AR-461: BUILDING SCIENCE
Department of Architecture and Planning
NED University of Engineering and Technology
LECTURE NO. 15
Human Ear and Hearing
Sound waves approaching the ear enter either directly or are reflected by the pinnae down the meatus and are conducted to the cochlea by the three auditory ossicles (i.e. the malleus, the incus and the stapes). The ossicular chain produces a pressure amplification of about 20:1. Their vibrations are conducted up the cochlea by the basilar fluid which excites about 30,000 small hair cells on the surface. It is from the motion of these hair cells that the brain interprets sound.
CROSS SECTION SHOWING COMPONENTS OF HUMAN EAR
How it Works:
Sound waves travel through the meatus (auditory canal) to the tympanic membrane (ear drum). The auditory canal can resonate and amplify sounds within a frequency range of about 2000 Hz to 5500 Hz by up to a factor of 10.
Successive compressions and rarefactions of air reaching the eardrum result in a change in pressure between the outer ear and the middle ear. The Eustachian tube helps to keep the middle ear at atmospheric pressure.
The difference in pressure between the sound wave striking the outer surface of the eardrum and normal atmospheric pressure on the inside of the eardrum causes the eardrum to vibrate.
Within the middle ear, vibrations travel through three small bones (the hammer, anvil, and stirrup) to the cochlea. The bones act as interlocking levers which amplify the force of the eardrum striking the hammer. The oval window of the cochlea is smaller than the eardrum. This causes a further amplification of the sound vibration, upt 20 times at some frequencies.
The semicircular canals act as miniature accelerometers. They also help to maintain a sense of balance by responding to gravity and changes in acceleration. The hair-like structures (dendrites) in the cochlea resonate at various different frequencies. The vibrations stimulate neurons to produce electrical impulses which are sent along the auditory nerve to the brain for processing.
The small pressure fluctuations we perceive as sound waves are imposed on a relatively stable atmospheric pressure. Given the way the ear is constructed, it is not sensitive to this constant pressure only the smaller fluctuations.
The brain is able to detect the relative direction of a sound using the following mechanisms;
Depends entirely upon time delays between similar excitement levels in each ear; the distance between each ear can be taken to be about 150mm. This means that there exists a vertical plane, running through the centre of the head, within which sound reach each ear simultaneously.
The Effects of the Pinnae
These are designed to collect frontal sound and reflect it down the aural canal. Sound entering from above and behind must have been diffracted by the pinea and, as a result, slight spectral changes to the sound will have occurred (more on diffraction later).
Subtle Head Movements
These assist in determining the height of a sound source in the medial plane (as described above).
The brain is also able to perceive the relative distance of a sound source as follows;
- Loss of intensity due to inverse square law and molecular absorption.
- Changes to the spectral content resulting from molecular absorption and diffraction around objects.
- The level of direct vs. indirect sound - the age old trick of increasing reverberation as a song finishes in order to give the impression it is fading away into the distance, sounds like its in a large cave.
The ear can hear sounds ranging from 20Hz to 20 kHz. It is most sensitive to frequencies between 500Hz and 4000Hz, which corresponds almost exactly to the speech band. Note that this threshold increases significantly with lower frequencies, an important point that will be the subject of more detailed discussion later in the course.
Just how can we measure a sound - what measurable characteristics do sounds have? The answer is power, pressure and intensity.
Measures energy output by a source, that sound's ability to do work
Measures fluctuations about the local atmospheric pressure Use of root-mean-square (rms) rather that peak-to-peak measures.
The amount of sound energy within a specific area normal to the direction of propagation
Through extensive empirical testing it has been clearly shown that the ear's response to a sound is proportionate, not to the absolute value of a stimulus, but to the ratio of the actual intensity of the sound to the threshold intensity. Further to this Fechner's law states that the relationship is a logarithmic one.
Response = Measured Intensity / Threshold Intensity
Sound level measurements are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:
Iref = 10E-12 W/m²
Or in terms of sound pressure:
Pref = 2E-5 Pa
Whilst these are the minimum thresholds, the table below details the range of audible sound. The upper limit actually represents the threshold of pain, where the sound it so loud it actually hurts the ear and may cause physical damage.
AUDIBLE RANGE OF THE HUMAN EAR:
Frequency: 20 Hz - 20,000 Hz
Intensity: 10E-12 to 10 W/m²
Pressure: 2E-5 to 200 Pa
As you can see from the table, human pressure perception ranges from 20 Micro Pascal up to 200 Pascal. This represents a considerable linear dynamic range (i.e.: 10E7). Because of this, and the way the ear works, it is convenient to firstly work with relative measurement scales rather than with absolute measurement scales, and secondly to logarithmically compress them.
