This article throws light upon the top seven properties of noise. The properties are: 1. Desirability for Noise 2. Stress of Noise 3. Subjective Qualities of Noise 4. Sound Levels of Noise 5. Loudness of Noise 6. Pitch of Noise 7. Physiological Response to Noise.
Property # 1. Desirability for Noise:
A comfortable environment may be defined as the one in which there is freedom from annoyance and distraction, so that working or pleasure tasks can be carried out unhindered physically or mentally. Not only do sound, lighting, colour, temperature, humidity, air movement and purity of air play a part in a comfortable environment, but psycho-sociological factors also have an important role.
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In addition, the attitudes of people around us, the social organisation in which we work, the organisation of space, colour schemes and many other factors all can have an influence on our mood and work output.
The problem is complicated still further due to the interaction among all these factors. Even if one of the physical factors is deficient, the balance of the environment may be spoiled. In the same way, surroundings which contain disturbing social or psychological aspects can be uncomfortable.
It is well-known that human beings are very adaptable; but the adaptation in itself may be no more than a philosophical acceptance of disturbing environmental conditions. Such conditions may still be harmful to the human system, although the person concerned may be unaware of this.
A well-known example in this regard in the workers in textile industries working in high levels of noise (ranging from 90 to 110 dB (A), such that the deterioration in their hearing at the age of 30 is more like that of someone aged over 70 who has worked in normal acoustic climates of, say, 40 to 60 dB (A).
It is well known that unfavorable environment imposes a stress on the nervous system and that this stress adversely affects the mood due to increased irritability and anxiety. Such stress, however, is much harder to measure than the physiological and pathological damage resulting from adverse environmental conditions.
On the other hand, noise is not always undesirable. Under certain circumstances, noise can be useful by masking other sounds, when desirable. Noise may also provide auditory clues in alarm systems. It may even increase the performance of some types of tasks by raising the arousal level of the individual to an optimum value. For aural comfort, an optimum (and not a minimum) amount of sound is required in a space.
Property # 2. Stress of Noise:
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Many investigations have been conducted to ascertain the effect of stress imposed on people by their environment. In this connection, it can be safely assumed that there are many cases of psychiatric disorders that are partly due to excessive noise.
Strong evidence now exists which demonstrates that noise adversely affects people mentally and socially.
In a report entitled “The Noise Around US” issued by the United States Department of Commerce in 1970, for example, reference is made to evidence in Europe, the (former) USSR and the US A, which suggests that noisy environments adversely affect the number of miscarriages, the death rate and the incidence of heart attacks.
It has been suggested by some workers that noise affects the human embryo and that there is a higher incidence of low-weight births in noisy areas. It has been shown that there is a significantly higher rate of admission to a psychiatric hospital from areas near the Heathrow International Airport (London) than from outside this noisy area.
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In a study carried out on telegraphists and mail sorters working in Australian cities, Ferguson (1973) found that the only occupational characteristics that could be identified strongly with neurosis were the negative attitudes to the job and supervision, and the noise in the work place.
Neurotic subjects were invited to state influences which they thought may have contributed to their symptoms. Multiple influences were commonly mentioned, as shown in Table 1, and noise was found to be the second largest stated influence (37%) after “job beyond ability” (44%).
As seen from Table 1, multiple environmental and other influences were commonly mentioned by most neurotic subjects. Only 4% of them could not account for their symptoms by some perceived external or internal stress.
Many authorities believe, on the basis of considerable experience and empirical evidence, that noise is a potent factor in the production and aggravation of stress. Noise does not just contribute towards occupational stress. It can also affect the quality of sleep and our attitudes to neighbours. Even the noise of one’s own children can be stressful.
Nemecek and Grandjean (1973) conducted a survey of the climatic conditions in fifteen landscaped offices throughout Switzerland and, at the same time, questioned 519 employees about their experiences of working in them. Their results regarding noise disturbance are shown in Table 2.
It was found by Nemecek and Grandjean that noise disturbance (particularly the content rather than the sound pressure level of conversations) was mentioned by 35% of the respondents. Moreover, managers and research staff were more disturbed than the administrative staff. It was found that people experience a loss of concentration and also a loss of privacy due mainly to noise.
