The Nervous System, Ear, Hearing and Balance — Page 67, Lesson 97

I want to walk you through the inner ear and how we actually hear sound, then move into the audible range of human hearing, how we measure sound, and finally the types of hearing impairment you need to know about as a professional pilot. Let's start with the inner ear. The diaphragm attached to the stapes — remember, the stapes is the last of the three tiny bones in the middle ear — causes the fluid inside the cochlea to vibrate. The cochlea is a spiral-shaped, fluid-filled structure deep in the inner ear. Inside the cochlea there is a fine membrane covered with tiny hair-like cells. These are sensory cells. Their movement depends on two characteristics of the original sound: volume and pitch. Volume is the intensity or loudness; pitch is the frequency, how high or low the sound is. The amount of displacement of these hair cells tells the brain about volume, and the frequency of displacement tells it about pitch. The auditory nerve detects this amount and frequency of displacement. That nerve leads directly to the cortex of the brain, where the tiny electrical currents generated by the hair cells are decoded into what we perceive as sound patterns. Now, here's an important point for aviation: hearing can also bypass the eardrum and the ossicular system — that's the chain of three bones: malleus, incus, and stapes. Sound can transmit through bone instead. This is because the cochlea is embedded in a bony cavity within the temporal bone of the skull. Vibrations of the entire skull can cause fluid vibrations directly inside the cochlea itself. So under certain conditions, if you place a tuning fork or a vibration device on the skull, the person will hear the note or sound even if their middle ear is damaged. That's called bone conduction. Let's move to the audible range of the human ear and how we measure sound. The frequency of sounds that a young person can hear is generally stated to be between 20 and 20,000 Hertz. Hertz is the unit of frequency, meaning cycles per second. However, the sound range depends to a great extent on intensity, which is measured in decibels. The human voice uses the frequency range of 500 Hz to 3,000 Hz — that's the range most important for speech communication, which matters for radio work. Sound intensities are expressed in terms of the logarithm of the actual intensities. That means the decibel scale is logarithmic, not linear — a small change in decibels represents a large change in actual sound energy. Let me walk you through the table of typical noise levels. The threshold of hearing is 0 dB — the quietest sound a healthy young ear can detect. A rustle of leaves in a gentle breeze is about 10 dB. An average whisper at 4 feet is about 20 dB. Quiet conversation is around 30 dB. Office noise is about 40 dB. Conversation in a noisy factory is about 50 dB. Loud street noises like trucks are about 60 dB. Standing close to heavy machinery is about 80 dB. Now, here are the critical limits for you as a pilot: the maximum recommended for 8 hours of exposure is 87 dB. The maximum recommended for 2 hours of exposure is 100 dB. The maximum recommended for 30 minutes of exposure is 110 dB. Standing near a piston engine aircraft — where noise becomes uncomfortable — is about 120 dB. Standing near a jet aircraft is about 140 dB, which is the threshold of pain. These limits are why we wear hearing protection on the ramp and why cockpit noise exposure is a real concern. Finally, let's cover hearing impairment. Hearing difficulties are broadly classified into three categories. The first is conductive deafness. This is any damage to the conducting system — that means the ossicles, the three small bones, or the eardrum. Damage here results in a degradation of hearing. Perforations of the eardrum can result in scarring of the tissue, which reduces its ability to vibrate freely. A blow to the ear may cause damage to the small bones in the middle ear, again limiting the transfer of vibrations. Modern surgery may help in some circumstances. Also, excessive wax or a tumour in the ear canal can cause conductive deafness by physically blocking sound from reaching the eardrum. That covers the inner ear mechanism, the audible range and decibel limits you need to know, and the first category of hearing impairment.