How do we hear sounds?

Sound is a mechanical wave in air. A sound wave is caused by a movement that imparts pressure to air molecules, causing them to form a wave in frequencies that our ears can detect, or should I say that our aural nerves can detect.

A sound wave has frequency, amplitude, and direction. Its speed of propagation is known as the speed of sound, which is 768 miles per hour in dry air. Sound can also propagate through liquids and solids at different speeds. We’ll stick to sound in air, because that’s how we hear sounds for the most part.

The speed of sound in air is dependent on the temperature and humidity. The lower the temperature, the lower the speed of sound, and this means that as one goes higher into the air, the speed of sounds decreases until one gets up to 11 kilometers where it increases in speed.

The way we hear sound is because of a marvelous mechanism in our ears. The outer ear is just a device to concentrate the sound wave and funnel it into our ear canal--that’s why it looks the way it does. The middle ear contains the tympanic membrane, better known as the eardrum. The sound wave causes this membrane to vibrate. Attached to the tympanic membrane is the malleus (hammer) bone, which is attached to the incus or anvil bonr, which is in turn attached to the stapes or stirrup bone. These three little bones function together to transmit the vibration of the eardrum to a little oval membrane window on the cochlea, which is filled with liquid. The oval window is one-eighteenth the size of the eardrum and thus exerts more force because of mechanical advantage, which amplifies the sound wave. The cochlea is where the sound wave, now a fluid wave, is transmitted to nerves that lead to the brain. The cochlea is a spiraled, hollow, conical bone chamber. There are several membranes inside it that help detect the sound wave and transmit it to the brain.

The inner ear also contains the semicircular ducts, part of the vestibular system, which allows us to keep our balance, but this has nothing to do with hearing.

The cochlea contains tiny hairs that are the nerve ends in the Organ of Corti, a structure that detect the sound wave and sends it through the vestibulocochlear nerve to the brain. The Organ of Corti is where hearing loss occurs. It seems that loud sounds damage the little hair cells, and if enough of them are damaged, hearing loss follows. Listen up you rock addicts! Actually, I attended rock concerts in the 70’s and I believe that I’ve suffered this kind of damage. Bummer!

Hearing is the result of us having a wonderfully complicated organ know as the ear. With this amazing organ, we can hear in stereo and delineate vocal sounds from background noise. Hearing range of frequencies goes from 20 Hz to 20,000 Hz. Sound happens in the brain and is a perception sort of like vision. In a sense we see by means of sound, and people who are blind use this sense of sound position to get around.

Sound, especially background noise, is measured in decibels or dB, a logarithmic scale used to measure sound. A whisper is around 30 dB while a jackhammer is at 110 dB. Any prolonged exposure to noise above 90 dB will cause permanent hearing loss. Many MP3 players or smart phones connected to earphones can produce noise loud enough to cause hearing damage. So turn down your music, please!

Thanks for reading.