Evaluating and treating the plant for noise control

Research continues to point to noisy machinery as a culprit in poor communication, lost-time accidents and higher insurance costs in plant operations. Constant exposure to noise creates employee stress and fatigue, and noisy operations can cause noise induced hearing loss (NIHL). But machines don't have to bear the blame.

By Eric Johnson, illbruck, inc., Minneapolis, MN April 15, 2002
Key concepts
Effective noise control requires thorough analysis.
Noise travels by both direct and indirect paths.
Noise can be reduced at the source, along the path, and at the receiver.
Listening: the first step
Direct and indirect noise sources
Sound evaluation
Source-Path-Receiver Model
At the source: contain noise
At the receiver: suppress noise
Noise terminology

Research continues to point to noisy machinery as a culprit in poor communication, lost-time accidents and higher insurance costs in plant operations. Constant exposure to noise creates employee stress and fatigue, and noisy operations can cause noise induced hearing loss (NIHL). But machines don’t have to bear the blame. The noise they make can be absorbed, contained, or otherwise isolated so employees can work in more comfortable — not to mention OSHA-compliant — facilities. Knowing how to evaluate your plant for excessive noise and learning how to make it quieter are value-added steps you can take to improve safety, comfort, and productivity.

Listening: the first step

According to OSHA, employees run the risk of hearing loss if they are exposed, without hearing protection, to more than 85 decibels (dB) averaged over 8-hr. Yet noise is not always considered a factor when employees burn out from fatigue or stress, or misunderstand instructions. Our ears may be so accustomed to audible safety violations that we don’t notice them.

Take a listening tour through your plant. Noise can be constant or intermittent, high-pitched or low, from one source or several. Regardless of its characteristics, noise can be contained or absorbed so employees don’t have to shout to communicate with, warn, or help one another.

Direct and indirect noise sources

Sound reaches workers two ways, directly or indirectly. Direct sound travels in a straight line from the source to the ear. A press operator standing at the machine hears its noise in a direct path before it bounces off walls and ceilings.

Indirect sound is what workers hear after sound has been reflected from surrounding surfaces. Generally, factories are constructed from hard, nonabsorbent surfaces such as concrete or brick, which cause excessive noise throughout a plant. Very little of the noise is absorbed by these walls, floors, or ceilings; most is reflected back into the room. A successful noise control program will effectively absorb or contain these reflections before they reach employees’ ears.

Sound evaluation

Once you know the source of noise, you can measure it to determine how much you need to reduce it. Though sound measurement is based on straightforward calculations, you might want to enlist the help of a sound control specialist to help you determine the acoustics of your workplace. Using a handheld sound level meter, a specialist will take specific sound measurements and analyze them to recommend the best results for your facility. This procedure avoids the problem of applying the wrong noise control solution, with the result that the room remains noisy.

The goal is to measure the noise source in decibels, Hertz, and reverberation time (see sidebar). These acoustical “dimensions” will determine how much acoustical control material is needed.

Source-Path-Receiver Model

Once you know the sources, quantity, and quality of noise in your plant, you can determine how to control it. An effective evaluation takes into account three different ways to control noise: at the source, along the path of the noise, or at the receiver.

At the source: contain noise

Containing noise keeps it confined within a space. An acoustic curtain, shield, or solid wall that physically blocks sound energy can cut noise levels by as much as 20 dB. The side of the curtain or barrier that faces the machine is usually covered with acoustic foam to absorb and dissipate the blocked energy. A successful sound-control curtain or enclosure contains noise while still allowing access for visibility, operations, or maintenance.

Containing noise at the source can also mean building a shield around a single piece of equipment. This is especially effective for mid-to-high-frequency noises, such as compressors and saws. Such shields can be made of plywood or sheet metal with noise-absorbing foam attached to them, or you can use flexible noise barrier curtains which isolate and contain noise.

In large, open shops with lots of small noise sources, the only way to control reverberating noise is through absorption. For example, the clinks, rattles and hisses in a bottling plant reflect off of walls and ceilings. Convoluted foam products applied to walls or suspended from ceilings soak up these reflections, reducing unwanted reverberation and echo. Noise levels can be decreased by 20% to 30% — a 4-to-6 dB reduction in noise — when walls or ceilings are treated with noise absorption material.

At the receiver: suppress noise

Finally, you can control noise at the receiver, which means providing employees with earplugs, earmuffs, or other hearing protection devices. Though economical, earplugs and earmuffs can be uncomfortable to wear; they can also prevent communication between employees. As a result, many employees tend to tolerate the noise rather than the discomfort or inconvenience of the ear protection. This is especially true in cases of intermittent noise, when a machine such as an air compressor cycles on and off.

An effective hearing protection program reduces noise without compromising communication or comfort. It shows employees you are concerned for their safety and well being and the quality of the workplace.

Hearing loss prevention programs reduce accident rates and lost time, and contribute to lower insurance rates. Employees recognize and appreciate employers who protect their health and well being. A successful hearing loss prevention program makes good sense and good business.

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

Noise terminology

Absorption coefficient – The amount of sound an acoustical material absorbs (and converts to heat) rather than reflects. An absorption coefficient of 0.90 means that the material absorbs about 90% of the noise that strikes it at that frequency.

Decibels (dB): The quantity measurement for sound; a measure of sound pressure expressed on a logarithmic scale.

The human ear can hear extremes in sound energy. Decibels were developed as a unit of measurement to encompass these extremes. Zero decibels corresponds to the threshold of audibility, while 130 dB corresponds to the threshold of pain. Calculated on a logarithmic scale, decibels measure the relative change in audible sound.

Basically, a 10-dB increase in noise sounds twice as loud, though it contains 10 times the energy in terms of sound pressure. For example, if Machine A produces 90 dB of noise and Machine B produces 80 dB of noise, then Machine A sounds twice as loud. A 3-dB difference is the smallest change we can hear, yet it represents a 50% increase or decrease in sound energy.

Hertz (Hz): The sound “quality” measurement; frequency of sound expressed in cycles per second.

The quality of sound is described as its pitch, which is dependent on frequency. Frequency is measured in units of Hertz (Hz), where 1 Hz is equal to one sound wave cycle per second. Low-frequency noise (under 250 Hz) is the most difficult to control. Middle-frequency noise (from 500 to 2500 Hz) coincides with human speech and, therefore, interferes with verbal communication. Higher frequencies (above 2500 Hz) are annoying because human ears are sensitive to high-pitched noises.

Noise reduction coefficient (NRC) – The average of the absorption coefficients at the most common frequencies (250, 500, 1000, and 2000 Hz). The NRC is often used to compare the acoustical performance of various materials.

Reflection – The amount of sound wave energy (sound) that is reflected off a surface.

Reverberation – Sound that continues to reflect off surfaces until the sound wave loses energy and eventually dies out.

Reverberation time — The number of seconds required for sound to decay 60 dB (1/1,000,000 of its original sound level) after the sound source has stopped. Reverberation time is the basic acoustical property of a room. It is dependent on the room’s dimensions and the absorptive properties of the room’s surfaces and contents.

Sabin – A unit of sound absorption based on 1 sq ft of material. 1 sabin = 100% absorption in 1 sq ft of material.

Sound Transmission Class (STC) – A measure of how much noise is stopped by a barrier or enclosure. It is an approximation of how much noise, in decibels, will be stopped by the material, assuming an airtight seal around the noise source.

Time-weighted average – An OSHA measurement, expressed in decibels, of a worker’s exposure to noise. If a worker is exposed to what OSHA considers to be the equivalent of over 85 dB in an 8-hr period, hearing tests and ear protection must be provided.