Every year a little less.

Four readings of one declining curve.

1. Why the highs go first.

   Sound enters the cochlea — a fluid-filled spiral in the inner ear — and travels along the basilar membrane. The membrane is stiff and narrow at the base and flexible and wide at the apex. High-frequency vibrations resonate at the base; low frequencies travel further inward. The hair cells at the base are exposed to every loud sound that passes, every concert, every motorbike, every leaf blower. They wear out first. By the time the lower-frequency cells deeper in the spiral show damage, the basal ones are already long gone — which is why high-frequency loss is the early signature of age-related hearing loss (Hertzano et al. 2020).

2. The infrasound world below us.

   Below 20 Hz, sound becomes physical pressure rather than recognisable pitch — for us. Blue whales communicate with calls between 10 and 40 Hz that travel thousands of kilometres through ocean SOFAR channels (Payne & Webb 1971); two whales on opposite sides of the Pacific could, in principle, hear each other. Elephants use ground-borne and air-borne infrasound around 14 Hz to coordinate family movement across kilometres of savannah, picked up through pads in their feet (O'Connell-Rodwell 2007). We feel the same frequencies — wind turbines, distant thunder, a pipe organ's lowest stops — but we don't quite hear them.

3. The ultrasonic world above us.

   Above 20 kHz lives the world we sealed off when we became mammals with limited high-frequency range. Bats emit clicks up to 110 kHz and read returning echoes with timing precision below a microsecond — they paint a three-dimensional acoustic image of their world. Bottlenose dolphins extend even further, to 150 kHz, with sonar that can distinguish a sardine from a herring at twenty metres. And the moths the bats hunt evolved their own counter-measure: ears tuned exactly to bat echolocation frequencies (Roeder & Treat 1957), and a startle response that drops them from the sky the moment a bat closes in.

4. The Mosquito tone — a generation's signature.

   In 2005 a British inventor commercialised a 17.4 kHz tone broadcast from small loudspeakers outside shops. Most adults could not hear it. Most teenagers could. It was sold as an anti-loitering device. Within months teenagers reverse-engineered it as a ringtone they could use in class without their teachers hearing — a frequency they owned, briefly, before age took it from them too. There is something tender in this: the upper end of our hearing is the part most quietly lost, and we usually only notice when someone younger points it out.