The units used to measure this ratio are called bels. Two variables differ by one bel if one is ten (1E1) times greater than the other and by two bels if one is one hundred (1E2) times greater than the other. The bel is still a very large unit and it is more convenient to divide it into 10 parts - hence the decibel.
The standard threshold values given above correspond to exactly 0 decibels. The actual average threshold of hearing at 1000 Hz is more like about 4 decibels, but zero decibels are a convenient reference.
LOUDNESS AND SOUND LEVELS
As opposed to a measurable characteristic, loudness is a subjective term describing the strength of the ear's perception of a sound. It is intimately related to sound intensity but can by no means be considered identical to intensity. The sound intensity must be factored by the ear's sensitivity to the particular frequencies contained in the sound. This is the kind of information contained in equal loudness curves for the human ear.
It must also be considered that the ear's response to increasing sound intensity is a "power of ten" or logarithmic relationship. This is one of the motivations for using the decibel scale to measure sound intensity. A general 'rule of thumb' for loudness is that the power must be increased by about a factor of ten to sound twice as loud.
Thus, a direct physical measurement of intensity, or even sound pressure, is almost meaningless in sensory terms unless it is referenced back to a threshold value. As the decibel is a relative measure and is used to quantify both, pressure and intensity levels, each measurement can be compared to a universally accepted standard threshold value to derive a Sound Level in decibels (dB). Sound levels are directly meaningful in sensory terms.
Sound Power refers to the absolute power of a sound source (in Watts) whereas Sound Power Level (SWL) refers to the magnitude of that power relative to a reference power (in dB). Thus;
SWL = 10 log (W / Wref) Where Wref = 1E-12 W/m²
Similarly, Sound Intensity refers to the absolute intensity (in Wm-2) whereas Sound Intensity Level (SIL) refers to the magnitude of the sound intensity relative to the reference intensity. Thus;
SIL = 10log (I/Iref) Where Iref = 1E-12W/ m²
For a plane wave, the intensity (I) of a sound field is proportional to the mean-square of the pressure fluctuation (p2) - the actual relationship is given by: I = p2/ (poc). Therefore, as spherical waves approximate plane waves in the far field, the Sound Pressure Level (SPL) becomes;
SPL = 10log (P²/P²ref) or SPL = 20log (P/Pref) Where Pref = 2E-5 Pa
Since pressure is far easier to physically measure than intensity, it is often useful to express SIL in terms of SPL. Since I = p2/ (po c), by taking logs and substituting reference values;
SIL = SPL + 10 log [(2E-5)²/(roc) * 1E-12
Obviously, given the density component (roc), the last term is pressure and temperature dependant. At 20'C and 1 atm it calculates out to around 0.1dB (Given that roc = 410 rayls). Thus:
SIL ~ SPL
The concept of acoustic power is not readily considered by most people, due in part to a preoccupation with sound pressure. To put it simply, sound pressure is the result of sound power. Sound power is the cause, sound pressure is the effect. The goal in loudspeaker design is to maximize the amount of acoustic power available from the device, given the constraints and trade-offs between the available parameters. Once the acoustic power has been realised, we then turn our focus to pattern control devices (horns) that will channel this power to a smaller unit area, increasing the sound intensity for that area, increasing the sound pressure (at a given listening position) beyond what it would have been if no pattern control was used.
In many conventional loudspeakers, it can take as much as 100 electrical watts to produce one acoustical watt, yielding an efficiency of about 1%. At the other extreme, a perfectly efficient transducer (that unfortunately does not and cannot exist) would produce one acoustic watt with the application of one electrical watt. Most cone-type transducers offer around 3% efficiency, with some advanced piezo-electric devices offering up to 20-25%. One acoustic watt is equivalent to 107.5 dBSPL at around a meter from an Omni-directional source. A device's directivity can be described with a term called "Q", which is basically an indication of its ability to confine the applied energy to a smaller unit area. Q, therefore, is a parameter of the horn, not the driver. If we take one acoustic watt and couple it to a horn with a Q of 2 (hemispherical), the intensity and therefore the pressure will increase by a factor of 2 to 1 in the area covered by the device. This is quite useful, since it allows more energy on the audience and less on the walls and ceiling. It is common practice to convert the Q rating into decibels by the formula:
DI = 10 log (Q)
Where DI is the directivity index
The power of different sources of sound can vary widely. Different sources can also produce sounds with varying spectral content. A very shrill gas leak may produce the same sound power as a lawnmower, but they will be two very different sounds. Later, we will see how to take this into consideration when determining sound levels.
