The capabilities of the human ear are remarkable. We can normally detect acoustic stimuli down to a threshold sound-pressure level of 0 dB (decibels) at the entrance to the external ear, which elicits eardrum vibrations in the picometer range. From this threshold up to the onset of pain, 120 dB, our ears can encompass sounds that differ in power by a trillionfold. The comprehension of speech and enjoyment of music result from our ability to distinguish between tones that differ in frequency by only 0.2%. All these capabilities vanish upon damage to the ear's receptors, the mechanoreceptive sensory hair cells. Each cochlea, the auditory organ of the inner ear, contains some 16,000 such cells that are frequency-tuned between ~20 Hz (cycles per second) and 20,000 Hz. Remarkably enough, hair cells do not simply capture sound energy: they can also exhibit an active process whereby sound signals are amplified, tuned, and scaled. This article describes the active process in detail and offers evidence that its striking features emerge from the operation of hair cells on the brink of an oscillatory instability—one example of the critical phenomena that are widespread in physics.

人耳的能力非常出色。我们通常可以在外耳入口处检测到低至 0 dB（分贝）阈值声压级的声学刺激，这会引起皮米范围内的鼓膜振动。从这个阈值到疼痛开始，即 120 分贝，我们的耳朵可以包含功率相差一万亿倍的声音。对语音的理解和对音乐的享受是由于我们能够区分频率仅相差 0.2% 的音调。所有这些能力在耳朵的受体（机械感受感觉毛细胞）受损时消失。每个耳蜗，即内耳的听觉器官，包含大约 16,000 个这样的细胞，这些细胞的频率在 ~20 Hz（每秒周期数）和 20,000 Hz 之间调谐。值得注意的是，毛细胞不仅捕获声能：它们还可以表现出一个主动过程，即声音信号被放大、调谐和缩放。本文详细描述了这一活动过程，并提供了证据，证明其显著特征来自处于振荡不稳定性边缘的毛细胞的运作——这是物理学中普遍存在的临界现象的一个例子。