Loudspeaker Driver Units

In the following, a description of the operating principle of loudspeaker units is given and then the limitations of REAL loudspeakers are explained. Finally, my opinions on what characteristics are important for the quality of loudspeaker units are presented.

Operating Principle

The purpose of any loudspeaker is to transform electrical energy into acoustical energy. This transformation of energy from the electrical domain to the acoustical is done via a mechanical system which is brought to oscillation by an electrical current. This movement of the mechanical system excites the surrounding air molecules, which start oscillating about their equilibrium state whereby sound arises.

Specifically for the electrodynamic loudspeaker, a current is conducted in a voice coil through a permanent magnetic field. The movement of the coil also moves the diaphragm attached to the conductor, thereby creating a ''piston like'' motion with position and velocity depending on the current running through the conductor (voice coil/inductor).

As illustrated above, an electrodynamic loudspeaker unit consists of a permanent magnet which ideally provides a uniform magnetic field for the voice coil. The strength of this magnetic system is usually given by the Bl product, which is the product of the magnetic flux density in the air gap and the length of the voice coil wire.

Together with the center pole piece and top plates, the magnet creates a closed magnetic circuit, in which the voice coil is enclosed and only separated from the magnet system via the air gap.

The coupling between the driving circuit of the loudspeaker and the chassis is done via the rear suspension, which also limits the motion of the diaphragm and voice coil to the forward and backward direction i.e. uni-directional movement.
The moving part of the loudspeaker is also attached to the chassis via the surround, which provides a separation of the sound radiated from the frontal side of the diaphragm from the sound radiated to the backside.

The position of the diaphragm is at the equilibrium state at  x = 0 when no force (voltage) is applied to the voice coil and the following relation exists between the applied force, F and the diaphragm position x.

F  = 1 / cm ∙ ∫ v dt  =  x / cm

cm is the mechanical compliance

v    is the velocity of the diaphragm.

This expression is valid assuming ideal transduction but a loss of energy will appear due to friction, whereby thermal energy is condensed.

Limitations of REAL Loudspeaker Units

The efficiency of a loudspeaker unit is defined by the ratio between the electrical power and the acoustically radiated power and this ratio is very small (around 0.04), indicating that a lot of energy is condensed / lost. Condensation of energy is undesirable because it introduces thermal buildup which alters the compliance and thereby the acoustical characteristic of the unit. Generally, three different approaches are used for minimizing condensation:

  1. Large diameter voice coils made of aluminum wire will ensure a large surface area and high thermal capacity.

  2. Thick top plates or encasing the coil in metal will provide good thermal conductivity of heat away from the voice coil.

  3. Use of the diaphragm motion in order to provide cooling for the voice coil and magnet system.

The diaphragm:

The center cap is mounted on the diaphragm in an attempt of avoiding standing waves on the surface of the diaphragm, but the center cap also scatters the radiated sound. Center caps have different shapes depending on the specific design criteria and sometimes it is even left out.

The conic shaped diaphram means that the middle of the diaphragm is positioned a further way back than the outer part of the diaphragm, and therefore high frequency signals will be cancelled or amplified uncontrollably, especially off axis.
The most commonly seen diaphragm material is paper because it is light, and rigid when used in a cone, but other materials such as kevlar, polystyrene, polypropylene and aluminum among others are used, because the diaphragm should also meet the criteria of being a good conductor of heat.

Limitations concerning transduction of high frequencies depends on the specific diaphragm design but primarily the geometry of the diaphragm is rather than the material is relevant. The limitation is illustrated, observing the radiated SPL at a distance 1m of a 20cm diaphragm loudspeaker mounted in an infinite baffle.