æiut（時間）Temporal ne structureEnvelopeThose with impaired hearing nd it difcultto perceive changes in sound.How hearing deteriorates with reductionin temporal resolution（Schematic illustration）81632641282565120-5-10-15-20-25-30-35Modulation frequency [Hz]Modulation detection threshold of person with normal hearingModulation detection threshold of person with impaired hearingModulation detection threshold[20log(m), dB]Temporal ModulationTransfer Function (TMTF)Variations insound pressureModulationfrequencyLargeFastSlowSmallLow pass lter characteristics L f L Lps= L + L fcutoff f fModulation detection threshold[20log(m), dB]Modulation frequency [Hz]Two measurementpointsSimple method for measuringtemporal modulation transfer function (TMTF) fWhat is temporal resolution?In the context of auditory perception, temporal resolution refers to the ability to detect temporal changes in the amplitude envelope of sound. Currently there are no hearing aids that incorporate technology to compensate for reduction in temporal resolution. However, simulations have been conducted to determine how hearing is affected when temporal resolution is impaired, and methods for processing sound to enhance the temporal amplitude envelope are being investigated.Morimoto of the Technical Development Center focused on the temporal modu-lation transfer function (TMTF), which uses an easy-to-understand formula to express the temporal response character-istics of the auditory system.“For frequency selectivity and recruit ment, we already have methods for measuring impairments in clinical settings. Hearing aids o er built-in com-pensation technologies. However, no hearing aid developed to date o ers built-in technologies for compensating for impairments in temporal resolution. ere are many reasons for this, but to develop and re ne a compensation tech-nology for this, we need to nd a quick way to measure indicators for real-time processing. One such indicator is TMTF. TMTF conventionally required long measurement times, so we proposed a simple TMTF measurement method that shortened measurement times to about 10 minutes.”He reduced the time required for conventional measurements from 30 to 40 minutes to about one-third.Two-Point Measurement: e Solution Everyone Missed e proposed simple TMTF measure-ment method is simple indeed. Normally, the expression for TMTF requires mea -s u r e m ent at seven or more points. But the curves for TMTF had been reported to resemble the waveform of rst-order Butterworth lters, implying it might be possible to express TMTF using just two parameters; TMTF could be estimated from the results of measurement at just two points: at the detection threshold of modulation (Lα in the gure) at low mod-ulation frequency (fα in the gure) and at the detection threshold of the modulation frequency (fβ in the gure) at modulations exceeding the former threshold (Lα + Lβ in the gure).To evaluate this simple measurement method, an experiment was performed involving 26 subjects with normal hear-ing and 21 subjects with impaired hearing by the conventional method and by the simple measurement method. TMTF estimates were made based on the results for the two methods.“To evaluate the validity and e ective-ness of this simple measurement method, we made TMTF estimates by the conven-tional method and the simple method on subjects with normal hearing and those with impaired hearing. We con rmed a positive correlation between the two estimation results and found no system-atic errors. e simple method took just about 10 minutes, nearly 1/3 of the time required with the conventional method. is showed the simple method would allow e ective measurements in clinical settings and during tting sessions of hearing aids. Since most hearing-im-paired people are elderly, prolonged measurement can be tiring, leading to loss of concentration and inaccurate mea-surements. And we need to measure not just temporal resolution, but the hearing threshold level and articulation score in speech sound. So, the less time it takes to measure one indicator, the better.”Morimoto’s achievement didn’t in vo-lve overcoming challenges in developing an advanced technology. Rather, it focused on making TMTF estimates based on measurement at only two points—a strat e gy not previously considered. Issues remain to be resolved before this simple TMTF measurement method is ready for widespread adoption in clinical settings and hearing aid tting.“Most instruments for subjective tests performed as part of clinical practice are built into audiometers. Ideally, then, the instrument designed to perform the sim-ple measurement method should be built into an audiometer. But common tests performed with audiometers require the manipulation of just one or two buttons, which is insu cient for performing this measurement method. Also, the mea su-rement time needs to be further reduced. Much research remains to be done, but I intend to continue making improvements to the proposed method to realize clinical usefulness.”13
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