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Course 751 - Hearing Conservation Program Management

Safety guides and audits to make your job as a safety professional easier

The Basics

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Sound generates pressure waves of compression and decompression.

Sound and Noise

Sound - Sound propagates as a wave of positive pressure disturbances (compressions) and negative pressure disturbances (rarefactions), as shown image to the right. Sound can travel through any elastic medium (e.g., air, water, wood, metal).

Noise - is nothing more than unwanted sound. Occupational noise can be any sound in any work environment. In the workplace, sound that is intense enough to damage hearing is unwanted and is considered to be noise.

Check out this short audio clip by Dan Clark of the that gives a quick overview of nature of sound, noise, and ways to prevent hearing loss in the workplace.

Read the material in each section to find the correct answers to each of the questions. After answering all questions, click the "Check Quiz Answers" button to see your score and a list of missed questions. To correct a question, return to the question, review the material, change your answer, and return to the last section page. Click the "Check Quiz Answers" again to recheck the results.

Do not refresh these pages or you'll have to answer all questions again.

Note: Videos and exercises in our courses are for information only and not required to view. Final exam questions will not be derived from the videos. OSHAcademy is not responsible for video content.

1. What is noise?

a. Pressure changes
b. Unwanted sound
c. Decibels
d. Varying frequencies

Next Section

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Parts of the outer, middle, and inner ear.

How does the ear work?

The function of the ear is to gather, transmit, and perceive sounds from the environment. This involves three stages:

  1. Outer ear. Modification of the acoustic wave by the outer ear, which receives the wave and directs it to the eardrum. Sound reaches the eardrum as variations in air pressure.
  2. Middle ear. Three small bones amplify and transmit the vibrations generated by the sound to the inner ear. They are called the:
    1. malleus (or hammer),
    2. incus (or anvil), and
    3. stapes (or stirrup)
  3. Inner ear. The vibrations from the middle ear are then transmitted as wave energy through the fluid within a snail-like structure called the cochlea in the inner ear. The cochlea is lined with about many thousands of microscopic hair-like cells that move with the vibrations in the fluid and convert the waves into nerve impulses - the result is the sound we hear. If the vibrations are too intense, over time, these microscopic hairs can be damaged, causing hearing loss.

2. How do the microscopic hairs in the cochlea produce the sound we hear?

a. They strike each other in a wave-like manner, producing sound
b. They lengthen and shorten changing the frequency of their output
c. They vibrate and convert sound waves into nerve impulses
d. They produce an electrical charge that is converted to sound

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Types of Hearing Loss

Types of Hearing Loss
Sensorineural hearing loss is more common in the workplace. Image Credit: Hough Ear Institute

Hearing loss can be described as either conductive or sensorineural, or a combination of the two.

Conductive hearing loss. This type of hearing loss results from any condition in the outer or middle ear that interferes with sound passing to the inner ear. Excessive wax in the auditory canal, a ruptured eardrum, and other conditions of the outer or middle ear can produce conductive hearing loss.

Although work-related conductive hearing loss is not common, it can occur when an accident results in a head injury or penetration of the eardrum by a sharp object, or by any event that ruptures the eardrum or breaks the ossicular chain formed by the small bones in the middle ear (e.g., impulsive noise caused by explosives or firearms).

Sensorineural hearing loss. Sensorineural hearing loss is the most common type of hearing loss on the job. This type of hearing loss is a permanent condition that usually cannot be treated medically or surgically. Sensorineural hearing loss is associated with a problem occurring in either the inner ear or the auditory nerve, which delivers sound to the brain. The normal aging process and excessive noise exposure are both notable causes of sensorineural hearing loss.

Studies show that exposure to noise damages the sensory hair cells that line the cochlea. Even moderate noise can cause twisting and swelling of hair cells and biochemical changes that reduce the hair cell sensitivity to mechanical motion, resulting in auditory fatigue. As the severity of the noise exposure increases, hair cells and supporting cells disintegrate and the associated nerve fibers eventually disappear.

3. Which type of hearing loss is the most common on the job?

a. Mixed types
b. Conductive
c. Short-term
d. Sensorineural

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How Sound is Measured

The Decibel (dB) is a measure of the amount of sound pressure. The dB is measured on a logarithmic scale which means that a sound with an intensity that is twice that of a reference sound corresponds to an increase of little more than 3 decibels.

