by Martin Polon
Man has received many blessings from the technology that propelled the 19th century towards the 21st-a lengthened life span with added material pleasures are ones to which urban man has be come accustomed. Many of his electronic entertainments use audio--from graphic-equalized car stereos to multichannel sound systems for disco.
The good life now encompasses high level audio in such diverse entertainments as indoor and outdoor concerts, multimedia displays, discotheques, etc. Quality high-level sound entertains around the world. Home entertainment has evolved from the table radio of 40 years ago to today's high-power home stereo system.
The difficulty with all this remarkable technology is that it bears a hid den and complex set of negatives.
There is just no such thing as a free lunch. The human body is a complete biological system that reached its current state of refinement by a process that did not include audio at high levels. The human life support mechanisms developed to keep the wolf from the door--literally! Man's physical and mental responses to various external stimuli evolved from the real threats and dangers of the primordial past. Today's threats and dangers are much more subtle than a pack of hungry wolves, yet pose dangers far greater to human life. Many stimuli from the technologies and life style of modern man act as analogs of basic threats, regularly triggering body reactions that would normally occur infrequently.
Worse still, electronically amplified music can easily exceed high sound levels found in manufacturing plants, subways, and at airports.
Man is by conception, terminal. The factors for each individual's life span are determined to some large extent by personal habits, including ingestion (food, beverages, alcohol, drugs, and tobacco), exercise and exertion or the lack thereof, and exposure to various polluting influences in the environment. While certain pollutants may be deadly, they also are so obviously toxic that avoidance is reflexive. Not many of us would breathe pure automobile exhaust or handle a beaker of corrosive fuming nitric acid. Less obvious and far more dangerous are polluting substances or energies that are quite attractive. High-level sound is in this category. "One person's treat is another person's poison," or so the old saying goes. Energy pollution effects do vary in susceptibility from individual to individual. The dosage and duration of exposure determines the ultimate effect of any pollutant. But there is one sobering fact to remember: The human body is an unforgiving storage medium with a permanent memory.
Anything that is carcinogenic, mutagenic or toxic, be it a substance or an energy form, can change the physical condition of those exposed. The only question is how much damage is done, and when the consequences of the damage become irreversible.
After World War II, medical technology was able to measure and treat disease caused by pollution. At the same time, pollutants became so complex as to defy the prediction of long term medical consequences. One ex ample from the benzene family is the polychlorinated biphenyls (PCBs) used in the manufacture of utility power transformers. These chemicals, utilized for the past 30 years, have been identified with serious disorders in people who live in proximity to the use or discard of these materials. This is "invisible pollution," because it cannot be seen or sensed, except by the toxic effect on human beings over a long time span. Another example of near invisible pollution is cigarette smoking. The introduction of commercial cigarettes for the male population was followed by a period of relative calm before a storm of lung cancer erupted. Similarly, the wide acceptance of cigarettes by women in the 1940s was followed 20 years later by a dramatic upward swing in the lung cancer rates for women.
What has all this invisible pollution to do with audio? The answer to that depends on what audio is. The science of epidemiology is literally translated as "the study of epidemics," in other words, the study of negative results to large segments of the population from causative agents. The dictionary de scribes audio as "audible sound" and noise as "unwanted sound." Audio is sound, and sound when unwanted is noise. Sound can cause extensive problems in terms of hearing and whole body damage. Sound is energy, and the effects of energy on the body are defined by dosage and duration, regardless if the energy is from solar, nuclear or electroacoustic sources.
Music changes to noise in a medical sense when the level exceeds 90 dB "A" SPL. Sound energy above 90 dB triggers systemic stress which can lead to body damage. Direct damage to the inner ear can take place with exposure in excess of 95 dB and, in the United States, OSHA says that daily exposure must be less than four hours. In the United Kingdom, medical experts recommend that exposure to 128 dB not exceed 28 seconds. The International Organization for Standardization (ISO) calls for a maximum exposure of 10 minutes a week to a level of 114 dB or greater. Musical content has no value if the energy present is in excess of what the body can tolerate. If established safe levels of exposure restrict a sound level of 120 dB to 28 seconds during a day, the content of the sound is not significant. The state of the art in audio has made levels of 90 to 120 dB very common in performance, recording, and playback. Higher levels are frequently encountered. Paradoxically, the medical arguments on the thresh old of hearing damage do not center on 90 dB. There is a consensus of opinion on damage at 90 dB. The current recommendations in the medical literature range between 65 and 85 dB for susceptible populations.
