How good is a dog's hearing?
Better than ours in two specific ways — frequency range and directionality — and not dramatically better in overall sensitivity, which is the part people get wrong.
Frequency range. Humans hear roughly 20Hz to 20kHz, declining considerably with age. Dogs hear from around 67Hz up to approximately 45kHz, well into what we call ultrasound. This is why dog whistles work, why some electronic devices distress dogs, and why a dog reacts to something apparently silent — rodents in a wall, ultrasonic pest repellers, or the high-frequency whine of electronics.
Why they evolved that range. Small prey animals communicate ultrasonically, so hearing them confers real advantage.
Directionality. Dogs have around 18 muscles controlling each ear, allowing independent movement and precise aiming — which is what the head tilt frequently accomplishes, adjusting ear position to locate a sound and, one hypothesis suggests, to see past the muzzle.
Sensitivity. At the frequencies both species share, the difference is smaller than assumed — dogs detect quieter sounds, but not by orders of magnitude. The popular claim that dogs hear "four times further" is imprecise; the advantage is mostly in the frequencies we simply cannot hear at all.
What follows practically:
Noise sensitivity is a welfare issue. Fireworks, thunder and alarms are considerably more intense for a dog, and noise phobia is common, treatable and frequently untreated.
Ultrasonic training and anti-bark devices are aversive by design and are not recommended by most modern behaviourists.
Household noise matters — televisions, vacuum cleaners and some smoke alarm frequencies can be genuinely uncomfortable.
Deafness is common in older dogs and congenital in some, particularly associated with white coats and merle colouring. Signs include sleeping through arrivals and not responding from behind. Hand signals should be taught early, which makes later deafness far easier to manage.
Cats hear higher still, to around 64kHz, the widest range of any common domestic mammal.