Department of Communication Sciences and Disorders, The University of Iowa, Iowa City, Iowa 52242, USA.
Ear Hear. 2012 Jan-Feb;33(1):44-56. doi: 10.1097/AUD.0b013e3182292107.
A number of clinical measures of directivity, including the front-to-back ratio (FBR) and front-to-side ratio (FSR), have been suggested to audiologists to monitor the functionality of hearing aids with directional microphones. These suggestions, however, are based on the assumption that directivity measured clinically changes monotonically when compared with changes measured using the directivity index (DI) and perceptual directional benefit. The objective of the present study was to empirically examine this assumption. In addition, the reliability of the clinical directivity measure was estimated to establish a referral threshold for defective directional microphone hearing aids.
The directivity of the directional microphones of two behind-the-ear hearing aids was systematically degraded by plugging the microphone ports. The directivity was measured using four clinical measures: the FBR and FSR performed in the test chamber of a hearing aid analyzer and in sound field. Each measure was repeated four times in each directivity-degraded condition. The degraded directivity was also assessed using the DI measure in an anechoic chamber. The perceptual directional benefit in each directivity-degraded condition was obtained by testing 10 hearing-impaired adults in a sound field with diffuse noise using the Hearing in Noise Test (HINT).
The results of the DI and HINT measures showed strong correlation between the two FSRs (test chamber and sound field), while the two FBRs showed no correlation. The directivity generated by the FBRs could remain unchanged even when the directional microphone had lost more than 50% of its directivity. The results further indicated that the measures performed in the sound field were more reliable than those performed in the test chamber. Based on the results of the reliability measures, a 30% change in directivity was suggested as the referral threshold signifying defective directional systems.
Because the FSR predicts the DI and HINT measurements more accurately than does the FBR, it is suggested that clinicians use the FSR to monitor hearing aid directivity. By using the FSR measure and informed by the suggested referral threshold, clinicians would be able to correctly identify defective directional microphone hearing aids at an early stage, rather than at a point when the directivity has been diminished completely or even reversed.
一些临床指向性指标，包括前后比（FBR）和前侧比（FSR），已经被推荐给使用指向性麦克风的助听器验配师，以监测助听器的功能。然而，这些建议是基于这样一种假设，即与使用指向性指数（DI）和感知指向性增益所测量的变化相比，临床测量的指向性会单调变化。本研究的目的是实证检验这一假设。此外，还评估了临床指向性测量的可靠性，以确定有缺陷的指向性麦克风助听器的转诊阈值。
通过堵塞麦克风端口，系统地降低了两个耳背式助听器的指向性麦克风的指向性。使用四种临床测量方法测量指向性：在助听器分析仪的测试室中和在声场中进行的 FBR 和 FSR。在每种指向性降低的条件下，每种测量方法重复四次。在消声室中使用 DI 测量评估了降低的指向性。在声场中使用弥散噪声对 10 名听力受损成年人进行测试，获得了每个指向性降低条件下的感知指向性增益，使用噪声中言语测试（HINT）。
DI 和 HINT 测量的结果表明，两个 FSR（测试室和声场）之间具有很强的相关性，而两个 FBR 之间没有相关性。即使指向性麦克风失去了超过 50%的指向性，FBR 产生的指向性也可能保持不变。结果进一步表明，声场中进行的测量比测试室中进行的测量更可靠。基于可靠性测量的结果，建议将 30%的指向性变化作为转诊阈值，表明指向系统有缺陷。
由于 FSR 比 FBR 更准确地预测 DI 和 HINT 测量值，因此建议临床医生使用 FSR 来监测助听器的指向性。通过使用 FSR 测量，并根据建议的转诊阈值，临床医生能够在早期正确识别有缺陷的指向性麦克风助听器，而不是在指向性已经完全降低甚至反转的情况下。
