Right to Quiet Society Noiseletter
Winter 2014, page 5

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Low-frequency noise: a biophysical phenomenon

Mireille Oud (medical physicist / consultant)
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The Netherlands

Abstract

Complaints on low-frequency noise were till recently fairly unexplained, but audiological research shed light on the mechanisms that enable perception of frequencies below the threshold of average normal hearing. It was shown that exposure to low-frequency sound may alter the inner ear. This results in an increase of sensitivity to low-frequency sounds, and as a result, previously imperceptible sounds become audible to the exposed person. Interactions between inner-ear responses to low and higher frequencies furthermore account for perception of low-frequency sound, as well as the property of the hearing system to perceive so-called difference tones.

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Perception-based protection from low-frequency sounds may not be enough

Alec N. Salt, Jeffery T. Lichtenhan;

Department of Otolaryngology, Washington University School of Medicine, St. Louis, MO, 63110, USA.

Hearing and perception in the mammalian ear are mediated by the inner hair cells (IHC). IHCs are fluid-coupled to mechanical vibrations and have been characterized as velocity-sensitive, making them quite insensitive to low-frequency sounds. But the ear also contains more numerous outer hair cells (OHC), which are not fluid coupled and are characterized as displacement-sensitive. The OHCs are more sensitive than IHCs to low frequencies and respond to very low-frequency sounds at levels below those that are perceived. OHC are connected to the brain by type II afferent fibres to networks that may further attenuate perception of low frequencies.

These same pathways are also involved in alerting and phantom sounds (tinnitus). Because of these anatomic configurations, low-frequency sounds that are not perceived may cause influence in ways that have not yet been adequately studied. We present data showing that the ear’s response to low-frequency sounds is influenced by the presence of higher-frequency sounds such as those in the speech frequency range, with substantially larger responses generated when higher-frequency components are absent. We conclude that the physiological effects of low-frequency sounds are more complex than is widely appreciated. Based on this knowledge, we have to be concerned that sounds that are not perceived are clearly transduced by the ear and may still affect people in ways that have yet to be fully understood.

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Loud. Louder. Lifeless

On "World Oceans Day”, June 8, 2013, an international coalition of conservation organisations launched “Silent Oceans”, an intensive campaign against underwater noise pollution. Spearheaded by OceanCare, the organisations aim is to achieve effective protection of marine life from anthropogenic noise. The campaign is focussing on the three major noise sources: military activities, the oil & gas industry and shipping.

Growing scientific evidence in recent years is providing us with a better understanding of the hazards and consequences of intensive noise to marine ecosystems: The communication of marine mammals is blurred and whales are stranding. Shoals of fish are collapsing, sea turtles are fleeing. Noise is life-threatening even to coral larvae. The main sources of underwater noise pollution are military sonars, oil & gas exploration, and the huge shipping traffic. In some ocean areas, noise levels doubled every decade since 1950. The UN acknowledges underwater noise pollution as one of the five major threats to marine mammals. Nonetheless, up to now there were hardly any efforts to reduce underwater noise.

“By referring to national security, the military is trying to shirk its responsibility, and the oil & gas industry is for financial reasons ignoring existing technology to reduce damaging noise levels.

That’s why we need the public to give their voice to the ocean animals”, says Sigrid Lüber, President of OceanCare and campaign head. “But this is a major challenge. It’s a very complex and intricate issue that we have to communicate in a comprehensible way in order to allow the public to understand the scale of the problem”, states Lüber.

The Silent Oceans campaign has a 10-step blueprint to reduce ocean noise pollution. This plan comprises - among other things - the following claims:

International regulatory framework to reduce underwater noise pollution.

Determine maximum noise threshold.

No intensive noise sources in highly sensitive regions and marine protected areas.
Obligatory use of the best available noise reducing technologies.

Comprehensive environmental impact assessments.

Special attention to pay to the developments in the Mediterranean region.

Millions of tourists are visiting the Mediterranean to find rest and peace - an idyll that has long been lost for the sea’s underwater world.

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