Property # 3. Subjective Qualities of Noise:
Noise is a sound unwanted by the recipient. Noise can be distracting, it can be annoying, and at high levels can produce physiological damage of a temporary or permanent nature. Music, on the other hand, is pleasing to the ear; but what is pleasing or agreeable to one person may not be so to another (or even to the same person at a different time).
To classify subjective qualities of sound and to correlate them with physical factors is, therefore, a major problem in economically establishing comfortable environments in buildings.
There are certain common elements which make up a sound (whether judged to be noise or music or something in between these two extremes). To discover these common elements, we must look at the spatial-temporal patterns of sound.
Let us consider a very simple example. A tuning fork emits a sinusoidal waveform which mirrors the movement of air particles caused by the acoustic pressure originating from the acoustic source.
Its spatial characteristics are wavelength and intensity, while the temporal characteristics are frequency and the duration of sound. Musical instruments, on the other hand, display a much more complex wave pattern. In this case, not only is the fundamental frequency present but the harmonics of this frequency too.
The harmonic content of a sound gives it the timber (or quality). It also enables us to make distinctions between one type of sound and another. A horn, for example, has a frequency spectrograph which will probably include the first four harmonics. A violin, on the other hand, may display even the sixteenth or eighteenth harmonic.
Tone quality of musical instruments may be judged from their spectral content. A decrease in the harmonic amplitude does not necessarily follow with an increase in the harmonic order number. For example, a quite different picture results when the sound from an air jet is recorded.
In this case, there is a random distribution of all frequencies. Such a sound is known as “white noise” (or “Gaussian noise”). Most industrial noise sources have discrete components superimposed on white noise characteristics.
Property # 4. Sound Levels of Noise:
The Weber-Fechner law proposed a logarithmic relationship between stimulus strength and perceived response. This law represents the auditory sensory modality fairly well in non-extreme conditions of sound frequency, intensity and duration.
Another good reason for using a logarithmic scale to express acoustical quantities is that the ear is a very sensitive receptor, and responds to acoustic power as low as 10-12 W and also as high as 102W. Thus the sound power level (Lw), sound intensity level (Li) and sound pressure level (Lp) may be defined in decibels as follows:
Lw = 10 log10 (W/W0) dB,…(1)
Li = 10 log10 (I/I0) dB, …(2)
Lp = 20 log10 (P/P0 ) dB, …(3)
where W0,10 and P0 are reference quantities (W0 = 10-12 W, I0 = 10-12 W/m2 and P0 = 2 x 10-5 Pa) corresponding to the average auditory thresholds of energy and pressure. We note here that decibels are not units or dimensions; they are simply logarithmic ratio scale values.
Property # 5. Loudness of Noise:
Evaluating the loudness of a complex noise is a difficult task. The reason is that the human attention may be shared between the various frequencies comprising the noise under consideration. In one type of experiment known as magnitude esumation for estimating the loudness of a sound, the experimental subjects are asked to make comparison judgements.
In such experiments, a pure 1 kHz tone is set to a given loudness level. Another tone is compared with this, by experimental subjects, until both tones are judged at the same loudness level.
The results of a large number of such magnitude estimation experiments are shown in Fig. 1 in the form of equal loudness level curves.
The following conclusions can be drawn from these curves:
(a) The human ear is a non-linear device having a maximum sensitivity in the region of 3-4 kHz.
(b) The ear can tolerate higher loudness levels at lower frequencies
(c) As the loudness level increases, the degree of non-linearity decreases
(d) The loudness level in phons is equal to the sound pressure level in decibels (reference level – 20µPa) at the frequency of 1 kHz. (Such experiments usually involve binaural listening to pure tones.)
Measuring instruments (known as “sound level meters”) are designed to respond in a similar way to the human ear by incorporating in them electronic weighting networks. These networks are known as A, B, C and D weightings, and they simulate the non-linearity of the human ear.
The frequency response of these weighting networks in a sound level meter is shown in Fig. 2. The A, B and C weightings are derived from the equal-loudness-level curves as indicated on Fig. 1 and 2. D weighting, on the other hand, is equivalent to the perceived noise level.
There is another type of experiment aimed at evaluating the loudness of a noise. In this type of experiment, known as “magnitude production”, the subject is required to estimate the ratios of loudness sensations. The loudness curve derived from this type of experiments is shown in Fig. 3.