Frequency (f) is a measure of the number of vibrations (i.e., sound pressure cycles) that occur per second. It is measured in hertz (Hz), where one Hz is equal to one cycle per second. The pitch of a sound - how high or low it seems - is how you perceive its frequency; the higher the pitch, the higher the frequency. High-frequency sounds are generally more annoying than low-frequency sounds and can be more harmful to hearing.

Human speech frequencies are in the range of 500 Hz to 4,000 Hz and is most sensitive to frequencies between 3,000 and 4,000 Hz. That's why people with damaged hearing have difficulty understanding higher-pitched voices and other sounds in the 3,000 to 4,000 Hz range.

Check out the CDC Noise Meter page to get a better idea how "loud" is loud.

Take this free online hearing test to see if you have hearing loss.

Click on the button if you want more comprehensive information on the various qualities of sound from OSHA's Technical Manual, Section III, Chapter 5.

Basic Qualities of Sound

Note: This discussion goes beyond the scope of training for this course and is for your information only, and is not testable.

Wavelength. The wavelength (λ) is the distance traveled by a sound wave during one sound pressure cycle, as shown in Figure 2. The wavelength of sound is usually measured in meters or feet. Wavelength is important for designing engineering controls. For example, a sound-absorbing material will perform most effectively if its thickness is at least one-quarter the wavelength.

Frequency. Frequency, f, is a measure of the number of vibrations (i.e., sound pressure cycles) that occur per second. It is measured in hertz (Hz), where one Hz is equal to one cycle per second.

Sound frequency is perceived as pitch (i.e., how high or low a tone is). The frequency range sensed by the ear varies considerably among individuals. A young person with normal hearing can hear frequencies between approximately 20 Hz and 20,000 Hz. As a person gets older, the highest frequency that he or she can detect tends to decrease.

Human speech frequencies are in the range of 500 Hz to 4,000 Hz. This is significant because hearing loss in this range will interfere with conversational speech. The portions of the ear that detect frequencies between 3,000 Hz and 4,000 Hz are the earliest to be affected by exposure to noise. Audiograms often display a 4,000-Hz "Notch" in patients who are developing the beginning stages of sensorineural hearing loss.

Speed. The speed at which sound travels, c, is determined primarily by the density and the compressibility of the medium through which it is traveling. The speed of sound is typically measured in meters per second or feet per second.

Speed increases as the density of the medium increases and its elasticity decreases. For example:

In air, the speed of sound is approximately 344 meters per second (1,130 feet per second) at standard temperature and pressure. In liquids and solids, the speed of sound is much higher. The speed of sound is about 1,500 meters per second in water and 5,000 meters per second in steel.

Sound Pressure. The vibrations associated with sound are detected as slight variations in pressure. The range of sound pressures perceived as sound is extremely large, beginning with a very weak pressure causing faint sounds and increasing to noise so loud that it causes pain.

The threshold of hearing is the quietest sound that can typically be heard by a young person with undamaged hearing. This varies somewhat among individuals but is typically in the micropascal range. The reference sound pressure is the standardized threshold of hearing and is defined as 20 micropascals (0.0002 microbars) at 1,000 Hz.

The threshold of pain, or the greatest sound pressure that can be perceived without pain, is approximately 10 million times greater than the threshold of hearing. It is, therefore, more convenient to use a relative (e.g., logarithmic) scale of sound pressure rather than an absolute scale (OTM/Driscoll).

Decibels. Noise is measured in units of sound pressure called decibels (dB), named after Alexander Graham Bell. The decibel notation is implied any time a "sound level" or "sound pressure level" is mentioned.

Decibels are measured on a logarithmic scale: a small change in the number of decibels indicates a huge change in the amount of noise and the potential damage to a person's hearing. It is not proper to add dB values by normal algebraic addition.

Sound Fields. Many noise-control problems require a practical knowledge of the relationships between:
  • A sound field. A region in which sound is propagating) and two related concepts.
  • Sound pressure. The intensity of sound pressure emitted from the sound source that is influenced by the distance from the sound source, and the surrounding environment.
  • Sound power. The sound energy emitted from a sound source that is not influenced by the surrounding environment.

Sound fields are categorized as near, far, and free fields.