The environment that man inhabits has never been totally silent, but the gradual rise in technology of the 20th century has markedly increased the daily noise dose. The advancements in recording and reproduction of sound give audio systems underestimated potential for generating high levels of sound in the working environment.
This "audio inflation" can also be heard in the home. Systems are equipped with efficient loudspeakers using large, heat-resistant voice-coils capable of handling 50 to 250 watts of power per channel. How many would expect that car stereos could produce sound pressure levels easily exceeding 90 dB? The car is a small, highly reflective area in which to deliver 10 to 100 watts rms per channel to multiple speakers. The headphone is one of the most "invisible" elements of sound pollution because it couples directly to the ear and can generate considerable energy. Concert entertainments regularly use large sound systems driven by 5,000 to 25,000 watts of power amplification, capable of constant levels in excess of 110 dB at several meters.
Level and exposure are the keys to susceptibility in sound-induced dam age. The body has evolved through time, and many aspects of hearing are functions of survival mechanisms. Our high-technology life style may have rendered these mechanisms obsolete.
The apparatus of human hearing features acute midrange sensitivity and selective accuracy. Through man's distant past, it was necessary to hear the approach of, say, a pack of wolves. To survive was to hear distant sound, al lowing for escape or defensive action.
The perception of threatening sounds would begin a series of physiological responses, involuntary functions which prepared man for reaction to the perceived threat. This complex defense mechanism, triggered by sound, allows man to have maximum levels of output from various body functions; when man is in the stressed mode, physical strength is enhanced and en durance for exertion is maximized.
This defense mechanism functions as well today as it did 50,000 years ago. A noise outside our living space at night will produce the same response as that of our ancestors, alerting for fight or flight. None of these physiological mechanisms evolved to trigger regularly as part of a life style. The changes in the body are to defend from infrequent threat, rather than frequent exposure to sound levels which mimic threats.
TYPICAL A-WEIGHTED SOUND LEVELS AT A GIVEN DISTANCE FROM ENVIRONMENTAL NOISE SOURCE DECIBELS
Living man is a complex bio-system which can defy precise explanation. However, medical information on the negative effects of high-level sound can be obtained in a number of ways.
Rats, gerbils, chinchillas, and certain monkeys are quite similar to man. De tailed examination after exposure has yielded significant confirmations of sound-related damage. Further information has come from human autopsy analysis and the medical histories of large populations known to have been exposed to high-level sound.
High-level sound exposure can produce negative effects in two major areas, auditory and non-auditory. Auditory damage from sound has been well documented, but there are some relationships between auditory and non-auditory damage that should be explored. Briefly stated, continued expo sure to high-level sound of variable threshold and duration will produce significant and permanent damage to the hearing mechanism in the inner ear. The damage is variable since each human being has a unique vulnerability to auditory and non-auditory damage, based on a variety of individual characteristics.
In the human hearing apparatus, the outer and middle ear exist principally to collect and transmit sound to the inner ear for sensory reception.
In the inner ear, the sensory mechanism consists of the cochlea, which is a coiled-up canal. It contains the basilar membrane which supports four rows of hair cells. These hair cells will bend with incoming sound impulses, sending bioelectrical signals towards the brain (via nerve fibers which are grouped to form the spiral ganglion and become the auditory branch of the cranial nerve).
Sound injury to the inner ear is based on the following mechanisms of damage:
1. Exposure to high-intensity noise produces a physical detachment of a portion of the hair cells from the basilar membrane. This injury is a consequence of mechanical stress which can develop during high level exposure.
2. The outer hair cells are damaged initially by sound exposure, while the inner hair cells die from a reduction in blood flow. Subsequent exposure in creases the extent of the damage.
3. The membrane structure changes, causing further damage during high level exposure. Nerve cells are dam aged as well as the hair cells.
4. The destruction of a number of cells at the same time during an expo sure episode can leak a minute amount of ear fluid. Possessing excessive potassium ions, this fluid causes swelling and rupturing of the uninjured sensory cells and nerve fibers which may continue long after expo sure ceases.
There are definite, well-documented patterns of exposure and measured damage in auditory injury. The use of audiological tests is empirical and they produce irrefutable information. Formulas exist which predict the physiological deteriorations. Approximately 96 percent of the working population can be protected from sound damage by limiting exposure to sounds at 90 dB and above. What of those who are not limited by the nature of their work or by recreational choice or both? There is a predictable pattern of dam age in the frequency range of 1,000 to 6,000 Hz, with peak damage at 4,000 Hz. The degree of loss can increase with subsequent exposure and can prove very severe in some individuals, assuming the natural loss with aging (presbycusis). The relationship to non-auditory damage is that the individual afflicted with a notching of hearing sensitivity will attempt to compensate by increasing sound levels to achieve perception. This will cause further hearing damage, especially at levels in excess of 100 dB, and accelerate the non-auditory damage.
