Abstract (summary)

TranslateAbstractThe purpose of the present study was to determine the feasibility of reducing intersubject variability of highf-requency hearing thresholds by usinginsert earphonesand frequency-modulated (warble) tones. Forty listeners with normal hearing in the conventional audiometric range (125-8000 Hz) participated in the study. Hearing thresholds were measured at 10, 12, 14, 16, and 18 kHz with circumaural (HD-250) andinsert(ER-1)earphonesusing pure tones and 2% and 5% frequency-modulated FM (warble) tones. Obtained results demonstrated significantly smaller intersubject variability of hearing thresholds measured withinsert earphonesand 5% FM tones in comparison to all other test conditions. [PUBLICATION ABSTRACT]

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The purpose of the present study was to determine the feasibility of reducing intersubject variability of highf-requency hearing thresholds by usinginsert earphonesand frequency-modulated (warble) tones. Forty listeners with normal hearing in the conventional audiometric range (125-8000 Hz) participated in the study. Hearing thresholds were measured at 10, 12, 14, 16, and 18 kHz with circumaural (HD-250) andinsert(ER-1)earphonesusing pure tones and 2% and 5% frequency-modulated FM (warble) tones. Obtained results demonstrated significantly smaller intersubject variability of hearing thresholds measured withinsert earphonesand 5% FM tones in comparison to all other test conditions.

Keywords

Audiometry, hearing threshold, high frequency

1. Introduction

High-frequency audiometry (HFA) refers to the hearing threshold measurements at frequencies above 8 kHz. It complements conventional audiometric measurements extending from low frequencies up to 8 kHz. Since frequencies in the order of 4-8 kHz are considered the high frequency region of conventional audiometric measurements, the audiometry above 8 kHz is also frequently referred to as extended high frequency audiometry (EHFA). In this paper we refer to frequencies beyond 8 kHz as the HFA range.

The HFA range lies beyond the basic range of speech communication frequencies but this range is important for acoustic signature recognition of many natural and military sources, spatial orientation, and music perception. Therefore, the HFA range is of great interest to human factors engineering. Human factors interest in HFA stems also from the fact that the first symptoms of noise-induced hearing loss (NIHL) frequently develop in the high frequency range before it spreads down toward lower frequencies. In addition, many non-NIHL hearing losses are progressive sensorineural high-frequency losses caused by various pathological agents. In both cases an early detection of a high-frequency hearing loss is important for prevention of more severe hearing loss affecting speech communication and perception of acoustic warnings as well as for identification of populations at risk. There are also studies indicating the importance of high-frequency hearing for speech perception and for proper selection of aural rehabilitation strategies [1][2].

Since high frequency hearing is important for human auditory awareness of the environment and general well-being it is important to determine normative values of high frequency hearing thresholds for the general population. Such values are needed in order to determine whether specific changes in high frequency sounds can be perceived by most of the population or to determine what a change in high frequency signal needs to be made by a signal designer in order to be detected and recognized under specific environmental conditions. However, the normative HFA data that can be used across large populations are very difficult to establish despite the fact that there are several documents and reports proposing normative high frequency thresholds for specific testing conditions and specific applications (e.g., [3]). The main factor making the development of general normative data a very difficult task is the large intersubject variability of high frequency thresholds. This variability results from large changes in high frequency hearing due to age, exposure and susceptibility to noise, and the dimensions of the ear canal, which greatly affect the behavior of high frequency acoustic waves. In addition, the proposed thresholds have been developed for pure tone signals under rigorous and time-consuming procedures using calibration equipment that is not easily available outside medical clinics and research laboratories. Such signals emphasize rather than deemphasize differences in people and require careful calibration of equipment. The medical community has limited interest of in developing the HFA norms for the general population since medical use of HFA is limited to serial measurements of the threshold of hearing in individual patients. The HFA is typically used to monitor to what extent specific pathologies, medications, and noise levels affect the threshold of hearing over time. In these applications patients serve as their own baselines. For that reason most of the efforts to develop appropriate methods to calibrate HFA equipment and to determine related normative hearing threshold values for the medical community was concerned with minimizing the intrasubject variability than intersubject variability of the data.

There are several studies reporting very good intrasubject variability of the HFA data that is comparable to that of conventional audiometry. Good intrasubject reliability reported in these studies also indicates that the random errors related to calibration procedures andearphonesfitting in HFA can be kept relatively small especially when the threshold of hearing is determined by measuring sound pressure in the ear canal using a probe tube microphone. The probe microphone technique is an excellent method for determining the normative HFA values and for general HFA measurements. However, this is a clinical and laboratory technique that is time consuming and requires medically trained personnel. In addition, while pure tones are important frequency-specific test signals for medical assessment of hearing, they are inappropriate signals for human factors and audio applications where more general information about the hearing threshold is needed. Thus, for all the above reasons, the existing HFA normative data and test procedures are not well suited for human factor applications.

In order to compare a person's hearing to established norms, the procedure and instrumentation used for testing must be compatible with the procedure and instrumentation that was used to determine the norms. Thus, in order to establish the normative HFA thresholds that can be used across wide ranges of medical, research, audio, and human factors applications it is important to first develop a procedure that is easily repeatable, can be used in various environments, and provides some averaging of the threshold values to provide more robust estimates of the HFA thresholds. This paper presents a description and an initial validation of a new HFA procedure that results in relatively small intersubject variability and could be used across both medical and non-medical applications of HFA. The signals proposed in this procedure are frequency-modulated tones that have never been reported before to be used for hearing threshold in high frequency range (HFA).

