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Noise and Hearing Conservation

construction worker wearing earmuffs
Noise is usually defined as unwanted sound. The following sections provide information related to the origins of noise and its impact on our ability to hear:

Physics of Sound
sound pulse drawingSound is the physical phenomenon that stimulates our sense of hearing. It is an acoustic wave that results when a vibrating source, such as machinery, disturbs an elastic medium, such as air.
  • In air, sound is usually described as variations of pressure above and below atmospheric pressure. These fluctuations, commonly called sound pressure, develop when a vibrating surface forms areas of high and low pressure, which transmit from the source as sound.
Additional information (App I:A) on the physics of sound, including basic qualities, sound fields, sound propagation, filtering, loudness, and sound pressure weighting is also available.

Anatomy and Physiology of the Ear
diagram of the ear The ear is the organ that makes hearing possible. It can be divided into three sections:
  • External outer ear
  • Air-filled middle ear
  • Fluid-filled inner ear
The function of the ear is to gather, transmit, and perceive sounds from the environment.

This involves three stages:
  1. Modification of the acoustic wave by the outer ear, which receives the wave and directs it to the eardrum.
  2. Conversion and amplification of the modified acoustic wave to a vibration of the eardrum (transmitted through the middle ear to the inner ear).
  3. Transformation of the mechanical movement of the wave into nerve impulses that will travel to the brain, which then perceives and interprets the impulse as sound.
Additional information (App I:B) on outer ear, middle ear and inner ear is also available.

Effects of Excessive Exposure
Although noise-induced hearing loss is one of the most common occupational illnesses, it is often ignored because there are no visible effects, it usually develops over a long period of time, and, except in very rare cases, there is no pain. What does occur is a progressive loss of communication, socialization, and responsiveness to the environment. In its early stages (when hearing loss is above 2,000 Hertz (Hz)) it affects the ability to understand or discriminate speech. As it progresses to the lower frequencies, it begins to affect the ability to hear sounds in general.

The three main types of hearing loss are conductive (App I:C-1), sensorineural (App I:C-2), or a combination of the two.

The effects of noise can be simplified into three general categories:
  • Primary Effects, which includes noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus.
  • Effects on Communication and Performance, which may include isolation, annoyance, difficulty concentrating, absenteeism, and accidents.
  • Other Effects, which may include stress, muscle tension, ulcers, increased blood pressure, and hypertension.
In some cases, the effects of hearing loss may be classified by cause.

Additional information (App I:C) about the effects of excessive noise exposure is also available.

Ultrasound is high-frequency sound that is inaudible, or cannot be heard, by the human ear. However, it may still affect hearing and produce other health effects.

Factors to consider regarding ultrasonics include:
  • The upper frequency of audibility of the human ear is approximately 15-20 kilo-Hertz (kHz).
    • This is not a set limit and some individuals may have higher or lower (usually lower) limits.
    • The frequency limit normally declines with age.
  • Most of the audible noise associated with ultrasonic sources, such as ultrasonic welders or ultrasonic cleaners, consists of subharmonics of the machine's major ultrasonic frequencies.
    • Example: Many ultrasonic welders have a fundamental operating frequency of 20 kHz, a sound that is at the upper frequency of audibility of the human ear. However, a good deal of noise may be present at 10 kHz, the first subharmonic frequency of the 20 kHz operating frequency, and is therefore audible to most persons.
Additional information (App I:D) on ultrasonics and the applicability of OSHA's Occupational Noise Exposure standard, 1910.95, health effects and the American Conference of Governmental Industrial Hygienists' (ACGIH) Threshold Limit Values (TLVs) is also available.
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