in this preferred embodiment the lfp of the fundamental frequency is related to hearing ability and the hearing impairment is linearly dependent to the lfp the foregoing relationship is as follows with reference to fig3 a and 3b the intensity of the fundamental frequency of a subject suffering from sensorineural hearing loss and the power of fundamental frequency of a subject suffering from sensorineural hearing loss are respectively shown with reference to fig3 c and 3d the intensity of the fundamental frequency of a subject having normal hearing ability and the power of fundamental frequency of a subject having normal hearing ability are respectively shown from the respective disclosure of fig3 a and fig3 c it is noted that the lfp of the subject suffering from sensorineural hearing loss is rather slow six 6 periods within five 5 seconds i e 1 2 hz and the magnitude is larger than the test result of a tested subject wearing a hearing aid as shown in fig3 b and 3d it is noted that the lfp mfp and hfp performances of a tested subject suffering from sensorineural hearing loss are larger than a tested subject with normal hearing ability among which the difference in the comparison between two lfps is even more prominent fig4 shows the comparison of powers of the fundamental frequency between a subject suffering from hearing impairment and a subject with normal hearing ability in the disclosure of fig4 it is to be noted that the tested subject suffering from sensorineural hearing loss.