Tech Talk

The ‘mouthpiece’ of a loudspeaker system are the transducers. A loudspeaker transducer converts electrical voltage input to the coil located in a magnetic circuit, which drives the diaphragm, producing acoustic output. However, when a loudspeaker moves from its rest position, distortion begins to increase due to inherit nonlinearities of a fundamental transducer design.

Most of the dominant nonlinearities in the transducer are directly related to the properties of the motor, suspension and cone geometry-material. In this Tech Talk we will focus on the following three dominant nonlinearities:

Force Factor BL(x): Strength of the the magnetic motor and voice coil in the gap, vs displacement.

Ideally, magnetic strength remains constant with displacement but in practice the level is reduced.

Stiffness of Suspension Kms (x): Nonlinear resistance due to the spider and surround, vs displacement.

Electrical Inductance L(x): Variation of inductance vs displacement in the voice coil. Ideally, inductance remains constant but varies due to position, and induced alternating currents through the coil.

How We Refine Loudspeaker Linearity:

An important parameter for minimal transducer distortion is symmetrical positive/negative displacement from rest, and maximum output linearity. IEC Standard 62458 defines the acceptable reduction percentage of Force Factor BL(x), Stiffness of Suspension Kms (x), and Electrical Inductance L(x).

The 8 inch woofer used in the JBL 4329P is a model for linearity refinement.

JBL 4329P 8 inch Woofer, Force Factor BL(x) vs Displacement

The goal is to have symmetrical positive-negative displacement from rest and minimize BL Force Factor loss as the coil moves along the gap. Proper voice coil positioning and advancing motor design is the goal. The graph below illustrates the impressive linearity of the JBL 4329 l with a linear peak-to-peak displacement of 18 mm according to IEC Standard 62458.