A sound localization method based on the adaptive estimation of inverse Ear Canal Transfer Functions (ECTFs) using a stereo earphone-microphone combination is proposed. This method can adaptively obtain the individual's transfer functions to fit the listener in real-time. We evaluate our sound localization method by studying the relationship between the estimation error of inverse ECTFs and the auditory sound localization score perceived by several listener. As a result, we clarified that the estimation error required of inverse ECTFs are less than -10 dB. In addition, we describe two adaptive inverse filtering methods in order to realize real-time signal processing implementation using affine projection algorithm and discusses the convergence time of an adaptive inverse filter to determine the initial value. It is clarified that method 2 based on copy weights with initial value is more effective than method 1 with filtered-x algorithm, in terms of convergence, if the initial value is the average of many listeners' impulse responses for our sound localization method.

Out-of-head sound localization achieved via binaural earphones is indispensable for a virtual sound system. It is necessary to measure the two transfer functions of each subject, Spatial Sound Transfer Function (SSTF) and Ear Canal Transfer Function (ECTF), for achieving sound localization. It is well known that the quality of sound localization may be poor if the individual transfer functions are not accurately reproduced. This is because each subject has his/her own transfer functions. It is very important to clarify which function includes more individual information, SSTF or ECTF, in order to implement a simpler model. Therefore, we introduce the quantity of "Personal differences" for investigating the subject's transfer functions included in SSTF and ECTF. We measure the transfer functions SSTF and ECTF of 60 subjects in a soundproofed room, and analysis of the data using Principal Component Analysis (PCA) and three subjective assessment tests. This study finds that ECTF differs more widely from person to person than SSTF.