Fast and Accurate Measurement of the Linear Transducer Parameters

A new measurement technique is presented for the estimation of the linear parameters of the lumped transducer model. It is based on the measurement of the electrical impedance and the voice coil displacement using a laser sensor. This technique identifies the electrical and mechanical parameters directly and dispenses with a second measurement of the driver using a test enclosure or an additional mass. Problems due to leakage of the enclosure or the attachment of the mass are avoided giving accurate and reliable results. The measurement of the displacement also allows identification of the mechanical compliance versus frequency (explaining suspension creep) which is the basis for predicting the radiated sound pressure response at low frequencies precisely. The linear parameters measured at various amplitudes are compared with the results of large signal parameter identification and the need for nonlinear transducer modelling is discussed. INTRODUCTION The determination of linear loudspeaker parameters belongs to the classical problems in driver design. It can be solved straight forward and computer programs that calculate the linear parameters are available for years. Are there any news? Traditionally the impedance function is measured and analyzed. The impedance is in fact a good basis for linear parameter identification. It is easy to measure, it is not affected by the acoustic system and does not contain any time delay. Unfortunately it describes only the electrical part of the model and additional information from the mechanical domain is required to determine the mechanical parameters. Therefore, a second measurement is performed where the transducer is either mounted in a test enclosure or an additional mass is attached to it (perturbation method). Apart from being time consuming the accuracy of the results may be deteriorated by leakage of the enclosure and problems due to the attachment of the mass. There are even transducers for which neither of the techniques can be applied. In the paper an identification technique is proposed that dispenses with a second measurement. The problems mentioned above are avoided giving accurate and reproducible results. Furthermore a procedure is described to validate the identification results. The linear parameters describe the loudspeaker adequately only if the excitation is sufficiently small. They fail to describe the large signal behavior of the speaker. The behavior at high amplitudes and the relationship between linear and nonlinear parameters are discussed in the second part of the paper. TRANSDUCER MODEL The paper presents a novel technique to identify the components (Thiele-Small Parameters) of the linear loudspeaker model below valid in the small signal domain. Mms Cms(f) Rms