At low frequencies. At low frequencies, the positive OE of the order of +5 to +10 dB means that the energy transmitted by the sum of the B1 and B2 paths is much more important in occluded ear than in opened ear: B1oc + B2oc B1op + B2op. As mentioned in section 1.2, the occlusion has an effect on the ear canal vibratory and acoustic system, hence, a reasonable assumption is that occlusion will affect only the B2 path and leave unchanged the B1 path, so that B1oc ≈ B1op. This equation put into the first inequality leads to B2oc B2op and, furthermore, on the left hand-side of the inequality B2oc B1oc. Indeed, if we had B2oc ± B1oc, then we would have B1oc B1oc + B2op which is false. So the assumption B2oc ± B1oc is false. Otherwise, if we had B2oc ≈ B1oc, then we would have B1oc · 2 B1oc + B2op which is false. So the assumption B2oc ≈ B1oc is false. In other words, for the VABC perception at low frequencies we can conclude: • Path identification: in an occluded ear the indirect path B2 transmits more energy than the direct path B1; • OE: a positive OE results from the fact that occlusion increases the energy transmitted by the B2 path which dominates the B1 path. As the indirect path B2 dominates, this is in agreement with the low frequency physical mech- anisms presented in section 1.2. In open ear, the low frequencies ear canal wall radiations are propagated out of the ear. In occluded ear, these low frequencies are blocked inside and so the SPL at low frequencies is increased.
At low frequencies. As can be seen in Fig. 1.9 at low frequencies (below 2000 Hz), the subjective BC OE level is much dependent on the earplug insertion and on the kind of earplug. However, the subjective BC OE is globally positive just like in the case of the subjective VABC OE, so the same rela- tions that have been introduced previously for the VABC case will be valid here, after the letter −10 250 500 2000 4000 8000 Occlusion effect [dB] −− −× ··· ··· ···
At low frequencies. At low frequencies, the OE is positive, so A2aoc A2aop. In other words, for the objective VBC at low frequencies we can conclude: • OE: a positive OE results from the fact that occlusion increases the energy transmitted by the A2a path. Like for the subjective BC OE, the objective one is in agreement with the low frequency phys- ical mechanism described in section 1.2. −10 250 500 2000 4000 8000 Occlusion effect [dB] −− −× ··· −· − At high frequencies, the OE is negative, so A2aoc ± A2aop. In other words, for the objective VBC at high frequencies we can conclude: • OE: a negative OE results from the fact that occlusion decreases the energy transmitted by the A2a path. For the same reasons as for the subjective VABC OE, these results are in agreement with the high frequency physical mechanism described in section 1.2. In this section, the relations derived in the previous sections on the subjective and objective BC OEs are brought together to characterize as completely as possible the path involved in the BC OE. An analysis of the physiological noise masking effect (PNME) that occurs when the subject is wearing a hearing protector is presented in a first sub-section. This analysis will be used to choose between the two possibilities which have been found for the subjective BC OE at high frequencies (cf. section 1.6.2.2). The two last sub-sections present the general results for BC OE.
At low frequencies. At low frequencies, from the analyses of both the subjective and the objective BC OEs as well as the physiological masking effect analysis, the following conclusions can de derived: • Path identification: From the subjective BC OE, it was found that in an occluded ear the indirect path A2 transmits more energy than the direct path A1 (A2oc A1oc); • OE: A positive OE results for both subjective and objective BC OE because occlusion in- creases the energy transmitted by the A2a path (A2aoc A2aop ) as well as the energy transmitted by the A2 path (A2oc A2op) which dominates the A1 path . The PNME analysis showed that the perceived energy (hearing threshold) will be higher than the ear canal measured energy (SPL), hence A2op ≤ A2aop, A2oc < A2aoc. As indicated in sections 1.6.2.1 and 1.6.3.1 for the subjective and objective BC OE, these conclusions are in agreement with the low frequency physical mechanism presented in section 1.2. At high frequencies, the relation A2aoc < A2aop from the objective BC OE and the relation A2 ≈ A2a from the PNME give: A2oc < A2op. This invalidates the first of the two possibilities found in section 1.6.2.2 that requires A2oc ≈ A2op, hence validating the second solution: X0xx X0xx and A1op A2op. The conclusions for the BC OE at high frequencies are: • Path identification: in open ear and in occluded ear the direct path A1 is dominant.