This curve enables the loudness of pure tones to be directly compared. We may draw the following conclusions from the curve shown in Fig. 3:
(a) A loudness of 1 sone is equivalent to a loudness level of 40 phons.
(b) Above the loudness level of 40 phons, doubling the loudness requires an increase of about 9-10 dB in the loudness level, i.e.,
Loudness level (phons)
= 40 + 10 log2 (L) …(4)
where L is the subjective loudness in sones.
(c) In contrast to the loudness level, the loudness values are additive. Subjective loudness decreases with time due to the phenomenon of auditory adaptation. Experiments have shown that some personality aspects such as introversion, neuroticism and anxiety do not correlate with loudness evaluation; but excitability may be a possible correlate.
It is too early, however, to derive any positive conclusions, from the available experimental evidence, on this aspect of loudness evaluation. It is much more certain, on the other hand, that noise annoyance depends on personality.
In practice, it is often required to calculate the loudness of a complex noise which may include several pure tones. The Stevens method (Stevens, 1957) is normally employed for this purpose. According to this method,
St = Smax + F (∑S-Smax ), …(5)
where St = total loudness (in sones) of a complex noise, Smax = the highest value of the loudness index, and ∑S is the sum of the loudness indices of all the octave (F = 0.3), one-third octave (F = 0.15), or one-half octave bands (F = 0.20).
The perception of a complex sound containing many pure tones is dependent not only on the loudness and pitch but also on the harmonic content of the tones, the transient behaviour of the harmonics, and any phase differences which may exist between the tones.
For example, if the frequencies of both tones (in a complex sound having two tones) lie within a critical bandwidth, then the resultant loudness is not the addition of that for each tone. The critical bandwidths for various frequencies are listed in Table 3.
The human ear acts as a frequency analyser of complex tones. The critical bandwidths listed in Table 3 correspond to the limit of its resolving power. This phenomenon is the subject of the well-known Ohm’s acoustical law, which simply states that the ear can distinguish many of the harmonic constituents in a complex tone as long as the critical bandwidths of the fundamental frequencies do not overlap.
When the sounds he in different critical bandwidths, they are heard simultaneously, but sensed as separate tones. When this is the case, the resultant loudness is the addition of that for each frequency. Masking occurs when one sound drowns another. Masking is particularly prevalent when the frequencies under consideration lie in a bandwidth equal to 0.4 of the critical bandwidths listed in Table 3.
Property # 6. Pitch of Noise:
Pitch may be defined as that subjective aspect of auditory sensation in terms of which sounds may be ordered on a scale primarily related to frequency. In the laboratory experiments for pitch determination, subjects are asked to make ratio judgements of pitch by adjusting the frequency of one audio oscillator until it is twice the pitch of another one.
In this way, a scale of pitch is developed, as shown in Fig. 4. A reference value of 1,000 mels has been chosen as the pitch of a 1,000 Hz tone with a sound pressure level of 60 db (references – 20 µPa).
Experiments have shown that pitch is not completely dependent on the frequency. It has been established by experimental research in this area that the effect of sound intensity on the pitch is negligible for some people. For others, however, pure tones at varying sound levels (in the range 40-120 dB) may cause pitch changes of as much as 35%. The pitch of a low pitched note is lowered when its intensity is raised.
A high-pitched note, on the other hand, will have its pitch raised with the increase in its intensity. Just as increasing or decreasing the intensity of most spectral lights produces shifts in hue (the “Bezold-Brucke effect” in vision), so low tones are lowered and high tones are raised in pitch when the sound pressure level of the tone concerned is increased.
So far, there is no clear explanation of this effect. This variation of pitch, fortunately, is only noticeable for pure tones. It is not detected in music, where the richness of the overtones in some way clouds any variation of pitch that might exist.
It can be concluded on the basis of experimental evidence that although the pitch of a sound is primarily a function of its frequency, it is also dependent on the sound pressure level and the spectral content of the sound concerned, besides the feedback and psychological effects. In general, a variation in frequency intensity or waveform of a sound can affect any of the subjective sensations of loudness, pitch or timbre.
Just as for loudness, the duration of a sound features in the subjective impression it leaves on the listener. Tones lasting for less than 200 ms lose their rich tonal character. If they last for only 100 ms or less, they seem more like a brittle, atonic click.
