AES British Section lectures - June 2007

Nonlinear Control of Loudspeaker Systems

Wolfgang Klippel, Klippel GmbH

On June 12th the UK Section of the AES hosted "Nonlinear Control of Loudspeaker Systems" by Wolfgang Klippel. Wolfgang is founder and Managing Director of Klippel GmbH, well known for its novel control and measurement systems for loudspeakers.

Wolfgang started by stating that the loudspeaker is normally the largest source of signal distortion in the audio chain, but operating large sized loudspeaker at medium levels the reproduced sound quality is just "good enough" for most applications. Control systems can be used to control loudspeaker distortions and to allow loudspeakers to be made smaller, lighter and cheaper. He noted that control systems have to be stable and robust, provide optimal performance while loudspeaker and ambient conditions are varying, be simple to use and be made of inexpensive hardware.

Wolfgang covered analogue and digital control systems. The main advantage digital systems have is that they enable simple and accurate multiplication of two time signals which is the most basic nonlinear operation. Both analogue and digital control systems can benefit significantly by using the back-EMF from the loudspeaker, instead of an external microphone.

Wolfgang discussed the dominant non-linearities for various types of loudspeakers including force factor, compliance, inductance and non-linear sound propagation. These are singled out because they limit acoustic output, generate audible distortion, indicate overload or cause instability. He noted that the distortion of the dominant nonlinearities are generated in the one-dimensional signal path close to the speaker input, which makes it possible to compensate the distortion actively.

Wolfgang covered the system design for a transducer-oriented digital control system. He explained how a simple model of the desired loudspeaker is used in the control loop and prevents the system from becoming unstable. He also demonstrated how the system is effected by different coil lengths.

Wolfgang finished with a discussion of how to split the design challenge between the loudspeaker designer and the DSP engineer. There followed a lively question and answer session. A recording of the lecture is available from the UK section website at www.aes.org/sections/uk/meetings/a0706.html.

Report by Tim Harris