Less is more? Loudness aspects of prescriptive methods for nonlinear hearing aids

University dissertation from Stockholm : Signaler, sensorer och system

Abstract: In Sweden, about 10% of the adult population experienceshearing problems that cause them difficulties in everydaycommunication, and approximately 60 000 people are providedwith hearing aids each year. Despite the fact that modernhearing aids can facilitate speech communication in a widerange of listening environments, many hearing-aid users aredissatisfied with their hearing aids. It is likely that theclinical methods used for individual fitting of the hearingaids are not optimal.The current study investigates prescriptive methods fornonlinear, wide dynamic range compression (WDRC) hearinginstruments. The goal is to draw general conclusions about thepreferences of hearing aid users. Therefore, the prescriptionsare evaluated using well-established models of loudness andspeech intelligibility.Current methods differed considerably in prescribed gain.Evaluations in a laboratory test, with 20 hearing-impairedlisteners, showed that these differences led to largedifferences in perceived and calculated loudness, but only tominor differences in measured and predicted speech recognitionscores. The difference in loudness was explored in a studywhere 21 first-time hearing-aid users compared twoprescriptions. One method led to normal and the other toless-than-normal overall calculated loudness (according to theloudness model of Moore and Glasberg (1997)). The prescriptionthat led to less-than-normal overall loudness was clearlypreferred in field and in laboratory tests.Preferred overall loudness was then quantified.Hearing-impaired participants with mild to moderate hearingloss preferred considerably less-than-normal overall calculatedloudness in both eld and laboratory tests. There were nosignificant differences between inexperienced and experiencedhearing aid users. Normal-hearing participants, on the otherhand, preferred close-to-normal overall calculated loudness. Inaddition, a potential problem with the loudness model wasencountered: despite the fact that the hearing-impairedlisteners were provided with less than normal overallcalculated loudness, they rated loudness higher than thenormal-hearing listeners.The results refute the most commonly adopted rationale forprescriptive methods for WDRC hearing aids - that overallloudness should be restored to normal. Hearing-impairedlisteners with mild to moderate hearing loss preferredconsiderably less than normal overall loudness. This should betaken into account when deriving new prescriptive methods, andwhen providing clients with hearing aids.Key words:hearing impairment, hearing aid, nonlinear,WDRC, hearing aid experience, prescription, loudness, loudnessmodel, speech intelligibility, preference.