Our ears provide us with two vital but very different senses: hearing and balance. Sound detected by the ears provides essential information about our external surroundings and allows us to communicate in highly sophisticated ways, such as through speech and music. In addition, our ears contribute to our sense of balance, the largely unconscious understanding of the body’s orientation in space that allows us to maintain an upright posture and move without falling over.
In this section
The ear contains separate organs of hearing and balance, which detect sound from the world around us and internal information about our posture and movement. Sensory structures inside our ears convert the different forms of information into nerve impulses, which travel along nerves to various parts of the brain where the information is analysed. Our ability to interpret sounds and use information about balance develops during infancy and childhood.
The qualities of sound
Sound is actually a vibration of the molecules in the air all around us. The pitch of a noise (how “high” or “low” it sounds) is determined by a property of sound waves called the frequency. Frequency is the number of vibrations per second and is measured in units called hertz (Hz). The higher the frequency, the higher the pitch.
The intensity or loudness of a sound depends on the power of the sound waves and is measured in units called decibels (dB). For every 10 dB increase in power, our ears hear double the loudness, so noises at 90 dB sound twice as loud as those at only 80 dB. Conversation is typically about 60 dB, and sound from nearby traffic is usually about 80 dB. Even brief exposure to noises over 120 dB can damage our hearing.
We all vary in the acuteness of our hearing (how loud a sound has to be for us to hear it) and in our ability to analyse complex sounds such as music. Young people can normally detect sounds with frequencies between about 20 Hz and 20,000 Hz, but our ability to hear high-frequency sounds tends to decline with age. Animals such as bats and dogs can hear sounds with frequencies much higher than the normal human range.
Balance and movement
The structures of the inner ear that are concerned with balance have two functions: awareness of the head’s orientation (where it is in space) and detection of the head’s rotation and movement in all directions. The brain combines information from the ears with that from position sensors in the muscles, tendons, and joints and visual information from the eyes. Taken together, this information enables us to move in many different ways without losing our balance.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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