Sound is a form of mechanical energy that travels through a medium, such as air or water, in waves of vibration. These vibrations create tiny fluctuations in pressure that our ears capture and translate into meaningful information. A sound wave has multiple physical properties, the most fundamental of which is frequency. Frequency is the primary determinant of the distinct qualities we experience when listening to the world.
The Physics of Sound Frequency
Frequency is the physical measurement of how quickly a sound wave oscillates or completes one full cycle of vibration. This characteristic is quantified using the scientific unit known as the Hertz (Hz). One Hertz represents one cycle of vibration occurring in a single second.
A sound wave with a high frequency completes many cycles per second, while a low-frequency wave completes fewer cycles in the same time frame. Frequency is distinct from the wave’s amplitude, which measures the intensity of the pressure fluctuation and is what we perceive as loudness. These properties are independent, meaning a sound can be high-frequency and quiet, or low-frequency and loud.
How Frequency Becomes Pitch
While frequency is a measurable, physical property of a sound wave, pitch is the subjective, psychological experience of that frequency by the human brain. The auditory system translates the objective rate of vibration into a perceived tone that we classify as either high or low. This conversion allows us to distinguish between the deep rumble of thunder and the sharp chirp of a bird.
A direct relationship exists between the two: a higher frequency corresponds to a higher perceived pitch, often called treble. Conversely, a lower frequency translates into a lower pitch, or bass. This distinction enables us to recognize different musical notes and differentiate between the unique timbres of human voices.
The Ear’s Frequency Processing Mechanism
The process of separating and analyzing incoming frequencies begins deep within the inner ear, specifically within the snail-shaped cochlea. Sound vibrations transferred from the middle ear create a traveling wave in the cochlear fluid. This wave causes a specialized structure called the basilar membrane to vibrate. The membrane is not uniform; its physical properties change along its length, allowing it to act like a natural frequency analyzer.
The part of the basilar membrane closest to the cochlea’s entrance, known as the base, is narrow and stiff, causing it to vibrate maximally in response to high-frequency sounds. As the membrane spirals toward the tip, or apex, it becomes wider and more flexible, responding best to low-frequency sounds. This systematic organization, where different frequencies are mapped to different physical locations, is known as tonotopy.
The mechanical movement of the basilar membrane stimulates thousands of tiny hair cells resting upon it, called the organ of Corti. Each hair cell location is tuned to a specific frequency, and when stimulated, it generates an electrical signal. These signals are carried along the auditory nerve to the brain, which interprets the location of the maximum vibration as a specific pitch.
The Limits of Human Hearing Range
The standard range of frequencies the healthy human ear can perceive spans from approximately 20 Hz up to 20,000 Hz (20 kHz). Sound waves below 20 Hz are called infrasound and are typically felt as vibrations rather than heard as a distinct pitch. Frequencies greater than 20,000 Hz are classified as ultrasound, which is also inaudible to humans.
The upper limit of 20 kHz is highly variable and tends to decrease significantly with age, a phenomenon known as presbycusis. For example, by middle age, the upper hearing limit may have dropped to 12,000 or 15,000 Hz. Prolonged exposure to loud noise can accelerate this high-frequency hearing loss by causing permanent damage to the hair cells in the cochlea’s high-frequency region.