What was that you said?

According to acoustician Arline Branzaft of New York's Council of the Environment, whereas sound can be pleasant and soothing, "noise" is a sound we do not want to hear. Her 30 years of acoustics research confirms that regular exposure to noise can impair learning, generate health disorders and promote aggressive and criminal behaviour.

More than 30 recent studies show noise exposure can significantly reduce the I.Q.s of school-aged children. In one study, students in a classroom facing noisy train tracks were found to have I.Q.s fully seven points lower than students in the same grade and school but in a quieter classroom.

Although most people live in an environment that is immersed in noise, its impact on hearing is an unseen threat that is progressive over time. Studies at Harvard University show that about 50 per cent of young North Americans currently have partial deafness due to regular listening to loud music, mainly on headsets. More than 75 per cent of farmers have some degree of hearing loss owing to their exposure to noisy machinery. New data shows about 44 million North Americans have impaired hearing, about 25 per cent of which results directly from exposure to excessive noise.

Hearing is a complicated process involving sound energy that passes through the outer ear, through the tympanic membrane and is transmitted into the cochlea, where it triggers tiny hairs. Electric signals transfer acoustic energy to the brain. But, habitual exposure to loud noises puts excessive pressure on cochlear hair structures called steriocilia, and they eventually die. The first ones to die are those that are sensitive to high-pitched sounds, and their loss is permanent.

Hazards to hearing have been well-documented, and they are usually expressed in decibel (dB) figures. Common everyday noises can have high decibel values: 65 dB can cause stress or heart damage, 85 dB can result in permanent hearing loss and 130 dB can be excruciatingly painful to listen to.

Many of the noises we regularly encounter have very high decibel values: jet taking off 140; stock car racing 130; fire alarms and most sirens 140; rock concerts 140; wailing babies 110.

A mere eight hours of exposure to a noisy bulldozer (85 dB) can cause permanent hearing impairment. Loud music played in earphones can result in permanent hearing loss with only 15 minutes of daily exposure. A single clap of thunder generates up to 120 dB, posing an immediate hearing risk.

Many of the most serious threats are encountered regularly, and most people are not aware of the risks. Most hearing loss results from steady exposure, over a long period, to noises such as power tools, heavy traffic on streets, music blaring through headsets, living in proximity to airports and other exposure that most people tend to take for granted. According to psycho-acoustician William Hartman at Michigan State University, habitual exposure is bound to cause hearing impairment and is toxic to the ear.

Exposure to noise is also a health risk for pets and other animals. Although humans are capable of hearing sounds up to 16,000 hertz (frequency of sound in cycles per second), dogs' ears are much more sensitive and can hear up to 45,000 hertz (mice up to 91,000 hertz). Such sensitivity makes some animals very vulnerable to excessive noise; coclear degeneration caused by regular exposure to loud noises is common in dogs.

According to the Canadian Veterinary Medical Association, noise-induced hearing loss is common in household pets living in noisy environments, and the damage is as irreversible in animals as in humans.

New evidence from the University of Buffalo suggests that new drugs might, to some degree, delay hearing loss caused by exposure to noise.

Robert Alison is a Victoria-based writer with a Ph.D. in zoology.

Republished from the Winnipeg Free Press print edition June 24, 2009 A15