At the cardiovascular system level, the effects of high-level sound are most prominent. At the onset of a high-level episode, the body goes from an alert state, known as the orienting response, to a sustained re action called the defense response.
Characteristically, these two responses cause an involuntary change in the body that begins with a tensing of the muscles (by the main motor nerves).
This is followed by the nervous system causing changes in heart activity, breathing, blood pressure, etc. Diseases of the heart and associated blockages of the veins and arteries can be complicated by the involvement of high-level sound, contrary to the optimum functioning of the cardiovascular system. A most apparent phenomenon is the change in size of the blood vessels (called vasoconstriction), which occurs at the onset of high-level sound. It may begin to disappear in some individuals when the sound ceases; in others it may persist for up to 25 minutes afterwards. Other heart-related changes include blood pres sure (especially diastolic), heart rate, cardiac output, and pulse volume. All of these components are interrelated, and the changes may create further effects. For example, sound does not seriously affect long-term blood pressure of a normal population group, but for those suffering from high blood pres sure (hypertension), the effect of high level sound is to cause a small but definite further increase in pressure. High sound levels also can increase the agents of blockage in arteries, that is elevated blood levels of triglycerides and cholesterol.
One interesting auditory and non-auditory correlation involves peripheral vasoconstriction. This changing in size of the blood vessels tends to re strict the supply of blood to the head.
The effect on the brain must be gauged in conjunction with the psychological and hormonal changes which also occur during high-level exposure. The ear, the very sensory organ which receives all of the high-level sound, is very much affected. The restriction of supply causes the ear to suffer cochlear anemia (ischemia), and this lack of blood flow occurs at the very time when the ear is under attack.
The sensory cells need an increased supply of blood to provide energy sources and remove metabolic products resulting from the high sound level and related auditory metabolic activity. The ear does not get the needed blood in sufficient volume to save all of the threatened cells.
Sound also acts on the digestive sys tem as a stress agent. A number of changes take place in the gastrointestinal system such as increased movement (motility), with waves of con tractions passing along the intestines causing the contents to increase speed. The result is diarrhea and/or frequent episodes of irregularity. Abnormal contractions of the stomach can take place along with the more usual outpouring of hydrochloric acid into the stomach; the two result in an unstable condition which can lead to peptic ulceration in susceptible individuals. There is an interesting note here. Those individuals who are able to control the level and duration of the sound source suffer greater motility problems than those who have no control on the sound at all. This is a complex reaction to sound and might be explained in terms of interaction between the brain and the psychological impact through the central nervous system.
The respiratory system of the body is a vital part of the metabolic process, and it is assumed that high-level sound has very little effect on the respiratory function. The only involvement is the inducement of slow, deep breathing, producing optimal ventilation of the lungs and high oxygenation of the blood. One fact of concern, however, is that sound can and does kill many laboratory animals under certain conditions. This is known as audiogenic seizure, and the cause of death is most usually respiratory failure (brought on by contractions of the respiratory muscles). Fortunately, man does not appear to be involved with this phenomenon at sound levels be low 160 dB SPL"A." The central nervous system is intimately involved with high-level sound inputs. We see the specific neurological involvements as the non-auditory response of the body. The central nervous system controls the voluntary nervous system, the autonomic nervous system, and the endocrine system through the hypothalamus in the brain. All of these are involved in a chain reaction. The management aspect of the central nervous system follows through the various systems. The second involvement of the central nervous system is with the psychological impact of high-level sound. Of greatest interest are the arousal and distraction effects. The effect of arousal is difficult to identify here since it is related to surprise, and the presence of high level sound in a listening environment is an expected quantity. The distraction effect is pronounced, and it manifests itself as follows in terms of human performance:
1. Sound can increase or decrease efficiency, but the effect will occur later rather than sooner.
2. Negative effect is normally associated with complex tasks that have multiple components or high information loads.
3. Tasks involving cognitive skills, such as memory, can be interfered with at levels as low as 70 or 80 dB.
4. Conversation may be rendered in comprehensible via distractive processes at levels far below those which physically mask conversation.
In other words, the ability to perform certain tasks is alterable by high-level sound.
Other psychopathological changes come with sound exposure. Studies of individuals working within a high sound level environment indicate a trend towards depression and neurotic behavior among susceptible subjects, especially single women. The interrelations of body functions come into play, and the increased output of various hormones may well account for this impact, especially given the hormonal change found during menstruation.
