They Have Ears But Hear Not
Published: 01st Jul 2005
People have ears. Fish don't but they can still hear. How? They have an organ called the lateral line that senses pressure changes in their environment.
The genes that spell out the lateral line in fish are closely related to the genes responsible for the ear and hearing in humans. Human hearing is highly developed and unusual among mammals: it is more important for communication between members of our species than for hunting prey or warning us of predators. The sense of hearing in humans is attuned to our own voices. We have in effect specialised in hearing ourselves talk.
The human voice range is from about 500 Hertz to 2000 Hz although children's voices are higher pitched. It comes as no surprise that we are able to hear best in this range although we have the ability to hear from a low of about 20 Hz up to 20,000 (2KHz). If you want an idea of what this means, a piano's A below middle C is 220 Hz, the A above C is 440 Hz, and continues to double each octave. The resonance of the musical chords are built into the mathematics of vibration.
Contrast human hearing to other mammals. Dogs range from 67 Hz to 44 KHz, cats from 55 Hz to 79 KHz and bats, the hunters of the night, have an astounding range of 600 Hz to 120 KHz. Bats have a special sensitivity at 60-62KHz, the frequency at which their vocal echoes produce excellent Doppler shifts for small objects like moths and mosquitoes. In essence, bats "see" with their ears, identifying location and motion with much greater accuracy than humans. In fact the typical insect-hunting bat's voice is so high pitched that we can only hear their occasional lowest squeaks. Listening to bats hunting at night with human ears is like listening to an opera where only the basso profundo sings and all others only mime. We are blind to their world. Yet we use the same neurobiology as these mammals.
There are three components of hearing. First there is the outer ear, an air filled system for gathering and amplifying sound waves. Second is the inner ear, a hydraulic system for converting sound into nerve impulses. Finally there is the brain, an organic computer for recognising and interpreting the nerve impulses. All three must function for hearing to take place.
The outer ear
The outer ear consists of the external ear or auricle, the external ear canal and middle ear. The auricle serves as a directional antenna, gathering and focusing sound from in front of us. Sound waves then pass into the external ear canal until they strike the external ear drum or tympanic membrane. This is a thin piece of skin, only three cells thick, stretched across the canal. In most people it is about the size of your little fingernail. The external ear canal is lined with hair to trap dust and prevent things from entering the canal. A cleaning mechanism of excreted wax, called cerumen, moves glacially out of the ear canal carrying debris with it. If the wax becomes too thick, it can plug the canal. Normally the wax is self cleaning. It should not be removed by swabs or solid objects. We have a saying that you should never stick anything in your ear except your elbow.
The ear drum divides the external ear from the middle ear, but both are air-filled cavities. The middle ear also contains three small bones: the malleus (hammer), the incus (anvil) and the stapes (stirrup). These three are called the ossicles and they connect the outer ear drum (tympanic membrane) to the inner ear drum (oval window). The footplate of the stirrup lies precisely on the oval window. Since the tympanic membrane is physically larger than the oval window, the middle ear ossicle chain serves as a 15:1 amplifier. Compare this to dogs, where the ratio is much higher, about 100:1, giving them much more sensitive hearing. Every mammal has these 3 inner ear ossicles from the minute shrew to the whale, which has ossicles the size of a human fist. The middle ear ossicles differentiate mammals from birds and reptiles, the latter having retained these same bones as part of their mobile jaws at the cost of hearing acuity.
Several factors affect the performance of the middle ear. Both chambers are air filled and must be at the same barometric pressure, otherwise the pressure difference will cause the ear drum to bulge in or out, becoming excessively taut and non-compliant. This prevents proper resonance. The air pressure is equalised by means of a valve at the back of the throat and a short connecting channel called the Eustachian tube. If this tube fails to open properly, such as when you have an ear or throat infection, sounds will be muffled.
To further control the compliance of the middle ear, two small muscles regulate tension. One is attached to the eardrum and adjusts the tension, the other is attached to the stirrup bone and can dampen the vibrations. These muscles respond reflexively to sound intensity. Consider when you step into a room where there is a radio blasting out a window-vibrating rock band. It is agonising, but in a few seconds, the two muscles reduce the amplification efficiency of the middle ear and the noise drops to merely painful levels. On the other hand, in an extraordinarily quiet situation, they will adjust the gain to maximum intensity. The nerves that control these muscles are responding to information from the brain as part of a feedback system. These nerves can be affected by trauma and infection, so damage to them can result in a permanent loss of function and inability to amplify sound efficiently.