  1. The near field is the space immediately around the noise source, sometimes defined as within the wavelength of the lowest frequency component (e.g., a little more than 4 feet for a 25-Hz tone, about 1 foot for a 1,000-Hz tone, and less than 7 inches for a 2,000-Hz tone). Sound pressure measurements obtained with standard instruments within the near field are not reliable because small changes in position can result in big differences in the readings.
  2. The far field is the space outside the near field, meaning that the far field begins at a point at least one wavelength distance from the noise source. Standard sound level meters (i.e., type I and type II) are reliable in this field, but the measurements are influenced by whether the noise is simply originating from a source (free field) or being reflected back from surrounding surfaces (reverberant field).
  3. A free field is a region in which there are no reflected sound waves. In a free field, sound radiates into space from a source uniformly in all directions. The sound pressure produced by the source is the same in every direction at equal distances from the point source. As a principle of physics, the sound pressure level decreases 6 dB, on a Z-weighted (i.e., unweighted) scale, each time the distance from the point source is doubled. This is a common way of expressing the inverse-square law in acoustics and is shown in Figure 4.

Sound Power. Up to this point, this discussion has focused on sound pressure. Sound power, however, is an equally important concept. Sound power, usually measured in watts, is the amount of energy per unit of time that radiates from a source in the form of an acoustic wave. Generally, sound power cannot be measured directly, but modern instruments make it possible to measure the output at a point that is a known distance from the source.

Understanding the relationship between sound pressure and sound power is essential to predicting what noise problems will be created when particular sound sources are placed in working environments. An important consideration might be how close workers will be working to the source of sound. As a general rule, doubling the sound power increases the noise level by 3 dB.

As sound power radiates from a point source in free space, it is distributed over a spherical surface so that at any given point, there exists a certain sound power per unit area. This is designated as intensity, I, and is expressed in units of watts per square meter.

Sound intensity is heard as loudness, which can be perceived differently depending on the individual and his or her distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases. The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense. In other words, the sound power level of a source is independent of the environment. However, the sound pressure level at some distance, r, from the source depends on that distance and the sound-absorbing characteristics of the environment (OTM/Driscoll).

Filtering. Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source. To properly represent the total noise of a source, it is usually necessary to break it down into its frequency components. One reason for this is that people react differently to low-frequency and high-frequency sounds. Tor the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise. Engineering solutions to reduce or control noise are different for low-frequency and high-frequency noise. As a general guideline, low-frequency noise is more difficult to control.

Loudness and Weighting Networks. Loudness is the subjective human response to sound. It depends primarily on sound pressure but is also influenced by frequency.

Three different internationally standardized characteristics are used for sound measurement: weighting networks A, C, and Z (or "zero" weighting). The A and C weighting networks are the sound level meter's means of responding to some frequencies more than others. The very low frequencies are discriminated against (attenuated) quite severely by the A-network and hardly attenuated at all by the C-network. Sound levels (dB) measured using these weighting scales are designated by the appropriate letter (i.e., dBA or dBC).

  • The A-weighted sound level measurement is thought to provide a rating of industrial noise that indicates the injurious effects such noise has on human hearing and has been adopted by OSHA in its noise standards (OTM/Driscoll).
  • In contrast, the Z-weighted measurement is an unweighted scale (introduced as an international standard in 2003), which provides a flat response across the entire frequency spectrum from 10 Hz to 20,000 Hz.
  • The C-weighted scale is used as an alternative to the Z-weighted measurement (on older sound level meters on which Z-weighting is not an option), particularly for characterizing low-frequency sounds capable of inducing vibrations in buildings or other structures.

4. Human hearing is most sensitive to frequencies between _____.

a. 1,000 and 2,000 Hz
b. 1,000 and 5,000 Hz
c. 3,000 and 4,000 Hz
d. 3,000 and 8,000 Hz

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The Perils of Exposure

The extent of damage to hearing depends primarily on the intensity of the noise and the duration of the exposure. Hearing loss caused by noise can be temporary or permanent.

  • Temporary hearing loss results from short-term exposures to noise, with normal hearing returning after a period of rest.
  • Permanent hearing loss is gradual, and can be caused by prolonged exposure to high noise levels and chemicals.

Loud noise can also create physical and psychological stress, reduce productivity, interfere with communication and concentration, and contribute to workplace accidents and injuries by making it difficult to hear warning signals.

Noise-induced hearing loss limits your ability to hear high-frequency sounds, understand speech, and seriously impairs your ability to communicate.

The effects of hearing loss can be profound because it can interfere with your ability to enjoy socializing with friends, playing with your children or grandchildren, or participating in other social activities you enjoy. It can also lead to psychological and social isolation.

