Why Is It So Noisy?
In a 1930s pulp magazine entitled The Avenger, a character walking down a street saw a building explode. He did not hear it
explode because, according to the narrative, the sound was so loud that his hearing was temporarily stunned. He watched as
the building silently collapsed, and his hearing returned to normal a few pages later.
Naturally, a pulp magazine—especially one with a central character who can transform to look like anyone else in the
world—does not have to adhere strictly to the facts. However, in the real world, excess noise and subsequent hearing damage
and loss pose a significant risk in the workplace.
In 1991, the Occupational Safety and Health Administration’s (OSHA’s) Directorate of Compliance Programs issued a memo
instructing regional offices to cite employers for failure to record instances of occupational hearing loss. This is defined
as “an average shift in hearing of 25 dB or more at 2,000, 3,000 and 4,000 Hz in either ear, if an exposure in the work
environment either caused, aggravated or contributed to the case.” The shift must be calculated by comparing the current
hearing test with the original baseline audiogram for the employee, adjusting for age if appropriate. OSHA formally codified
hearing loss recordability as part of its record-keeping rule in 2001, and it was implemented on January 1, 2003. Hearing
loss is now officially recognized as a workplace hazard.
Loud Noise Can Affect More Than Hearing
Catherine Palmer, Ph.D., is director of the Center for Audiology and Hearing Aids at the University of Pittsburgh Eye and Ear
Institute and is an associate professor of otolaryngology and of communications science and disorders at the university. Dr.
Palmer notes there are many reasons why noise can be a problem. Audiologists worry about noise when it interferes with
communication or is loud enough to cause permanent hearing loss. Noise also contributes to the perception of tinnitus
(ringing in the ears), which can be an annoying or disabling condition. In addition, loud noise is known to elevate blood
pressure in some individuals.
David C. Byrne, a research audiologist for the National Institute for Occupational Safety and Health (NIOSH), agrees. “Noise
is often defined as ‘unwanted sound,'” he says. Sometimes the noise level is not loud enough to produce a health hazard, but
it may present a nuisance to nearby individuals. Everyday noise (i.e., loud music, recreational activities, barking dogs,
road/air traffic, etc.) can be annoying and is exacerbated by factors such as time of day, particular location (e.g.,
urban/rural) and the duration, volume and character of the noise. Too much noise can also be an issue in indoor office areas,
particularly where the noise is comprised of conversations that can be overheard between cubicles.
Is this as big a problem as OSHA believes? Byrne thinks it is. “Exposure to high sound levels results in the development of
noise-induced hearing loss, which can be a serious physical, psychological and social problem. Occupational hearing loss is
the most common occupational disease in the United States and is listed among the 21 priority research areas, as described in
the NIOSH National Occupational Research Agenda. Efforts to prevent occupational hearing loss appear to be hindered because
the problem is insidious and occurs without causing pain in affected workers. One consequence of noise-induced hearing loss
is a reduced quality of life due to the inability to communicate with family, friends and the general public. However, this
normally occurs after the hearing loss has progressed too far and the damage is irreversible.”
Absorption, Isolation and the Combination of Both
Dr. Palmer believes that there are several solutions to the problem of excess noise: “Remove oneself from the situation; wear
hearing protection; reduce the actual noise source; or treat the environment to reduce the noise that impacts the
individual.” Many sources of noise in the workplace cannot easily be reduced; therefore, environmental treatment must be
Byrne discussed the use of insulation in combating sound, saying, “The important concept to remember here is that there is a
distinction between absorption and attenuation (isolation) of sound.”
Absorption, as measured by the absorption coefficient (a), is desirable for reducing noise within a space, he went on to
explain. Sound absorption is realized by materials—usually porous and often lightweight—that dissipate the acoustical energy
as heat (in negligible amounts) when the sound waves propagate through. Tables containing frequency-by-frequency absorption
coefficients for various materials are printed in acoustical textbooks and manufacturer’s product data. The absorption
coefficient is always a number between 0 and 1 and represents the percentage of sound absorbed by the material (e.g., 0 = no
acoustical energy absorbed; 0.5 = 50 percent absorbed and 0.99 = 99 percent absorbed).
A high sound isolation factor, measured by the number of decibels lost in transmission, is desirable for preventing sound
transmission. This is achieved using materials that sound waves cannot easily penetrate. A good attenuating material is
typically nonporous, dense and relatively heavy. Unlike sound-absorption materials (e.g., soft foam), sound-isolation
materials usually have structural functions (e.g., concrete or bricks). Tables/charts are available that list the sound
transmission class (STC) of common construction materials.