The brain is the computer system receiving nerve impulses from the inner ear and interpreting their meaning. Sounds must be learned for them to have meaning. If a human does not hear in their first few years of life, the brain will never learn to recognise sound and interpret it properly. Some new implantable electronic ears can be placed in deaf children which allow them to "hear" in a limited sense, but it must be done at an early age when the brain can still learn to make effective use of information. Damage to the brain, such as from a stroke, can cause the person to be unable to remember sounds or words, despite the fact that they can hear and both the inner and outer ear is still working.
The inner ear
The inner ear is fluid filled and the interface with the middle ear takes place at the oval window. Vibrations from the stirrup bone send impulses across the oval window and they enter a long coiling tube called the cochlea, due to its snail shape. Since you cannot compress a liquid, there must be another outlet and there is. The fluid-filled tube curls around two and a half times, spiraling inward to the tip, then bends around in an 180o turn and reverses its track until it comes to the end of the cochlea, again at the middle ear. There, just next to the oval window, is another membrane called the round window. It does not touch anything, it merely allows the vibrations to move the inner ear fluid, called perilymph. Perilymph is pretty much water with small amounts of salt and glucose. The perilymph tube has doubled itself, which allows the vibrations to resonate and find their own amplitude doubling point along the course of the cochlea. Laid in the space between the two branches of perilymph are the sensors of hearing. These are called hair cells: nerves with small hair-like projections. The "hairs" vibrate when the perilymph vibrations are in resonance. So each section of the cochlea is tuned to a specific frequency. All the hair cells are the same, but they detect the vibration in their area of the cochlea. We have about 30,000 hair cells in each ear, a lot but finite. Other animals, such as dogs and bats have many times more. More sensors mean greater sensitivity.
The cochlea is a very sensitive organ. It must remain sealed and fluid filled at all times. Any damage from impact, infection or chemicals can severely and permanently destroy the hair cells. They are nerves and if injured, cannot regenerate. Many chemicals, such as the antibiotics streptomycin and gentamicin, metals like mercury and lead, some solvents like hexane and benzene, can lead to cochlear damage. Injury in the form of a skull fracture with the crack going through the bone and cochlea itself will irreversibly destroy the nerve.
The leading cause of cochlea damage is acoustic trauma. Sound energy is measured in terms of the unit decibel (dB) after the inventor Alexander Graham Bell. This is a logarithmic unit rather than an absolute unit due to the ability of human ears to scale tremendous ranges of sound intensity. One decibel is a pressure of 1 dyne X 10-7 /cm2 and considered to be the lower limit of human hearing. We can hear from 1 dB to about 140 dB at which point our hearing is overloaded. Typically sound intensity is measured in decibels and not pressure and a sound dosimeter is utilised.
There is a threshold for damage. Sustained sound levels of greater than 90 decibels have sufficient energy to fracture the hairs off the cells or kill hair cells outright. Sudden impact or explosion levels of over 120 dB can have the same effect. Broken hairs can drift around in the perilymph. If they drift onto other hair cells, they can continuously stimulate them, giving the person a sensation of a continuous ringing sound in the ear: tinnitus. It is important to protect the ears from these high sound levels because the damage, called noise-induced hearing loss, is cumulative over a lifetime.
Noise-induced hearing loss
There are over 30 million workers in the United States alone annually exposed to hazardous noise, defined as an average sound level (time weighted average) of 90 dB. The cost of noise-induced hearing loss (NIHL) is huge. The United States workers' compensation systems (we have over 50, one for every state, territory, plus several federal government, military, railroad, and postal worker organisation) have a number of schemes for calculating hearing loss. The most common one, called the AMA Guides, calculates hearing loss by means of decibel levels.
A person's hearing is tested at a series of pure tone frequencies: 500 Hertz, 1000 Hz and 2000 Hz. The minimum sound level for hearing is determined using the decibel. As already mentioned, decibels are a log rhythmic scale, so 20 dB is ten times the energy level of 10 dB. 20 dB plus 20 dB equals 24 dB. The lowest energy level is expressed as the threshold of hearing. A normal person should hear a tone at only 5 or 10 dB. A normal conversation takes place at about 60 dB. Hazardous noise is defined as a level greater than 90 dB. A tooth-rattling jet engine can put out 140 dB. Hearing loss costs amounted to $242,000,000 in US workers in 2002. Up to 70% of hazardous noise workers will have significant and compensable hearing loss by age 60 if not prevented.