Both the female reproductive sys tem and the immunologic system can be significantly influenced by the hormonal output triggered by sound.
Immunological agents such as the eosinophils (white blood cells involved in allergic reactions) and gamma globulin (plasma protein which protects against certain diseases), among others, appear to be depressed in availability during the periods of high sound levels. The presence of certain hormones in the body of the human female during pregnancy can affect the rate and form of fetal development and the effect of sound produces stimulation of the breasts and uterus, yielding additional hormone activity. The fetus of any mammal is very susceptible to any kind of change from a stabilized condition, so the presence of sound-induced hormones cannot be viewed positively in any context. There is even some research to suggest that the fetus itself will undergo physiological changes as growth brings it to the point where it can physically sense either the sound or the mother's reaction to the sound.
The major source of chemicals in the brain is from the hormonal output triggered by the initial response to sound (in the hypothalamus to the pituitary gland), and then to other hormone-involved glands. This stimulated production of hormones (such as primary steroids like 17-hydroxycorticosterone), involves virtually all body functions not previously discussed.
During a high-level sound episode, for example, male sexual drive seems to be increased while sexual potency is decreased because the gonads are signaled with a hormone. The basis of much of the tensing during the orienting and defense reactions involves the release of adrenaline which causes the increase in neuromuscular tension, nervousness, irritability, and anxiety.
Other hormones affect the function of the kidneys and the amounts of substances in the blood. The trigger hormones, the corticosteroids, can retain their high bloodstream level over a long time frame after episodes of high sound levels have concluded.
It is important to note that all of these radical hormonal changes occur when sound levels above body tolerance are perceived. The effects tend to be extended due to the time required for the various chemicals to be dissipated in the body.
The effect of high-level sound on the mental state is documented in terms of the presence in the brain of chemicals also found in those with schizophrenia and psychosis. High level sound stimulates these chemicals, such as norepinephrine, in susceptible individuals. There is even some suggestion of specific involvement between high-level sound and a rare form of epileptic seizure.
The special senses of vision and balance are influenced by the presence of high sound levels, but with out any apparent health negatives past temporary dysfunctions. One change is a tendency in susceptible individuals for dizziness and vertigo via acoustic stimulation of the vestibular labyrinth or canal link in the ears.
Sound also can cause susceptible individuals several alterations from normal vision. These include a narrowing of the angle of the visual field, impaired acuity and color vision, and interference with the critical flicker-fusion frequency. The brain's ability to use relevant visual information can also be disturbed. Perhaps the most interesting fact associated with high sound levels and vision is that the pupil of the eye dilates with high sound levels (dilation increases with the increase of the stimulus).
All of the changes that the body undergoes during episodes of high-level sound exposure suggest that the total effect is less than positive. In fact, it would seem that for a certain percentage of the population, such exposure to levels in excess of 90 dB could pose a substantial hazard to long-term health. The real focal point of the problem with high-level sound is the invisibility of the effects. There are very few members of a susceptible population group who are going to realize that damage is taking place, and that in various ways, the ability to perform all life-related tasks is deteriorating. The bottom line in exposure to high levels is the total lack of identification of those most susceptible.
Certainly, the amount of exposure time tends to shift the possibility of severe health and hearing damage to those audio professionals who are involved on a daily basis or those enthusiasts who use their systems daily and have the very large sound systems necessary to reach levels in excess of 110 dB. (There may be benefit to the urban dilemma in housing, in that the presence of neighbors tends to limit the levels.) Alternatives are many, but few will work practically because those who al ready have impaired hearing cannot tolerate attenuation without seriously compromising their perceptions. For undamaged individuals, two solutions seem reasonable. Those using audio at levels in excess of 90 or 100 dB can use some kind of hearing protectors. There is some frequency response alteration, but it may be acceptable as an alternative to health damage. Secondly, the use of small, inexpensive sound level meters can provide a constant means for keeping levels at or below 90 dB.
Doing nothing is unacceptable be cause it ends with negative changes in the body. No one can know how susceptible they might really be to the physiological impacts of high-level sound, and the question demands thoughtful answers. Lastly, it is foolish to assume that the well-established (and restrictive) governmental safety standards for sound will never reach the audio profession. The recording and reproduction of audio will suffer from start to finish if solutions are imposed as regulations.
(Adapted from: Audio magazine, Jan. 1981)
Also see:
Build a Headphone Crossfeed Circuit (May 1980)
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