied
but not for AC.
n the following we'll apply AC; the variables I, U are complex amplitudes.
_{M} is the length of the conductor within the magnetic field,
B is the flux density of the magnet and v is the velocity of the voice
coil.
Therefore, the induced voltage is dependant on the electromagnetic force factor and the velocity of the voice coil.
_{L} runs ahead of I by 90 degrees
and ω is the angular frequency, therefore 2 * π * frequency
f.
When voltage is applied to the driver, this voltage is divided into
three different "activities" (please refer to the picture on the left): The less power in a) is turned into heat, the higher the efficiency of the driver might be. b) puts power into the driver and out of it (that's the characteristic of a coil) and c) is responsible for the drive; this electrical energy is tuned into mechanical energy. Considering a coil with infinite low resistance (superconductor), having hardly any inductance, only part c) would be effective. That implies that:
It's not easy to work with equations and, moreover, the sound transition
from the diaphragm to the ambient air also plays a role, Thiele and
Small developed their model to determine the low frequency properties
with the following parameters: |