Noise control is the basis for effective hearing loss prevention; reducing or eliminating the hazard is the best way to control exposure. An effective noise control plan reduces liability and hearing conservation costs by controlling noise where it starts — at the source.

Buy quiet

Most new equipment manufacturers can provide noise-controlled equipment, but it is the responsibility of the purchaser to ask for it. All new equipment decision-making and bid specification development should include a comprehensive new equipment sound level specification. It does not need to be complex.

A purchase specification should contain measurement criteria. ANSI is developing a new technical report (ANSI B11-TR5) to help design effective and repeatable machine measurement procedures. Typically, measurement locations are defined at a one-meter envelope around the machine under test, about 1.5 meters from the floor (to approximate a worker’s hearing zone), no further than three meters apart. Include any operator console or workstation as well. Examples of measurement locations are reflected in Figure 1. The appropriate specification sound level is a function of the desired end. Specifying emissions of less than 90 dBA meets compliance requirements. But many companies who want to eliminate hearing conservation program requirements and minimize the risk of noise-induced hearing loss (NIHL) choose an 80 dB(A) purchase specification level, as the addition of multiple 80 dB sources should not add up to produce worker exposure exceeding 85 dB in most situations. For facilities with open floor plans and team management orientation, the 80 dB limit may result in ambient sound levels too loud for effective verbal communication to conduct team meetings; a lower limit or enclosed meeting spaces may be appropriate.

In some situations, vendors may be unable to provide required equipment at the desired sound level. You should include a documented means of providing variance from the purchase specification, describing the measured noise emitted by the machine, the responsibilities of each party to pursue further noise reduction, and the level of documentation required. This step will enable you to develop appropriate expectations and to plan for effects the equipment will have on workers before it comes into the plant.

What’s making all the racket?

Source identification is one of the most challenging parts of noise control engineering. Most industrial environments are comprised of a complex set of noise generators, so determining which one(s) to prioritize for control can be difficult.

When a process or noise problem is made up of a number of parts, try to operate them separately to assess how each contributes to the overall noise level. Run the fan without the motor, the motor without the fan, the pump alone, and so on. Octave band noise measurements of the overall noise generated by the system, compared to each of these individual measurements, can help identify the offending noise source and point out the right direction for pursuing controls.

Sometimes temporary noise barriers can help find the noise generator in question. Plywood boxes over the pump assembly, for example, may reduce noise sufficiently from that source to help determine its contribution to overall noise. A well-placed cardboard shipping box could accomplish the same end for some applications.

Plan the work — work the plan

Select and prioritize noise issues for control. Once noise problems find their way into the plant, it can be challenging to decide which issues to pursue from the myriad of noise problems in a large facility. Start with noise control projects that provide “double payoffs”; many noise controls have secondary benefits.

Minimizing and controlling the use of compressed air can reduce noise while saving energy. Your first priority is to repair air and steam leaks that provide no benefit to the process. Second, reduce process compressed air flow to the minimum required for the process. Third, make sure you’re using air nozzles specially designed for noise reduction. Replacing compressed air part movers with gravity-fed designs can provide significant benefits.

Reducing part fall distance and treating work surfaces for noise can also improve part quality and reduce in-process damage. Sliding parts rather than dropping them from machines and onto work surfaces can significantly reduce impact noise.

Several methods for control prioritization are available. Chapter 9 of the AIHA Noise Manual (AIHA Press, 2000) provides a system that takes into account most of the essential variables, resulting in a noise control priority factor derived from:

• Number of employees affected by the noise problem under consideration;
• Potential hearing damage from the level of noise generated by the source;
• Nature and spectrum of the noise;
• Potential for success of the control;
• Effect on productivity;
• Cost, both immediate and long-term.

This approach weighs each aspect to help determine which problems should be addressed first. Such a systematic approach can clarify the most problematic issues and help maximize return both in terms of noise reduction benefits and dollars spent.

Keep it up

Maintenance workers are key assets to any noise control program. Involve maintenance staff in every aspect of the noise control effort to ensure that controls put in place are maintained to provide the same benefits they had when new. A reasonable inventory of replaceable noise control air nozzles should be in maintenance stores, for example, so that the overworked pipefitter is not tempted to replace a carefully selected noise control with a nozzle that provides no noise control, or worse, an open air pipe.

Noise enclosures are typically the most easily defeated means of achieving noise control. Expensive enclosures are often removed from equipment, only to be set aside and not reassembled. Enclosures should be designed with the full understanding that access to equipment for maintenance is essential, and with durable seals so the noise reduction evident when the enclosure was new is maintained on reassembly.

Noise control technology is, for the most part, well understood and mature. Systematic application of a few fundamental principles, and appropriate professional help when warranted, is the best way to reduce noise and eliminate hearing loss from the workplace.