Noise-Induced Hearing Loss is 100% preventable! Hearing loss is not normal with aging. Noise from any source, machines or loudspeakers, can damage the ear. Prevention is also cost-effective. The US Army began a NIHL prevention program of surveillance and passive hearing protection in 1974. By 1994, the program had saved over $504,000,000.
Who does this? Critical personnel are the Industrial Hygienist, who assays the work place environment and performs critical measurements, and the Audiologist, who tests, evaluates and educates the workers. Physicians may also evaluate workers if injury or other conditions have occurred. If you have a workplace where the sound levels are so intense, that at arm's length distance, you have to shout to be heard, you probably have a hazardous noise environment and need an industrial noise survey. If the survey confirms hazardous noise, surveillance of the workers is required.
Surveillance consists of monitoring hearing and tracking the individual over time. For workers exposed to hazardous noise, a loss of up to 25 dB is considered normal and acceptable. Losses above this level are measured and calculated, but only the 500, 1000 and 2000 Hz frequencies are measured. The formula for calculating loss is quite complicated but the results are millions of dollars of payouts to noise injured workers. Note that this does not exclude the worker who listens to heavy metal music and hunts skeet on the weekends. This damage can accumulate and be incorporated into the work-related loss. Most, but not all, compensation systems assume that there is an age-related hearing loss called presbycusis, and after age 40, the 25 dB limit creeps higher at 1 or 2 dB per year. However scientific data demonstrates that there is little if any natural hearing loss with age. Most presbycusis is really NISL or arthritis of the ossicles.
How do we prevent NISL?
The same way we prevent any occupational disease. We can remove the offending source, we can isolate it from the worker or we can protect the workers on an individual basis. It is hard to make quiet jet engines and jack hammers, so removing the noise source rarely works. We can insolate the noise sources with dampening materials. However our major effort is to provide each and every worker with education and hearing protection. In the US, the Occupational Safety and Health Administration (OSHA) also requires us to monitor all those who work in hazardous noise areas, defined as anyplace where the average sound levels exceed 90 dB over an 8 hour period.
Hearing protection works if you wear it. There are several kinds. Passive hearing protection consists of disposable ear plugs or reusable earmuffs and headsets. Both work well and will provide about 20 dB of sound attenuation. They can even be used in tandem, headsets over plugs. Remember, 20 + 20 = 24.
Passive hearing protection may be either small rolled cylinders of foam or the newer "mushroom on a stick" where a small plug of foam is on the end of a short piece of plastic, grasped for easy insertion and removal. I personally use these most of all. The older cylinders must be rolled and compressed into a smaller cylinder and then inserted into the external ear canal, then allowed to expand and seal. They are very good for comfortable all day noise exposure but have problems if you need to remove them frequently or cannot use clean fingers to roll up and insert.
Passive protection also consists of using earmuffs, large externally worn devices that are bulky and hot. The major advantages are ease of donning and doffing as well as supervisors knowing when the protection is being worn. Good seals around the ears are a must both for protection and comfort. Spectacles can interfere with the seal.
Hearing protection actually makes it easier to communicate in the workplace. With the background noises reduced, speech and warning signals are easier to recognise and understand.
Active Noise Reduction (ANR) hearing protection requires sophisticated noise-canceling systems. In effect, the system senses the sound waves and generates a "counter wave", a sound pulse of equal intensity and frequency, but exactly out of phase, resulting in destructive interference. Clearly this requires a lot of computing power. If the waves are precisely out of phase, they can reinforce rather than destroy each other. Most active noise reduction systems are well received and very comfortable to wear in contrast to many of the passive systems. Once a person tries an ANR system, it is almost impossible to get them to use anything else! The down side is that they are much more expensive. One unusual problem recently occurred in a private aircraft. A pilot wearing ANR had such good protection that he could not hear the weak warning buzzer for his airplane's landing gear! The result was most embarrassing.
Area active protection has even been created: SAAB has a system for the cabins of small passenger jets. This may be the future of technology. Now if they can just fit one for each taxicab's horn, we shall have a quiet future indeed.
For more information on hearing protection please visit http://www.osedirectory.com/product.php?type=health&product_id=6
Published: 01st Jul 2005 in Health and Safety International