2. Method

2.1 Listeners

Forty listeners, 20 males and 20 females, ranging in age from 20 to 30 years (mean = 26.2 years, SD = 2.6 years) participated in this study. Each participant had bilateral air conduction hearing thresholds less than 20 dB HL at all audiometric frequencies within the 0.25 to 8 kHz frequency range (ANSI S3.6-1989 [4]), normal bilateral tympanograms, no history of recent otologic pathology, and no history of any prolonged noise exposure.

2.2 Instrumentation

All hearing screening and subsequent experimental testing were conducted in an audiometric test booth (Suttle, SEB1) complying with ANSI S3.1-1991 [5] noise requirements for open ear testing. The test signals were high frequency pure tones and 2% and 5% FM tones at 10, 12, 14, 16 and 18 kHz produced by a Beltone 2000 audiometer. All signals were presented for 1.5 s with a 25 ms rise/fall time. The FM tones were obtained by linear (triangular) frequency modulation with a 5 Hz modulation rate. The audiometer calibration was verified before and after the collection of the data. The test stimuli were delivered to the subject's ears monaurally through either aninsert earphone(Etymotic Research ER-1; 10 ohms) or circumauralearphone(Sennheiser HD-250; 50 ohms). A custom-built impedance matching device was inserted between the 50 ohms Beltone 2000 output and the ER-1 10 ohmsinsert earphoneduring both calibration and testing.

The ER-1insert earphoneis a shoulder-mounted transducer coupled to the ear via plastic tubing ending with a foam eartip surrounding the internal plastic tube. Each ER-1insert earphonewas calibrated using an ANSI S3.25-1989 [6] occluded ear simulator (Knowles DB-100 Zwislocki Coupler) with the nipple of the ER-1 sound tube attached to the coupler through ER1-08 adapter. The Sennheiser HD-250 circumauralearphoneswere calibrated with a flat-plate configuration of the IEC 318-2 (1998) [7] artificial ear (B&K 4153). Bothearphonesused in this study had gradual frequency response roll-offs above 15 kHz and steep roll-offs above 17 kHz. Therefore, the high-frequency FM tones at 16, and 18 kHz were actually modulated in both frequency (FM) and amplitude (AM). Consequently, the additional amplitude modulation of FM signals might have influenced threshold data reported in the study. This situation, however, is common for reproduction of FM signals by the transducers with non-ideal frequency responses and was not accounted for in this study.

2.3 Procedures

Each listener participated in two, 1.5-hour long, experimental sessions: test session (Session 1) and retest session (Session 2). Each session included six test conditions presented as the test blocks: (1)insert earphoneswith pure tone stimuli; (2)insert earphoneswith 2% FM tone stimuli; (3)insert earphoneswith 5% FM tone stimuli; (4) circumauralearphoneswith pure tone stimuli; (5) circumauralearphoneswith 2% FM tone stimuli; and (6) circumauralearphoneswith 5% FM tone stimuli. In each test block, both ears were tested and the test signals were always presented in the 10 to 18 kHz sequence. The order of test blocks as well as ears tested were counterbalanced in Session 1 and reversed in Session 2. There was a minimum interval of two and a maximum of seven days between the test and the retest session for each listener.

The HD-250 circumauralearphoneswere fitted on each listener by an experimenter to ensure that eachearphonediaphragm was centered over the opening of the ear canal. Then the headband assembly was tightened. After theearphoneshad been placed, the subject was not allowed to touch or reposition theearphones. In the case of the ER-1insert earphones, the foam eartips were inserted by an experimenter into the listener's ear canal. This was done by compressing the foam eartip and inserting it into the ear canal so that the outer edge of the earplug was flush with the floor of the subject's concha (12 mm insertion depth). The eartip was then held in place for 30 seconds to allow it to expand. The eartip was checked for insertion depth and reinserted if necessary. These fitting procedures are common in HFA testing [8]. .

During the experiment, each listener was seated comfortably and provided with a response button. Instructions for responding at threshold were identical to those routinely given for conventional audiometry. The listener's task was to press the response button every time the signal was heard even if the signal was very faint. In each test block, all test stimuli were initially demonstrated at suprathreshold level to familiarize the listener with the type of stimuli used in this block. During testing, the stimulus was initially presented at a subtreshold level and increased gradually in 2 dB steps until the listener responded. Once an initial response occurred, the level was decreased in 10 dB steps until no response occurred and then increased in 2 dB steps until the subject responded again. This procedure (10 dB down and 2 dB up) was continued until six ascending thresholds were obtained. Threshold was defined as the mean of the six ascending thresholds [9].

3. Results and Discussion

3.1 Intrasubject Reliability

Intrasubject reliability was assessed by comparing test and retest data at all experimental conditions. Mean test-retest differences observed for both types ofearphones, both ears, and all test signals were all very small. For the HD-250earphonesthey were in the range from 0.1 dB for the 5% FM tone at 16 kHz to 1.4 dB for the pure tone at 10 kHz. These values are similar to those reported by Frank [9] for HD-250 data measured across four test sessions and agree with the data reported by Frank and Dreisbach [10]. The test-retest differences observed for the ER-1earphoneswere slightly smaller than those reported for the HD-250earphones. Average left-right ear differences were below 3 dB for all conditions tested and bothearphones. To assess statistical significance of the observed differences, ten three-factor repeated measures ANOVAs on SESSION (test, retest), EAR (left, right), and STIMULUS (pure tone, 2% FM tone, 5% FM tone) were conducted for individual frequencies (5) andearphonetypes (2). The ANOVA results did not reveal any significant effects at p