March 7, 2024 / Joshua Leasure

Noise Control Engineering in Hearing Conservation

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Organizations often initiate noise control in response to noise dosimetry surveys that have identified groups of workers who are at increased risk of noise-induced hearing loss. In an industrial environment, noise control means reducing the time-weighted average of noise levels a worker is exposed to throughout their shift.

Acoustics and noise control are not intuitive subjects. They involve logarithmic math that turns seemingly simple operations like addition and subtraction into multi-step formulas. Adding to the confusion, many quantities in this domain, including sound pressure, sound power, sound intensity, noise exposure, barrier insertion loss, and material transmission loss, are typically reported in decibels.

The role of a noise control engineer is to apply an understanding of acoustics to the industrial environment: first to identify the noise sources that present the highest risk to workers’ hearing and, second, to invent solutions that reduce that risk.

Noise Perception vs. Noise Impact

Human hearing is naturally tuned to certain types of sounds. Periodic noise, tones, or sounds dominant in low frequencies often catch our attention due to their distinctiveness. Loud, impulsive noises do present a serious risk to hearing, but these more perceptible noises do not drive workers’ noise exposures in the same way as constant, broadband sounds do. Constant noise, particularly in the 1,000 to 2,000 Hertz range, contributes the most to time-weighted exposures in many industrial environments. Continuous exposure accumulates to produce a higher dose over time and thereby poses a greater risk to hearing health.

Relying solely on auditory perception can lead to misjudging primary noise sources in the workplace. A noise control engineer must go beyond what their ears tell them and positively identify primary noise sources through methodical measurement and analysis. Noise sources have unique, identifying spectral signatures that can be matched to measurements taken at the locations where workers spend time. These signatures can be measured directly or inferred from characteristics of the noise source, such as how it is powered or at what speed it operates.

Prioritizing Source Controls

When many people think of noise controls, they immediately think of sound absorption panels. While panels are certainly among the most visible acoustic tools, they are not typically the most effective for lowering worker noise exposures and represent just one of many noise control strategies. Noise controls can be implemented at the source, along the path, or at the receiver. Source controls involve modifications to whatever is creating the noise, meaning that less sound energy enters the environment. Path controls, such as barriers, enclosures, and absorption panels, interrupt sound as it travels to the receiver. When a worker is the receiver of the noise, implementing controls at the receiver can mean creating a protected space for the worker to operate in or even relocating the worker to a safer location.

Controls that address the source are always preferable to controls along the path or at the receiver because source controls address the root cause of the problem. These controls are less susceptible to tampering by workers and generally do not hinder workers’ tasks. Additionally, source controls are often more cost effective compared to path controls. A prime example of an effective source control is adding a muffler to a machine’s pneumatic exhaust, which significantly reduces the noise emitted by the machine without impacting its operation.

It is not unusual for source controls to pay for themselves over time in material or energy costs. For example, quiet-design pneumatic nozzles operate with less air compared to their open-pipe counterparts and tend to generate energy savings that offset the cost of the nozzle in a matter of months. Other common source control strategies, like lowering operating pressures, adding regular maintenance inspections, or implementing vibration isolation, can result in equipment lasting longer and requiring less energy to operate.

Joshua Leasure will host a virtual professional development course (PDC) on noise control engineering from 8 a.m. to 5 p.m. Eastern time on March 26, 2024. To learn more about the PDC or to register, please visit AIHA's website.

Joshua Leasure

Joshua Leasure, PE, is the founder and principal consultant at Colorado Analytics LLC in Fort Collins, Colorado. He is also a contributing author of Chapter 15, "Community Noise," in the sixth edition of The Noise Manual by AIHA Press.


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