Evaluation of early hearing damage in personal listening device users using extended high-frequency audiometry and otoacoustic emissions.

Although sound exposure from personal listening devices (PLDs) could potentially lead to noise-induced hearing loss (NIHL), the actual hearing risk associated with the use of these devices is still unclear. In this study, early hearing effects related to PLD usage were evaluated in 35 young adult PLD users (listening for >1 h/day, at >50% of the maximum volume setting of their devices) and their age- and sex-matched controls using a combination of conventional and extended high-frequency audiometry as well as transient-evoked otoacoustic emission (TEOAE) and distortion product of otoacoustic emission (DPOAE) measurements. The mean listening duration of the PLD users was 2.7 ± 1.0 h/day while their estimated average listening volume was 81.3 ± 9.0 dBA (free-field corrected). Typical signs of NIHL were not detected in the audiogram of PLD users and their audiometric thresholds at most of the conventional test frequencies (0.25-8 kHz) were comparable with those obtained from controls. However, compared with the controls, mean hearing thresholds of PLD users at many of the extended high-frequencies (9-16 kHz) were significantly higher. In addition, TEOAE and DPOAE amplitudes in users were reduced compared with controls. The deterioration of extended high-frequency thresholds and the decrease in DPOAE amplitudes were more evident in the users' right ears. These results indicate the presence of an early stage of hearing damage in the PLD user group. Preventive steps should be taken as the initial hearing damage in these users could eventually progress into permanent NIHL after many years of PLD use.

Hearing risk associated with the usage of personal listening devices among urban high school students in Malaysia.

OBJECTIVES: To investigate listening habits and hearing risks associated with the use of personal listening devices among urban high school students in Malaysia. STUDY DESIGN: Cross-sectional, descriptive study. METHODS: In total, 177 personal listening device users (13-16 years old) were interviewed to elicit their listening habits (e.g. listening duration, volume setting) and symptoms of hearing loss. Their listening levels were also determined by asking them to set their usual listening volume on an Apple iPod TM playing a pre-selected song. The iPod's sound output was measured with an artificial ear connected to a sound level meter. Subjects also underwent pure tone audiometry to ascertain their hearing thresholds at standard frequencies (0.5-8 kHz) and extended high frequencies (9-16 kHz). RESULTS: The mean measured listening level and listening duration for all subjects were 72.2 dBA and 1.2 h/day, respectively. Their self-reported listening levels were highly correlated with the measured levels (P < 0.001). Subjects who listened at higher volumes also tend to listen for longer durations (P = 0.012). Male subjects listened at a significantly higher volume than female subjects (P = 0.008). When sound exposure levels were compared with the recommended occupational noise exposure limit, 4.5% of subjects were found to be listening at levels which require mandatory hearing protection in the occupational setting. Hearing loss (≥25 dB hearing level at one or more standard test frequencies) was detected in 7.3% of subjects. Subjects' sound exposure levels from the devices were positively correlated with their hearing thresholds at two of the extended high frequencies (11.2 and 14 kHz), which could indicate an early stage of noise-induced hearing loss. CONCLUSIONS: Although the average high school student listened at safe levels, a small percentage of listeners were exposed to harmful sound levels. Preventive measures are needed to avoid permanent hearing damage in high-risk listeners.

Effects of centrifugal pathways on responses of cochlear nucleus neurons to signals in noise.

The medial olivocochlear (MOC) system, which originates in the brainstem and projects to the outer hair cells in the cochlea, is thought to be involved in improving signal detection in noisy backgrounds. This proposition arises from the observation that the input-output functions of auditory primary afferent fibres to pure tones recorded in a continuous background noise are unmasked by MOC activation, improving the dynamic range, and is supported by both animal and human behavioural experiments. However, it is not known how the unmasking effects observed in the cochlea are translated into higher auditory brain centres, such as the cochlear nucleus, where intrinsic circuitry can potentially modulate any effect. In this study we have investigated the effects of continuous background noise without and with MOC system activation, on responses of different neuron types in the ventral cochlear nucleus of the guinea pig. Results show that the unmasking effects of MOC system activation on tone responses in continuous background noise are present in the cochlear nucleus. These unmasking effects manifest themselves as decompression of input-output functions as well as an improved slope, which results in an improvement in intensity discrimination of the tones. The data show, however, that the strength of the unmasking effects of MOC system activation varies between the different neuronal types. Unmasking was not detected in onset chopper neurons despite its demonstrable presence in other neuronal types in the same animals. These observations may reflect the level of involvement of different neuronal types in intensity discrimination.