quation (3) indicates that only differences between the normal and client magnitude response are needed to develop the hearing aid circuit specification. This suggests a process that is simpler than actually getting the TF of the ear, which involves only measuring the client TOH at a discrete number of frequencies. The TOH is defined as the smallest audio power level at which a person can a still hear signal, based on an absolute scale. These levels over the audio spectrum form a function that corresponds to the reciprocal of the TF. Since calibrated equipment will not be used on the test, only relative level will be determine. To examine the relationship between absolution and relative TOH functions, let be the absolute normal TOH, and be the absolute TOH of the client. The transfer function from a relative TOH measurement is described by the following equations: (4) where and are proportionality constants that have no impact on the shape (relative changes) of the TF. These result from different volume level settings on the playback devices and listening station. While the true TF values require absolute measurement of sound, the absolute scale is not needed in our case since the device will have a volume adjustment for the client to change overall level of the signal. So the important aspect of our design is the relative differences or shape of the circuit TF. This is observed by substituting the TOH function from q. (4) into q. (3) to obtain: (5) where the last term is a constant resulting from the proportionality factors and would be directly affected by the overall volume control and can be ignored for our circuit. So Eq. (5) indicates that the measured TOH level of the client’s ear in dB, minus the normal TOH level is our circuit specification. For this project the client’s threshold of hearing will be measured at discrete frequencies contained in the Table 1. Table 1. Test Frequency hearing thresholds Test Frequencies (Hz) Absolute TOH (dB) 300 9 600 6 1200 4 2400 0 4000 -4 5000 0 7000 10 9000 18 12000 11 14000 20 The set of frequencies are assumed to be dense enough in the critical part of the spectrum, such that they provide sufficient information for the continuous reconstruction of the filter
quation (3) indicates that only differences between the normal and client magnitude response are needed to develop the hearing aid circuit specification. This suggests a process that is simpler than actually getting the TF of the ear, which involves only measuring the client TOH at a discrete number of frequencies. The TOH is defined as the smallest audio power level at which a person can still hear a signal, based on an absolute scale. These levels over the audio spectrum form a function that corresponds to the reciprocal of the TF. Since calibrated equipment will not be used on the test, only relative level will be determine. To examine the relationship between absolution and relative TOH functions, let be the absolute normal TOH, and be the absolute TOH of the client. The transfer function from a relative TOH measurement is described by the following equations: (4) where and are proportionality constants that have no impact on the shape (relative changes) of the TF. These result from different volume level settings on the playback devices and listening station. While the true TF values require absolute measurement of sound, the absolute scale is not needed in our case since the device will have a volume adjustment for the client to change overall level of the signal. So the important aspect of our design is the relative differences or shape of the circuit TF. This is observed by substituting the TOH function from q. (4) into q. (3) to obtain: (5) where the last term is a constant resulting from the proportionality factors and would be directly affected by the overall volume control and can be ignored for our circuit. So Eq. (5) indicates that the measured TOH level of the client’s ear in dB, minus the normal TOH level is our circuit specification. For this project the client’s threshold of hearing will be measured at discrete frequencies contained in the Table 1. Table 1. Test Frequency hearing thresholds Test Frequencies (Hz) Absolute TOH (dB) 300 9 600 6 1200 4 2400 0 4000 -4 5000 0 7000 10 9000 18 12000 11 14000 20 The set of frequencies are assumed to be dense enough in the critical part of the spectrum, such that they provide sufficient information for the continuous reconstruction of the filter
EE221Project.docx