Understanding industrial noise

Noise is unwanted sound. That statement seems pretty simple. But sound, in turn, is pressure, frequency, intensity, amplitude, wavelength, and a few other things. Noise is also aggravating, dangerous, and expensive. And if not properly controlled in the workplace, noise can be illegal. Excessive exposure can result in noise-induced hearing loss (NIHL), hypertension, and elevated blood pressur...

By Richard L. Dunn, Chief Editor, Plant Engineering Magazine June 1, 2001

Noise is unwanted sound. That statement seems pretty simple. But sound, in turn, is pressure, frequency, intensity, amplitude, wavelength, and a few other things. Noise is also aggravating, dangerous, and expensive. And if not properly controlled in the workplace, noise can be illegal.

Excessive exposure can result in noise-induced hearing loss (NIHL), hypertension, and elevated blood pressure levels. According to OSHA, occupationally induced hearing loss is one of the leading occupational illnesses in the U.S. Chronic NIHL is a permanent sensorineural condition that cannot be treated medically. It is initially characterized by a declining sensitivity to high-frequency sounds, usually above 2000 Hz.

In addition to its effects on hearing, noise:

Interferes with understanding speech

Causes stress

Interferes with sleep

Lowers morale

Reduces efficiency

Causes annoyance

Interferes with concentration

Causes fatigue.

Noise measurement

Noise is generally measured in decibels (dB), which is a unit expressing the ratio of the sound pressure level being measured to the reference sound pressure level, usually 0.0002 microbars (dyn/sq cm). The equation for decibels is:

Sound pressure level (dB) =

10 log 10 (Measured pressure)2/ (Reference pressure)2

Where: Reference pressure = 0.0002 dyn/sq cm

Most discussions of sound include a number of terms of questionable meaning to the novice, such as amplitude, loudness, gain, volume, pressure, intensity, frequency, pitch, quality, spectral distribution, octave bands, impedance, phons, and sones. Some of these terms are objective; others are subjective.

Human ears have variable sensitivity to frequency as well as to pressure. Although no two people’s ears respond in exactly the same way, the sensitivity curve shown indicates that most people hear best in the 500-5000 Hz range — the range in which speech falls.

The reason the decibel has become the basic unit of measure is that it is an objective measurement that roughly corresponds to the subjective measurement made by the human ear. Because sound is composed of several distinct and measurable parts, and since the human ear does not differentiate between these parts, measuring scales which approximate the ear’s reaction have been adopted. These are the A, B, and C scales on a standard sound level meter.

The A, B, and C scales are frequency weighted through filtering systems to more closely duplicate the ear’s response over the scales’ respective frequency ranges. The A scale, which is the most important because of its adoption as the standard in OSHA regulations, approximates the ear’s response to frequencies over the lower range of sound pressure level; that is, below 55 dB.

The B and C scales do the same for 55—85 dB and above 85 dB, respectively. The C scale is the only one that indicates the actual sound pressure level. Scales are important, because the human ear’s response to sound varies with the sound’s frequency.

Zero decibel has been rather arbitrarily adopted as the threshold of hearing for the “average” individual. The actual threshold varies as much as

Because of the logarithmic nature of decibels, they are not additive. Combining a 60-dBA sound level with another 60-dBA sound results in a 63-dBA total sound level — not 120 dBA. Doubling the sound pressure level increases a dBA reading by six.

A noise dose is the length of time a person is exposed to a noise above a specified dBA level. Noise dosage is important because the danger to hearing is both time and pressure-level dependent. As the dBA level increases, the safe amount of exposure time decreases. OSHA specifies a threshold level for noise measurement purposes of 80 dBA.

Loudness, volume, and intensity

Loudness and volume are terms used to describe the effect of a combination of factors on the human ear. The most important factors are sound pressure (dB) and frequency (Hz). Frequency is most closely related to what the ear hears as pitch, which we think of as “high” (like squeaks) or “low” (like thunder). The important concept to remember is that loudness varies with frequency. A high-frequency sound at 80 dB seems louder to the listener than a low-frequency sound at 80 dB.

Equal loudness contours indicate the sound pressure level necessary at each frequency to produce the same loudness response in the average listener.

Humans can hear sounds between 16—20,000 Hz. Most speech falls between 500—2000 Hz. This range is where the ear is most sensitive and, consequently, the range of most significance in noise control.

As with light, sound also has an intensity factor that follows the inverse square law — as the distance is doubled, the intensity is quartered. Since sound intensity is so closely related to sound pressure level and both are expressed in decibels, there is little need to differentiate between the two for practical purposes. Intensity is commonly referred to as constant (steady state) or variable — the latter being sporadic, cyclic, or impulsive. It is possible for a number of combined variable-intensity sounds to create the effect of a steady-state noise.

Octave bands

Since frequency is an important component of any sound, it is usually necessary to study a sound in terms of its sound pressure level at various frequencies. It is practically impossible to measure the pressure level at each frequency, so the audible frequency range has been divided into sections called octave bands .

In each octave band, the upper cutoff frequency is twice the lower cutoff frequency. The preferred octave bands have center frequencies of 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, and 16,000 Hz.

Permissible noise exposures(Table G-16, OSHA Regulations)

Duration/day, hr
Sound level, dBA slow response









1/4 or less

When the daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: C(1)/T(1) + C(2)/T(2)…C(n)/T(n) exceeds unity, then the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140-dB peak sound pressure level.

Richard L. Dunn, Chief Editor, 630-320-7141, rdunn@cahners.com

Important noise-related references

OSHA Technical Manual, Sec III: Chap 5. Noise Measurement.

osha-slc.gov/dts/osta/ otm/otm_iii otm_iii_5.html

OSHA Technical Link. Noise and Hearing Conservation.


OSHA Regulations (Standards – 29 CFR) Occupational Noise Exposure. – 1910.95.


Also search site for “noise” for numerous other references.

NIOSH recommendations and other material on “Noise and Hearing Protection” (list).


Acoustical Society of America compact disc, Auditory Demonstrations.

For ordering information, see asa.aip.org/discs.html

Sound measuring instruments sources

Ametek Test&Calibration Instrumentsametek.com

Analogic Corp.analogic.com

AVO Intl.avointl.com

Balmac, Inc.balmac.thomasregister.com


Cole-Parmer Instrument Co.coleparmer.com

Davis Instrument Mfg. Co., Inc.davisontheweb.com

Extech Instruments Corp.extech.com

Fieldpiece Instruments, Inc.fieldpiece.com

FindMRO.com, a Grainger Co.findmro.com

HMC-Hub Material Co.hubmaterial.com

Industrial Process Measurement, Inc.instrumentation2000.com

Industrial Test Equipment Rentals (ITER)iterents.com

J&W Instruments, Inc.jwinst.com

Kernco Instrument Co., Inc.kerncoinstr.com

Lab Safety Supply, Inc.labsafety.com


Milltronics, Inc.milltronics.com

Noise Control Co.noisecontrolco.com

Olympus America, IPGolympusipg.com

Ono Sokki Technology, Inc.onosokki.net


Pacer Industries, Inc.pacer-ind.com


Response Rentalsresponserentals.com


Speck Industrial Controls, Inc.speckvc.com

Superior Signal Co., Inc.superiorsignal.com

Techni-Tool, Inc.techni-tool.com

UE Systems, Inc.uesystems.com


Yokogawa Corp. of Americayca.com

Zack/Joseph Electronicszackinc.com