Noise is ubiquitous and man-made (or anthropogenic). Noise is increasing in many environments due to increases in transportation and the exploration for and exploitation of energy sources.
While humans and animals are adapted to deal with natural levels of ambient noise, increased ambient noise levels from human activities can potentially have a variety of adverse effects. These could include noise-induced hearing loss and auditory system damage, temporary hearing loss (called Temporary Threshold Shift or TTS), and masking of communication and other important biological sounds.
Increased noise can also cause behavioural and/or physiological changes such as increased stress, sleep loss, and hormonal changes. Moreover, in water, high intensity signals can damage non-auditory tissues and result in death, although death has rarely been documented except when animals are exposed to very high intensity impulsive sound.
While relatively little is known about the effects of man-made sounds on fishes, a good deal may be inferred about the potential effects, and approaches to the study of those effects, from the wealth of data available from humans and laboratory animals. Over the past decades it has become apparent that man-made noise also has the potential to impact the lives of aquatic organisms, including sharks, bony fishes, marine turtles, and marine mammals. Indeed, the potential effects of man-made noise described above for humans, birds and other animals are likely to be encountered in fishes and other aquatic animals.
One difficulty in understanding the effects on fishes, or any nonhuman animal, is that while most humans have very similar auditory capabilities and sensitivities, the same is not true for all species or even for closely related species. Thus, sound sources that may have negative consequences for one species may not have similar consequences for another.
Another problem is that it is sometimes difficult in animal studies, and in particular for terrestrial species, to definitively identify noise as the principal source of non-auditory effects, such as stress-related behavioral or physiological effects. The general effects of noise, and the relationships among these effects, vary with distance from the noise source.
Fixed-wing aircraft operations at airports typically involve a large number of flights per day and, because the noise characteristics of most of the large jets are similar to one another, the noise climate is relatively uniform. Away from airports, aircraft fly at very high altitude so that noise levels on the ground are low. In addition, there is little concern over aircraft safety.
Helicopter operations are very different. In general, the paths, unlike those used by fixed-wing aircraft, vary widely and so at any one location the noise pattern is much less consistent. The noise levels have peak values when aircrafts fly low and overhead or take off and land at airports. The noise limits prescribed by UK airports for take-offs are 110 PNdB (1 PNdB=dB scale+13) during day and 102 PNdB during night.
Helicopter operations around the world, and in particular in the United States, are giving rise to objections over noise from the general public; in the last two or three years there have been major objections to helicopter use in the New York and Los Angeles areas.
The development of new heliports, and changes to services at existing facilities, tend to be controversial and are often rejected as a result of public opposition. Such negative response to helicopter operations as a result of noise is a little difficult to understand because most helicopters generate less noise than the noise certification standards and, in most cases, meet established community noise rating criteria and guidelines.
Also, the same rating methods are generally considered to be successful in understanding the environmental impact of large commercial aircraft and other forms of transportation, so there appears to be something different about the way in which helicopters are perceived.
When the helicopter noise and complaint information available was examined initially, a number of observations could not be explained. Thus a few noisy helicopters can create adverse response which will then affect the public response to all helicopters. If, however, the number of operations of noisy helicopters is very low, this may not always be the case.
The sound quality of the noise at levels 20 dB or more below the maximum level provides the initial audible cues that alert an individual to the presence of a helicopter, i.e. provide the trigger for the virtual noise component.
It follows that improvements to the noise signature by reducing or eliminating the impulsive sources will result in greater public acceptance irrespective of the absolute noise level generated. It also implies that many of today’s small and medium size helicopters will need to fly at 2,000 – 4000 ft (600 – 1200 m) or more, and that the use of noise abatement procedures for normal operations are essential. Also, they will need to fly much slower than anticipated if impulsive noise is not to create a problem.
Even so, there is certainly a need for more research into the subjective response to helicopter noise – the main activity in this area was over 20 years ago and there has been little examination of these aspects since then. From the industry point of view (operators and manufacturers), it is essential to establish what really needs to be done to improve public acceptance.
(Dr Senapati is Professor and Head, Department of Chemistry, Trident Academy of Technology, Bhubaneswar-751024)