Sound Power Level of Typical Sources
Sound Power, Watts
dB Re 10-12 Wats
After burning jet engine
Centrifugal fan at 500,000 cfm (849,000 cu m/hr)
75 piece orchestra
Vane axial fan at 100,000 cfm (169,900 cu m/hr)
Large chipping hammer
Centrifugal fan at 13,000 cfm (22,087 cu m/hr)
Auto on highway
Food blenders-upper range
Quiet-Duct silencer, self-noise at +1000 fpm
Voice-very soft whisper
Lowest audible sound for persons with excellent hearing
Reference: From the IAC Noise Control Reference Handbook, 1989 Edition by Martin Hirschorn © Copyright 1982, revised January 1989 by Industrial Acoustics Company.
EFFECTS OF NOISE ON HEALTH AND PERFORMANCE:
The perception of sounds in day-to-day life is of major importance for human well-being. Communication through speech, sounds from playing children, music, and natural sounds in parklands, parks and gardens are all examples of sounds essential for satisfaction in everyday life. Conversely, the adverse effects of sound (noise) may also be taken into consideration.
According to the International Program on Chemical Safety (WHO 1994) an adverse effect of noise is defined as a change in the morphology and physiology of an organism that results in impairment of functional capacity, or an impairment of capacity to compensate for additional stress, or increases the susceptibility of an organism to the harmful effects of other environmental influences. This definition includes any temporary or long-term lowering of the physical, psychological or social functioning of humans or human organs.
HEALTH EFFECTS FROM NOISE:
Noise health effects are the health consequences of elevated sound levels. Elevated workplace or other noise can cause hearing impairment, hypertension, ischemic heart disease, annoyance and sleep disturbance. Changes in the immune system and birth defects have been attributed to noise exposure, but evidence is limited. Although some presbycusis may occur naturally with age, in many developed nations the cumulative impact of noise is sufficient to impair the hearing of a large fraction of the population over the course of a lifetime. Noise exposure has also been known to induce tinnitus, hypertension, vasoconstriction and other cardiovascular impacts. Beyond these effects, elevated noise levels can create stress, increase workplace accident rates, and stimulate aggression and other anti-social behaviors. The most significant causes are vehicle and aircraft noise, prolonged exposure to loud music, and industrial noise. Road traffic causes almost 80% of the noise annoyances in Norway. The social costs of traffic noise in EU22 are over €40 billion per year, and passenger cars and Lorries (trucks) are responsible for bulk of costs. Traffic noise alone is harming the health of almost every third person in the WHO European Region. One in five Europeans is regularly exposed to sound levels at night that could significantly damage health. Noise is also a threat to marine and terrestrial ecosystems.
EFFECTS OF NOISE ON RESIDENTIAL BEHAVIOR AND ANNOYANCE:
Noise annoyance is a global phenomenon. A definition of annoyance is "a feeling of displeasure associated with any agent or condition, known or believed by an individual or group to adversely affect them". However, apart from "annoyance", people may feel a variety of negative emotions when exposed to community noise, and may report anger, disappointment, dissatisfaction, withdrawal, helplessness, depression, anxiety, distraction, agitation, or exhaustion. Noise can produce a number of social and behavioral effects in residents, besides annoyance.
The social and behavioral effects are often complex, subtle and indirect. Many of the effects are assumed to be the result of interactions with a number of non-auditory variables. Social and behavioral effects include:
- Changes in overt everyday behavior patterns (e.g. closing windows, not using balconies, turning TV and radio to louder levels, writing petitions, complaining to authorities);
- Adverse changes in social behavior (e.g. aggression, unfriendliness, disengagement, non-participation);
- Adverse changes in social indicators (e.g. residential mobility, hospital admissions, drug consumption, accident rates); and
- Changes in mood (e.g. less happy, more depressed).
