Audible sound refers to the range of sound frequencies and intensities the average human ear can perceive. This allows us to interact with our environment, from understanding speech to enjoying music. Our auditory system translates physical vibrations into meaningful information.
The Physics of Sound
Sound originates from vibrations, which create disturbances that travel through a medium, such as air or water, as waves. These waves are essentially pressure variations, where regions of higher pressure (compressions) alternate with regions of lower pressure (rarefactions). The movement of molecules within the medium is necessary for sound waves to propagate, which explains why sound cannot travel through a vacuum.
Two primary characteristics define these sound waves: frequency and amplitude. Frequency, measured in Hertz (Hz), describes the number of wave cycles that pass a point per second, which our ears interpret as pitch. A higher frequency corresponds to a higher pitch, like a whistle, while a lower frequency results in a lower pitch, similar to a drum beat. Amplitude, on the other hand, measures the maximum displacement of the wave from its resting position, which we perceive as loudness or volume. A larger amplitude indicates a louder sound, measured in decibels (dB).
How We Hear
The process of human hearing begins when sound waves enter the outer ear, specifically the pinna or auricle. This funnel-shaped structure collects and amplifies sound waves, directing them into the ear canal. These sound waves then reach the eardrum, or tympanic membrane, causing it to vibrate.
The vibrations from the eardrum are then transferred to the middle ear, where they are amplified further by a chain of three tiny bones called ossicles: the malleus, incus, and stapes. The stapes, the innermost ossicle, pushes on the oval window, a membrane that separates the middle ear from the inner ear. This action sets up pressure waves in the fluid within the cochlea, a snail-shaped organ in the inner ear. Inside the cochlea, approximately 16,000 tiny hair cells (stereocilia) convert these fluid vibrations into electrical signals. These electrical impulses then travel along the auditory nerve to the brain, where they are processed and interpreted as the sounds we recognize.
The Human Hearing Range
The human hearing range extends from 20 Hertz (Hz) to 20,000 Hz. Sounds below 20 Hz are known as infrasound, while frequencies above 20,000 Hz are termed ultrasound; both are inaudible to humans. For instance, elephants use infrasound for long-distance communication, and bats utilize ultrasound for echolocation.
The intensity or loudness of sound is measured in decibels (dB). The human hearing range spans from 0 dB, the quietest perceptible sound, up to 130 dB, the threshold of pain. Normal conversation is around 60 dB, while a vacuum cleaner measures 70 dB. As individuals age, their hearing ability declines, a condition known as presbycusis, which affects the perception of higher frequencies. This natural process can reduce the upper limit of hearing to 14,000 Hz or lower.