5. What are the two factors that determine the amount of hearing loss a person experiences when exposed to loud noises?

a. Decibels and frequency
b. dBA and A-weighted sound levels
c. Intensity and duration
d. Sound and noise

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Exposure to chemicals

Damaged earhair cells.
Repeated exposures to loud sounds and chemicals can have serious effect on hearing.

Research demonstrates exposure to certain chemicals, called ototoxicants, may cause hearing loss or balance problems, regardless of noise exposure. Substances, including certain pesticides, solvents, and pharmaceuticals that contain ototoxicants can negatively affect how the ear functions, causing hearing loss, and/or affect balance.

Combined exposures to repeated loud sounds and chemicals can cause more hearing loss than exposure to either agent alone. This combination often results in hearing loss that can be temporary or permanent, depending on the level of noise, the dose of the chemical, and the duration of the exposure. This hearing impairment affects many occupations and industries, from machinists to firefighters.

Effects on Hearing

Harmful exposure to ototoxicants may occur through inhalation, ingestion, or skin absorption. Health effects caused by ototoxic chemicals vary based on exposure frequency, intensity, duration, workplace exposure to other hazards, and individual factors such as age.

Effects may be temporary or permanent, can affect hearing sensitivity, and result in a standard threshold shift. Since chemicals can affect central portions of the auditory system, not only do sounds need to be louder to be detected, but also they lose clarity.


The first step in preventing exposure to ototoxicants is to know if they are in the workplace. One way to identify ototoxicants in the workplace is by reviewing Safety Data Sheets (SDSs). Other strategies to help prevent exposure includes:

  • Providing health and safety information as well as training to workers exposed to hazardous materials, including ototoxic chemicals.
  • Replacing a hazardous chemical with a less toxic chemical is an effective way to reduce exposure when ototoxicants are identified in the workplace.
  • Using engineering controls, such as isolation and enclosures to control exposure to ototoxicants and noise
  • Eliminating unnecessary tasks that cause noise or ototoxicant exposure, or operating noisy equipment when workers are not near.
  • Using appropriate personal protective equipment (PPE) including chemical-protective gloves, arm
  • sleeves, aprons, and other appropriate clothing.

6. Which two combined exposures can cause more hearing loss than exposure to either agent alone?

a. Repeated loud noise and chemicals
b. Intense light and loud sound
c. Constant wind and intense light
d. Intense light and high-frequency sound

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Exposure to Noise


Sometimes, overexposure to loud noise can trigger phantom noise such as ringing, humming, roaring, or other sounds in your ears. The condition is called tinnitus and it may be a symptom of damaged hearing caused by repeated loud noise, infections, medications, or earwax.

There are two kinds of tinnitus:

  1. Subjective tinnitus is the condition in which only you can hear the phantom noise. This is the most common type of tinnitus. It can be caused by outer, middle, or inner ear problems, or by problems with the hearing (auditory) nerves.
  2. Objective tinnitus is the condition in which the person examining you can hear the noise. This type of tinnitus is rare and caused blood vessel problems, a middle ear bone condition, or muscle contractions.

To know if noise has damaged your hearing, have a hearing examination that is conducted by a certified audiometric technician, audiologist, otolaryngologist, or physician.

Click on the button to see the questions you should ask if you suspect you might be suffering from tinnitus.

If you answer "yes" to any of the following questions, your hearing may be at risk:

  • Do you frequently ask people to repeat sentences?
  • Do you think your hearing is worse than it was 10 years ago?
  • Have family members noticed a problem with your hearing?
  • Are you exposed to loud noise without hearing protection where you work?
  • Do you have to shout to a co-worker because of the noise around you?
  • Are you exposed to noise from firearms, motorcycles, snowmobiles, power tools, or loud music without hearing protection?

7. If an audiologist cannot hear the ringing in your ear, you are most likely suffering from _____.

a. permanent tinnitus
b. subjective tinnitus
c. objective tinnitus
d. temporary tinnitus

Check your Work

Read the material in each section to find the correct answer to each quiz question. After answering all the questions, click on the "Check Quiz Answers" button to grade your quiz and see your score. You will receive a message if you forgot to answer one of the questions. After clicking the button, the questions you missed will be listed below. You can correct any missed questions and check your answers again.


The Hearing Video

Produced in the style of a TV science show, The WorkSafeBC Hearing Video uses vintage films, computer animation, and noise-defying stunts to demonstrate how your ears work and the effects of hazardous noise on your hearing. The video also demonstrates the proper use and maintenance of hearing protection and how to chose the right protection for you.

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