Psychoacoustics

Psychoacoustics is the science of how people perceive sound: the link between the physical properties of a sound wave (its frequency, measured in hertz, and its amplitude, or size) and what a listener actually experiences (pitch, loudness, timbre, and where the sound seems to come from). A few core ideas shape the field. The audible range for healthy human hearing runs from roughly 20 Hz to 20,000 Hz, and we do not perceive it evenly: loudness is related to amplitude but not in a simple one-to-one way, which is why sound is measured on the decibel scale rather than in raw pressure. Frequency masking is the effect where a louder sound hides a quieter one that is close to it in pitch; this is the principle behind sound masking (covering up distracting noise) and behind lossy audio compression such as MP3, which saves space by discarding sound the ear would not have heard anyway. Sound localisation is how two ears, working together, let the brain place where a sound is coming from. See Wikipedia: Psychoacoustics for an overview, and the US National Institute on Deafness and Other Communication Disorders explainer NIDCD: How Do We Hear?, which describes how hair cells along the cochlea respond to different pitches and pass signals through the auditory nerve to the brain.

The journey from a sound wave to a perception happens in stages. Sound enters the ear and reaches the cochlea, the fluid-filled, snail-shaped structure of the inner ear, where it is sorted by frequency: tiny hair cells at one end respond to high pitches and those at the other end respond to low pitches, an orderly layout known as tonotopy (a frequency map). Those hair cells convert the movement into nerve signals that travel up to the brain. To work out direction, the brain compares the tiny differences in timing and loudness between the two ears, since a sound off to one side reaches the near ear a fraction sooner and a touch louder. Perception is non-linear throughout: equal physical steps do not produce equal perceived steps, which is why doubling a sound's intensity does not make it seem twice as loud. This descriptive science is also where "psychoacoustic therapy" sits as a marketing term: it means applying what we know about hearing to design audio (for example, sound masking to support sleep or focus, or comfortable, hearing-safe listening levels), not a proven medical treatment. See NCBI Bookshelf (StatPearls): Neuroanatomy, Auditory Pathway, which describes the tonotopic gradient that lets the brain map distinct sound frequencies. For where it connects to the wider library, see our guide to white noise (masking in practice) and the sound healing pillar.