Besides pitch, loudness and timbre, psychologists have attempted to quantify several other attributes of auditory sensation such as volume (related to the spatial quality of sound), brightness, tonality, density (related to the compactness of sound), and vocality (vowel similarity). These features, however, are not yet clearly understood.
Property # 7. Physiological Response to Noise:
Detailed physiological explorations and attempted explanations about the response of human ear to sound waves with varying intensities and frequencies have often appeared in books and journals during the last 60 years (see, for example, Bekesy, 1960,1964).
We are mainly concerned here with annoyance, disturbance and distractive effects of noise on people, so the theories of hearing which try to explain the ear’s response to sound will not be discussed.
The human ear is a very sensitive instrument. If the hearing mechanism is damaged in any way either by excessive noise levels or by diseases which affect the brain, the auditory nerve or the auditory ossicles, then hearing will De impaired.
Intense noise levels, for example, are encountered in many industries, and they can cause temporary or progressively permanent loss of hearing. In many advanced countries, even premature babies experience loud noise.
In the United Kingdom, for example, the average sound level of many incubators is about 57 dB (A), whereas a maximum value of 35 dB (A) is recommended for people sleeping. Since hardly any work has been done so far in this area, we can only conjecture as to the deleterious effects of this noise level on the neonates.
It is essential, therefore, to observe carefully the hearing performance of such neonates over the first few years of their lives.
For detailed accounts of deafness and physiological hearing damage, the interested readers should refer to the excellent treatises by Burns (1968) and Kryter (1970).
Most of us live and work in environments having noise levels less than 5 dB (A) – and this is the starting level where hearing damage risk is at present thought to be imminent. In this connection, however, we note here that there is some experimental evidence which shows that the hearing at a level of 75 dB (A) may take two hours or more to return to normal.
There is need, therefore, to consider the lowering of this 85 dB (A) starting point for the purpose of noise control. For most people not associated with noisy industries, the danger of exposure to high noise levels arises mainly from traffic and, for the teenagers, from pop music as well.
As a general rule, people experience some hearing loss (particularly at high frequencies) which increases with age. Rosen (1962), however, has noted that the Mabaan tribesmen in Sudan show hardly any hearing loss with age. This suggests that presbycusis (the hearing loss with age) is at least partly caused by occupational noise.
At lower noise levels, there may be no hearing loss, and psychological effects of noise become more prominent in such cases. Much about the psychological effects of noise, however, still remains unknown, and there is much scope for further research in this area.
Various attempts have been made to measure the factors controlling the stress-strain relationship between man and his environment. Some of these measures are listed in Table 4, while the factors governing the stress-strain relationship are shown in Table 5.
It is evident from these Tables that there is a wide. variety of possible measures; but there is no clearly-defined basic measure so far that can be used to predict the effect of noise on people part of this difficulty results from the fact each stress experiment superimposes the stress under investigation on stresses already present within the individual.
Moreover, we are often subject to not one stress but several of them in combination.
Some of the research that has been carried out on the physiological effects of noise on human beings is summarised in Table 6. This table, however, excludes the work on temporary or permanent deafness.
Among the physiological effects of noise, vasoconstriction seems to depend on the sound level alone (irrespective of whether the sound concerned is noise or music); it also seems to be independent of the age or the past noise conditioning of the individual. This suggests that vasoconstriction is an unlikely indicator of the subjective response to noise.
When considering the somatic responses to noise (and other environmental factors such as light or heat), however, we should always keep in mind the fact that the attitude of the individual is probably more important than the environmental factors.
Physiologically, the attitude of the individual determines the base adrenaline levels Psychologically high motivation makes the individual less dependent upon the environment. Boredom, on the other hand, makes the individual more dependent on them.
The nature of the task also has an equal or even large effect than the environmental factors on such physiological measures as the muscle tension and the skin resistance.
Another point to be borne in mind is that the physiological measures (of the effects of noise on man) have tended to emphasize the physical aspects of man‘s make-up, whereas the mental health reflected by the psychological state of the individual is just as important.
The physical and mental states have common starting points in the brain and the autonomic nervous system. For this reason there is need for further research relating stress to the brain and the nervous system.
Physiological response to various levels of sound is summarised in Table 7. We note in this connection that the psychological response is also important in partly determining the arousal level. However, the effects of stress on the system are unknown in many situations. For noise levels above 115 dB (A), the physiological effects become more important than psychological effects.