Acoustical absorbers are usually poor attenuators or sound barriers. A good attenuator reflects sound waves and thus has a
low absorption coefficient. Manufacturers often combine materials with different properties to achieve both absorption and
isolation (e.g., urethane foam bonded to a dense substrate).
The Best Type of Insulation for the Job
When asked whether mineral wool or fiberglass is better for sound control, Byrne had mixed feelings. “The answer depends on
exactly what you want the material for,” he said. In general, both materials work on the same acoustical principle of
Open-cell or open-structure products are better than closed-cell or closed-structure for acoustical purposes. Some examples
of open-cell products are commonly known. “Mineral wool” refers to three types of insulation that are basically the same:
glass wool or fiberglass, which is made from recycled glass; rock wool, which is made from basalt (an igneous rock); and slag
wool, which is made from steel-mill slag. For many years, mineral wool was the most widely used insulation in the United
States, Canada and Europe. Although mineral wool is much heavier and costs more than fiberglass and cellulose, it offers some
substantial benefits such as being more heat resistant than fiberglass.
Regarding acoustics, the combined effect of the surface openings, internal structure, flow resistance and thickness
determines the absorption coefficient of the material. Proper installation is necessary for optimal performance of acoustical
materials. For example, unless approved by the manufacturer, a material should not be compressed to less than the original
thickness when being installed. Likewise, paint or other surface treatments/coverings may severely degrade a material’s
absorptive properties. To determine the best material for the job, it is helpful to consult the manufacturer or a table of
the representative physical characteristics and absorption coefficients for different forms/thicknesses of fiberglass and
mineral wool. The best-suited material for a particular application may depend on environmental conditions rather than
Important Elements of Sound Control
STC and noise reduction coefficient (NRC) are important elements of sound control. Everything in acoustics, particularly
regarding noise-control issues, is frequency dependent. Significantly different treatments/solutions may be required,
depending on whether the offending noise is at a very high or very low frequency. It can be helpful to condense a set of
acoustical properties into a single number.
STC is a single-number rating used primarily to measure the speech privacy of a barrier or other structure (e.g., walls,
doors, windows, office partitions, etc.). It is determined from a plot of frequency-specific transmission loss data measured
in 1/3-octave bands from 125 to 4,000 Hz. STC is often specified as a performance criterion by architects and engineers for
places where speech privacy is of primary interest. It is important to note that STC is measured in an acoustical laboratory
(where the test specimen is mounted between two reverberation chambers); therefore, the labeled STC value of a product will
be fully realized only if it is installed correctly.
NRC is also a single-number rating. It is typically used when specifying the desired and/or required absorptive
characteristics of a material. The NRC for a material can be obtained easily by taking the mathematical average of the
absorption coefficients at 250, 500, 1,000 and 2,000 Hz. Careful consideration is necessary when using NRC values to compare
or select materials because absorption coefficients can vary widely in frequency even if NRC values are similar.
Sound insulation comes in many forms, including panels with perforated steel coverings that help trap sound and fiberglass
cloth, which can be draped over an object and sealed with vinyl, for a less permanent control attempt.
When asked about a new advance in mastic, or insulating paint, Byrne said, “If you are referring to spray-on
thermal/acoustical (typically cellulose) insulation, then everything [we have] discussed regarding acoustical absorption
applies here as well. Again, the exact characteristics of the material and the final thickness will govern its acoustical
He continued, saying, “It looks like this particular sound-reduction paint coating also works on the principle of acoustical
absorption. In other words, this paint apparently adds an absorptive layer to the wall surface, where the incident sound
waves are dissipated (absorbed) by the coating instead of just bouncing back off the wall. In theory, this should work fine.
However, I would question its usefulness for a couple of reasons. First, the resulting ‘thickness’ is actually so thin that
only the very highest audio frequencies would likely be absorbed (typically, the lower the frequency, the greater the
thickness required). Also, they advertise a 30-percent reduction of sound. I’m not sure exactly what they are referring to or
how they calculated it, but a 30-percent reduction may only be measured as a 1- to 2-decibel difference, which is barely
noticeable to most people.”
Insulation Is Working
Excess sound remains a danger in commercial work areas. Even in an office, free from the sounds of spinning turbines and
pounding jackhammers, excess noise can be a problem. If not for insulation around us, the constant whine of a Xerox machine
or dot-matrix printer would be significantly more grating. At home, a teenager’s loud stereo can contribute to discomfort or
partial hearing loss. To combat such noises, we have the acoustical engineers of the insulation industry. They may not be
able to stop exploding buildings from causing temporary deafness, but they can help keep noise in the workplace under