Although changes in social behavior, such as a reduction in helpfulness and increased aggressiveness, are associated with noise exposure, noise exposure alone is not believed to be sufficient to produce aggression. However, in combination with provocation or pre-existing anger or hostility, it may trigger aggression. It has also been suspected that people are less willing to help, both during exposure and for a period after exposure. Fairly consistent evidence shows that noise above 80 dBA is associated with reduced helping behavior and increased aggressive behavior. Particularly, there is concern that high-level continuous noise exposures may contribute to the susceptibility of schoolchildren to feelings of helplessness. The effects of community noise can be evaluated by assessing the extent of annoyance (low, moderate, high) among exposed individuals; or by assessing the disturbance of specific activities, such as reading, watching television and communication. The relationship between annoyance and activity disturbances is not necessarily direct and there are examples of situations where the extent of annoyance is low, despite a high level of activity disturbance. For aircraft noise, the most important effects are interference with rest, recreation and watching television. This is in contrast to road traffic noise, where sleep disturbance is the predominant effect.
A number of studies have shown that equal levels of traffic and industrial noises result in different magnitudes of annoyance. This has led to criticism of averaged dose-response curves determined by meta-analysis, which assumed that all traffic noises are the same. Some scientists have synthesized curves of annoyance associated with three types of traffic noise (road, air, railways). In these curves, the percentage of people highly or moderately annoyed was related to the day and night continuous equivalent sound level. For each of the three types of traffic noise, the percentage of highly annoyed persons in a population started to increase at the value of 42 dBA, and the percentage of moderately annoyed persons at 37 dBA. Aircraft noise produced a stronger annoyance response than road traffic. However, caution should be exercised when interpreting synthesized data from different studies, since five major parameters should be randomly distributed for the analyses to be valid: personal, demographic, and lifestyle factors, as well as the duration of noise exposure and the population experience with noise. Annoyance in populations exposed to environmental noise varies not only with the acoustical characteristics of the noise (source, exposure), but also with many non-acoustical factors of social, psychological, or economic nature. These factors include fear associated with the noise source, conviction that the noise could be reduced by third parties, individual noise sensitivity, the degree to which an individual feels able to control the noise (coping strategies), and whether the noise originates from an important economic activity. Demographic variables such as age, sex and socioeconomic status, are less strongly associated with annoyance. The correlation between noise exposure and general annoyance is much higher at the group level than at the individual level, as might be expected. Data from 42 surveys showed that at the group level about 70% of the variance in annoyance is explained by noise exposure characteristics, whereas at the individual level it is typically about 20%.
When the type and amount of noise exposure is kept constant in the meta-analyses, differences between communities, regions and countries still exist. This is well demonstrated by a comparison of the dose-response curve determined for road-traffic noise and that obtained in a survey along the North-South transportation route through the Austrian Alps. The differences may be explained in terms of the influence of topography and meteorological factors on acoustical measures, as well as the low background noise level on the mountain slopes. Stronger reactions have been observed when noise is accompanied by vibrations and contains low frequency components, or when the noise contains impulses, such as shooting noise. Stronger, but temporary, reactions also occur when noise exposure is increased over time, in comparison to situations with constant noise exposure. Conversely, for road traffic noise, the introduction of noise protection barriers in residential areas resulted in smaller reductions in annoyance than expected for a stationary situation. To obtain an indicator for annoyance, other methods of combining parameters of noise exposure have been extensively tested, in addition to metrics. When used for a set of community noises, these indicators correlate well both among themselves. There is a growing concern that all the component parameters of the noise should be individually assessed in noise exposure investigations, at least in the complex cases.
THE EFFECTS OF NOISE:
"A comfortable environment is one in which there is freedom from annoyance and distraction, so that working or pleasure can be carried out unhindered physically or mentally" D.J. Croome Noise, Buildings and People. In broad terms, the sound pressure level of any broad-band noise can be categorised as follows:
- 150 dB causes instant loss of hearing.
- 120 dB is physically painful and should be avoided.
- 100 dB short periods of exposure causes a temporary loss of acuity (threshold shift) with prolonged exposure likely to cause irreparable damage to auditory organs.
- 90 dB long term exposure at this level normally causes permanent hearing loss.
- 65dB long periods of exposure cause both mental and bodily fatigue.
Significant research has been carried out in this area (Shepard (1975), Freeman (1975), Ando and Hattori(1970, 1973), Gatonni and Tarnopolsky (1973), Winder(1976) and Tarzi (1976)). It has been shown that noise is a potent factor in the production and exacerbation of stress. There are even suggestions that the general level of background noise is responsible for the comparative nervousness of city-dwellers. Excessive noise clearly affects concentration, particularly if it has some sort of information content (such as speech, a radio, etc). In a work situation, switching attention to and from such a noise may take up to 2 seconds and may be done quite often.
