Responses of the ear to low frequency sounds, infrasound and wind turbines

Abstract

Infrasonic sounds are generated internally in the body (by respiration, heartbeat, coughing, etc) and by external sources, such as air conditioning systems, inside vehicles, some industrial processes and, now becoming increasingly prevalent, wind turbines. It is widely assumed that infrasound presented at an amplitude below what is audible has no influence on the ear. In this review, we consider possible ways that low frequency sounds, at levels that may or may not be heard, could influence the function of the ear. The inner ear has elaborate mechanisms to attenuate low frequency sound components before they are transmitted to the brain. The auditory portion of the ear, the cochlea, has two types of sensory cells, inner hair cells (IHC) and outer hair cells (OHC), of which the IHC are coupled to the afferent fibers that transmit “hearing” to the brain. The sensory stereocilia (“hairs”) on the IHC are “fluid coupled” to mechanical stimuli, so their responses depend on stimulus velocity and their sensitivity decreases as sound frequency is lowered. In contrast, the OHC are directly coupled to mechanical stimuli, so their input remains greater than for IHC at low frequencies. At very low frequencies the OHC are stimulated by sounds at levels below those that are heard. Although the hair cells in other sensory structures such as the saccule may be tuned to infrasonic frequencies, auditory stimulus coupling to these structures is inefficient so that they are unlikely to be influenced by airborne infrasound. Structures that are involved in endolymph volume regulation are also known to be influenced by infrasound, but their sensitivity is also thought to be low. There are, however, abnormal states in which the ear becomes hypersensitive to infrasound. In most cases, the inner ear’s responses to infrasound can be considered normal, but they could be associated with unfamiliar sensations or subtle changes in physiology. This raises the possibility that exposure to the infrasound component of wind turbine noise could influence the physiology of the ear.

Systematic Review 2013: Association between Wind Turbines and Human Distress

“…Although three of the studies [6][7]24] suggested that low-frequency sound energy wind turbines (i.e., infrasound below 20 Hz) may directly and negatively affect health, the level of evidence for these studies is also weak (expert opinions [7,24] and a review [6] citing these two studies).…”

“…Complaints resulting from the proximity to wind turbines vary in their nature, and distress is often attributed to different mechanisms, such as noise, visual impact, sleep disturbance, infrasound, and others [5][6][7]. Noise is the complaint that has been studied most often, especially given that environmental noise has become one of the major public health concerns of the 21st century [8].…”

‘Wind turbine syndrome’: fact or fiction?

“…[5][6][7] Low frequency noise is generated by many everyday sources such as household appliances, transportation and industrial machinery. 8,9 There are natural sources of infrasound such as ocean waves and volcanoes, as well as internal sources such as the heartbeat and respiration. A child playing on a swing experiences levels of infrasound of approximately 110 dB.…”

“…3 Under good conditions with low wind speeds, the sound of wind turbines can appear benign or 'difficult to discern above the rustling of trees', as described by the British Wind Energy Association. 8 However, in suboptimal conditions the aerodynamic noise varies as the blades turn with changes in the strength and direction of wind. 3 Several studies have indicated that this amplitude modulated sound, rather than being a constant, steady type of noise, is more easily perceivable, and hence more likely to cause annoyance or health symptoms.…”

“…8,13 To become audible, the sound pressure level for a 20 Hz sound needs to be approximately 75 dB, compared with 4 dB for a 1000 Hz sound. 8,13 Historically, infrasound and low frequency noise have attracted much adverse publicity mainly based on misunderstanding and media hype, with stories such as the worker who 'fell dead on the spot' and the ability of infrasound to 'knock down buildings'. 12,13 Wind turbine syndrome has reinvigorated these anxieties about possible adverse health effects of infrasound.…”