How Do We Hear?

When we detect sounds our ear is changing the energy in sound waves into nerve impulses that our brain can interpret. When we hear a sound, waves enter the ear canal of the outer ear and contact the eardrum (shown in green). The human eardrum is a stretched membrane, like the skin of a drum, and when the sound waves hit your eardrum, it vibrates. These vibrations pass through 3 bones in the middle ear called the malleus, the incus and the stapes. As the sound wave is propagated through the bones (shown in blue). The stapes passes those vibrations along a coiled tube in the inner ear called the cochlea (shown in purple). In the cochlea, this vibration sets fluid in motion that bends thousands of delicate hair-like cells in the inner ear that convert the vibrations into nerve impulses. These impulses are carried to the brain by the auditory nerve (shown in yellow on the right) and these impulses are converted into what we "hear" as sound.

As we have learned, sound waves are produced when air is compressed and a wave is generated as a result. We measure sound in several ways. As we already know, one aspect of sound is frequency. The frequency of a sound determines its pitch. We perceive pitch as a high sound or a low sound and this is what makes a sound pleasing or unpleasing to the ear. When a singer is performing a song, they have to be able to control the pitch in order to carry certain notes. If you have ever heard a person sing who cannot carry a note, you know how unpleasing those sounds can be.

High frequency sounds are more damaging to the hearing process than low frequency sounds because they carry more energy.

If you have ever watched an opera singer hit a high note that can break a drinking glass, you can see how much energy a sound wave can carry. When a high frequency sound enters the ear, the sound wave can damage the inner ear and produce hearing loss. Many performers, especially those that play high frequency loud music, wear ear protection for this reason. Sometimes people who are exposed to loud noises suffer from a condition known as tinnitus, a ringing in the ear. People who work around loud machinery, loud music, firearms and other noise producing devices have a constant ringing in the ear that can interrupt sleep and prevent them from hearing certain sounds. This can make life uncomfortable, which is why it is always important to wear ear protection.

Unlike pitch, loudness is based on intensity, which is the amount of sound produced. This property of sound is measured in decibels (dB), where loud sounds register higher on the decibel scale and softer sounds register lower on the decibel scale. If we use our voice as an example, the more air you expel from the lungs, the louder your voice. We can also use machines to amplify sounds electronically. These amplifiers are commonly used by musicians to make an instrument or a person's voice louder for an audience. Just the opposite may also be desirable, where sounds need to be muffled so that it does not disturb other people in certain places. In order to muffle a sound, people have designed sound proof rooms and sound proof materials. These are designed by trapping gases within solid materials so that the gases slow down sound travel. In doing so, the sounds are minimized allowing a person to remain undisturbed when a loud noise is nearby.

The science of sound is called acoustics, and scientists that employ these techniques work with amphitheaters and movie companies to control sound. These scientists work with various shapes and materials that can amplify or reduce sound. This can also be employed in devices that use sound to warn people or animals of danger. Sirens, warning signals and even high pitched whistles used to alert deer and other animals are amplified by the shape of the instrument. These shapes concentrate sounds so that they are easier to hear by the target. This makes these sounds easier to detect so that immediate action can be taken.

The perception of sound in any organism is limited to a certain range of frequencies. Humans can typically hear frequencies between about 20 Hz and 20,000 Hz. The upper frequency limit usually decreases with age due to damage that occurs over a lifetime. Other organisms have a different frequency range that they can detect based on their habitat and hearing needs. Your dog, for example, can hear higher frequencies than humans, but cannot hear frequencies below 40 Hz. This is why a dog can hear a high pitched dog whistle and we cannot.

Many animals produce sounds through special organs that they use to communicate, warn of danger and attract mates. Some animals that you are familiar with that use sound for communication are frogs, birds, dolphins and whales. In fact, dolphins have very intricate communication sounds and the best sense of hearing amongst all animals. They are capable of hearing 14 times better than humans and can communicate over much longer distances. Many animals can communicate using various sounds, but they do not have a distinct language. Humans are the only animals to have complex language skills that separate us from other animal species.

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