## 4 Discussion  
Advances in medical technology have led to the development of several devices that can be used in patients with hearing loss. Selecting devices for patients has become moredifficult, as more devices are available for patients with conductive or mixed hearing loss. Although, AC-HA remains the most commonly used device, and CC-HA, Baha, and ADHEAR are also indicated when AC-HA is difficult to use. There are many overlaps in hearing levels and indicated diseases for each device. Since factors other than patient factors can determine which device is deemed suitable for each patient by an otologist and audiologist, we examined the results using a comparative trial. The hearing gains of Baha and CC-HA tended to be greater than those of ADHEAR, particularly at low and middle frequencies. There was a report comparing ADHEAR with another bone conduction implant, Bonebridge (MED-EL, Innsbruck, Austria). In [PERSON]'s study, the gain of Bonebridge is larger than that of ADHEAR [17], meaning that, the gain of Baha, one of a transcutaneous bone conduction implant, is more likely to be larger than that of ADHEAR. Although, it is difficult to determine the cause of the lower hearing gain observed with ADHEAR, but several mechanisms are conceived. First, ADHEAR uses less pressure than conventional bone conduction hearing aids. Increased comfort is associated with ADHEAR usage, however, it diminishes low-frequency hearing gains by compressing the temporal bone. Second, the gain of ADHEAR is set in a readymade manner that cannot be adjusted or fine-tuned to the patient's hearing level in Japan. On the other hand, the other two hearing devices (CC-HA and Baha) can fine-tune the hearing gain and accelerate the low-frequency hearing function. In a previous report by [PERSON], it was suspected that amplification is smaller with ADHEAR, as working principles were considered. The manufacturer suggests that cases with bone conduction hearing thresholds within 25 dB HL are good candidates for ADHEAR [18], so the limits of ADHEAR's gain might be smaller than those of the other two devices. However, we believe that it would be meaningful for patients who cannot use AC-HAs for some reason, such as ear discharge or itchy ears, to experience the ADHEAR device, which does not need to be placed in the ear, and is noninvasive. Hearing thresholds at 4 kHz in each type of hearing aid are worse than without in cases 1, 3, and 4. Since the sound sources used in sound field audiometry are not pure-tone, but a warble-tone, this difference may have caused a measurement error for high frequencies. Although no subjective hearing loss was observed in any of the cases, we believe that we need to pay attention to this difference. Regarding the device fixation, ADHEAR immediately peeled off and could not be tested in two out of the six patients, suggesting that the condition of the skin limits ADHEAR usage in some patients. Baha was not tested in two out of the six patients because they had negative reactions to the operation. Even so, all patients attempted to use CC-HA, which is thought to be advantageous because it is easy to use in daily life and does not require surgery. Regarding the results of the sound localization test, there was no clear advantage for any of the hearing devices. In cases where the score worsened, patients reported that it was because the sound from all speakers stood out equally, and claimed that it felt like all the sounds were coming from the side with the hearing aid. Previous studies have also reported that wearing Baha does not improve the sense of sound localization. Similar to the results of this study, some cases improved while others worsened [19]. To date, there have been few reports on the improvements in sound localization using ADHEAR or CC-HA. We believe that in order to make accurate evaluations for these devices, a few issues will need to be clarified, such as the setting of the presentation sound pressure level in examinations.