Method and apparatus for the measurement of the acoustic impedance

A method for the determination of the acoustic impedance of cavities, such as an ear in connection with a hearing aid, comprising the steps of:

• arranging a probe with a microphone and a speaker at the area to be measured;
• sending out acoustic signals over the speaker into the cavity and receiving again over the microphone;
• transforming the received signals by the microphone into electrical signals and transferring them to an analysis unit;
• using a previously defined stimulation input to a twoport chain transfer matrix as a calculation base for the impedance Z,
• wherein the voltage ratio between the stimulation and the impedance is described as a dimensionless transfer function in a form of a complex function of the stimulation frequency;
• generating a series of acoustic calibration signals by a number of known acoustic impedances covering different calibration scopes by means of the defined stimulation;
• recording the calibration signals received by the microphone and merging the electric values together with the respective voltage values of the stimulation for the an evaluation of the results of the respective transfer functions;
• merging together the transfer functions of the calibration signals into an over-determined linear system of equations and solving the system of equations for calculating and storing two coefficients;
• determining the impedance Z to be calculated by evaluating the transfer function by use of the two coefficients;
• and
• outputting the determined impedance Z for use by an external entity.

An apparatus for the determination of an acoustic impedance Z comprising:

• a probe;
• a microphone;
• a speaker;
• an acoustic resistor arranged between the speaker and an exit opening within a connecting channel connecting to one of the microphone and an exit of the probe;
• and
• an analysis unit for receiving electrical signals from the microphone, and for determining an impedance Z, wherein
• a series of acoustic calibration signals of a number of known acoustic impedances covering different calibration scopes are generated by means of a predefined stimulation for output by the probe for reception by the microphone, and wherein
• the analysis unit comprises a function for solving an over-determined linear system of equations in terms of minimum squares by use of at least three of said acoustic calibration signals.