LOW FREQUENCY NOISE:
To a degree, it is useful that we have a reduced sensitivity to the lower frequency range as it relieves us of being annoyed by low frequency sounds within and around us. However, very low frequencies, subsonic or infra-sounds can be produced in buildings by long air-conditioning ducts and wind effects. Subsonic sounds generally act on the inner ear, the position-sensing organ, and significant levels may cause disorientation, seasickness, digestive disorders, troubled sight and dizziness. Frequencies of between 1 and 8Hz can correspond to resonant frequencies within the body itself, 8Hz being thought to cause circulatory resonance phenomena, overloading the heart or bursting blood vessels. 7Hz also seems particularly unpleasant as it has been shown to cause tiredness, headache and nausea. It is suggested that 7 Hz corresponds to the median frequency of the brain's 'alpha' waves.
Although a subdued voice is understandable in a quiet room, it is difficult to understand even a raised voice above the roar of an aircraft engine. This drowning out, or masking, occurs as a result of the general excitation of the auditory nerve cells. For a sound to be picked out of this background level of excitation, it must produce a corresponding increase in its threshold. This is generally known as a problem with signal-to-noise ratio. Low frequency sounds produce a considerable masking effect over higher frequency sounds. Excessive low frequency sounds therefore constitute a serious source of interference for speech and music which rely on higher frequency sounds to impart most of their auditory information. Higher frequency sounds can mask lower frequencies to a degree, but the effect is most pronounced in sounds of similar frequency. This phenomenon can be exploited in environmental noise control by using uninterrupted, non-information carrying sounds. These can provide an acceptable background noise to suppress other objectionable noises, making them psychologically quieter. Such noise may include air-conditioning noise, traffic flow on a highway or even a water fountain.
PRESBYCOUSIS AND HEARING DEFECTS:
Normally people experience some hearing loss with age, particularly at high frequencies. Whether this is simply due to the aging process or to the effects of present levels of environmental noise seems to be a matter of speculation: Croome (1977) and Rossen 1962) describe Mabaan tribesmen in the Sudan who do not exhibit this. It is, therefore, very difficult to separate completely noise induced hearing loss from presbycusis.
TEMPORARY THRESHOLD SHIFT
When a person with normal hearing is exposed to intense noise for an extended period, they first suffer a temporary loss of sensitivity or accuity called a temporary threshold shift. After a certain period the ear gradually recovers it's sensitivity. If insufficient recovery time is given before the next exposure (as in the case of a workplace), it appears that a persistant threshold shift is eventually followed by permanent threshold shift, ie: partial deafness at higher frequencies.
This defect is experienced by most people, usually in the form of a high pitched ringing in the ears. It can occur spontaneously and last for only a few seconds or, in some cases, persist for long periods of time.
The potential health effects of community noise include hearing impairment; startle and defense reactions; aural pain; ear discomfort speech interference; sleep disturbance; cardiovascular effects; performance reduction; and annoyance responses. These health effects, in turn, can lead to social handicap; reduced productivity; decreased performance in learning; absenteeism in the workplace and school; increased drug use; and accidents. In addition to health effects of community noise, other impacts are important such as loss of property value. In these guidelines the international literature on the health effects of community noise was reviewed and used to derive guideline values for community noise. Besides the health effects of noise, the issues of noise assessment and noise management were also addressed. Other issues considered were priority setting in noise management; quality assurance plans; and the cost-efficiency of control actions. The aim of the guidelines is to protect populations from the adverse health impacts of noise. The following recommendations were considered appropriate:
· Governments should consider the protection of populations from community noise as an integral part of their policy for environmental protection.
· Governments should consider implementing action plans with short-term, medium-term and long-term objectives for reducing noise levels.
· Governments should adopt the health guidelines for community noise as targets to be achieved in the long-term process.
· Governments should include noise as an important issue when assessing public health matters and support more research related to the health effects of noise exposure.
· Legislation should be enacted to reduce sound pressure levels, and existing laws should be enforced.
· Municipalities should develop low-noise implementation plans.
· Cost-effectiveness and cost-benefit analyses should be considered as potential instruments when making management decisions.
· Governments should support more policy-relevant research into noise pollution.
This lecture is not possible without the support and data taken from: http://www.kemt.fei.tuke.sk/Predmety/KEMT320_EA/_web/Online_Course_on_Acoustics/index_acoustics.html (Retrieved April 9, 2011) Copyright © Andrew Marsh, UWA, 1999. The School of Architecture and Fine Arts The University of Western Australia
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CONCLUSIONS AND RECOMMENDATIONS: