D1 Pure Tonal Audiometry: Air and Bone Thresholds

Topic

A calibrated audiometer generates a pure tone at a known frequency and intensity; this calibration—adjusted according to international standards that define the exact sound pressure level corresponding to "zero" on the audiometer's scale—enables the comparison of results between different individuals, or between results from the same individual at different times, as all measurements are based on the same reference pure tone.

The calibrated audiometer delivers this pure tone to the listener via two distinct pathways that reach the cochlea through different routes; each pathway yields a specific monaural detection threshold, referring to one ear at a time. The air-conduction pathway delivers the pure tone via a transducer placed over the external auditory canal, allowing the tone to travel through the external auditory canal, the tympanic membrane, and the ossicular chain before reaching the cochlea—mimicking the path of any ambient sound. In contrast, the bone-conduction pathway delivers the pure tone via a vibrator placed directly against the skull; the tone reaches the cochlea by vibrating the bone itself, bypassing the external auditory canal, the tympanic membrane, and the ossicular chain.

The air-conduction monaural detection threshold depends on all the structures the pure tone traverses on its way to the cochlea—the external auditory canal, the tympanic membrane, the ossicular chain, and the cochlea itself—meaning that any impairment at any of these stages raises the air-conduction monaural detection threshold. In contrast, the monaural bone-conduction detection threshold depends solely on the cochlea, because the pure tone delivered via bone conduction never passes through the external auditory canal, the tympanic membrane, or the ossicular chain; a defect in any of those three structures does not raise the monaural bone-conduction detection threshold—it only raises the monaural air-conduction detection threshold.

The difference between the monaural air-conduction detection threshold and the monaural bone-conduction detection threshold for the same ear defines that ear's conductive gap. When the monaural air-conduction detection threshold is elevated but the monaural bone-conduction detection threshold remains normal, the resulting conductive gap indicates that the defect lies in the external auditory canal, the tympanic membrane, or the ossicular chain, rather than in the cochlea. Conversely, when both thresholds—air-conduction and bone-conduction—are equally elevated, the conductive gap shrinks to almost zero, because the defect is no longer located prior to the cochlea but rather within the cochlea itself or beyond it.

Since a calibrated audiometer measures the monaural air-conduction detection threshold at several distinct frequencies rather than just one, the pure-tone average summarizes these various thresholds—measured at the frequencies that contribute most to speech comprehension—into a single figure describing the severity of the ear's hearing loss for practical conversational purposes. A low pure-tone average indicates that the ear requires very little additional intensity to detect speech-relevant frequencies, whereas a high pure-tone average indicates that the same ear requires much more intensity than usual to detect those same frequencies, and will therefore have greater difficulty following a conversation at a normal volume.

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