How loud is too loud?
Hearing loss from workplace sound can gradually sneak up on individuals, starting with the bright young things who are the future of industry and stealthily advancing to the most experienced proportion over 45 who may have had a lifetime of exposure to date.
These valuable people, young and older, are the ones industry wants to protect and retain, because they have the vital skills and experience that are in very short supply amidst post-COVID global skills shortages occurring both in Australia and worldwide.
A US Chamber of Commerce analysis of labour shortages, for example, found recently that manufacturing is one of the sectors most affected by the ongoing dearth of talent, with about 55% of job offerings remaining unfilled.
Continued exposure to sound over just 70 dBA can cause hearing loss over time. That’s not much more than normal conversation or background in a busy office, and typically much less than vacuum cleaning equipment used within workplaces.
But sound levels exceeding 85 dBA — much less than the 100 dBA produced by many industrial machines — are considered hazardous and potentially damaging to human hearing, depending on the duration of the exposure, according to international authorities such as the US Environmental Protection Agency and the provisions of Australia’s Work, Health, and Safety Act.
“It is at this level of continued exposure for periods of eight hours or more that OH&S measures can become a statutory obligation, affecting business owners and plant, designers, manufacturers, importers, suppliers and installers,” said James Maslin, Technical Product Manager for NVH (noise, vibration and harness) prevention technology supplier Air Springs Supply Pty Ltd.
“It is important to realise that every increase of 3 dB represents a doubling of sound intensity or acoustic power,” he added. “So even what might appear to be slight increases are in fact substantial increases, because doubling of sound intensity can sneak up on a workplace very easily where heavy equipment is involved. The maximum exposure recommended for 100 dBA, for example, is just 15 minutes.
“As workplace noise OH&S regulations increase across national and state jurisdictions, industrial operations and engineering staff are being challenged to find the most effective, cost-efficient and reliable solutions to the issue of vibrating and intrusive machinery.”
The challenge exists across a broad spectrum of industry, from heavy industry — including mining, energy, bulk handling and metals manufacturing — through to processing industries, including food, beverage and primary processing. Typical NVH hotspots include:
- Compressors and generators
- Pumps and drives
- Sorters, sizers and vibrating bins
- Vibrating screens, conveyors and elevators
- Crushers and blenders
- Bulk handling and minerals processing equipment
- HVAC and refrigeration equipment
- Garbage compactors and waste-to-energy equipment.
The choice often lies between less expensive and less complex passive vibration isolation — which makes use of materials and mechanical linkages that absorb and dampen mechanical waves — or active vibration isolation, which often involves sensors and actuators that produce disruptive interference that cancels out incoming vibration.
Active vibration can be expensive, but passive vibration can produce results cost-efficiently. One cost-effective passive solution that achieves high isolation efficiencies is offered by air springs, which are the only passive isolator that operates on the principle of compressing a gas rather than deflecting a solid.
Air springs are, in essence, heavy-duty balloons that contain no internal moving parts to break or wear. Because of their differences (compared with isolators such as coil springs and solid rubber pads), air springs are the passive isolator with the lowest natural frequency — and the lower an isolator’s natural frequency, the better the isolation effectiveness. Typically inflated at 3–6 bar, air springs will deliver isolation efficiencies exceeding 90%.
With the use of an auxiliary reservoir, air springs can lower the natural frequency to further improve isolation effectiveness to even finer degrees (which can exceed 99.9% vibration isolation in demanding applications).
Air springs are also the only passive isolator that can achieve a reduction in the natural frequency, which is the frequency at which a system tends to oscillate in the absence of any driving or damping force.
An even simpler passive isolation option is presented by solid no-maintenance rubber and fabric-reinforced bias ply isolators (or die springs), which offer high load capacity with constant vibration isolation through changing loads. They will not bottom out like coil springs and offer low natural frequencies to provide high isolation, including forced frequencies in the range of 13–20 Hz.
The basic construction of the die spring includes a solid rubber core with a hollow centre, and fabric-reinforced body. They can eliminate potential problems of broken coil springs causing fragment damage to vibrating, stamping and metal forming equipment.
Die springs can be compressed by up to 40% of their free height at a cycling rate of 50 cycles per minute or less, and cycling rates of up to 300 cpm can be attained.
“Both air springs and die springs have built-in advantages, being compact, easy to install, maintenance-free and proven effectiveness. Plus, they cannot break or fail catastrophically in the way that some metal springs might,” Maslin said.
“The essence of their effectiveness as isolators is their simplicity. Safety officers and engineers understand there is no one technology that solves all workplace NVH issues, because this challenge varies immensely with the workplace and the issues being addressed. But having a simple solution available for many issues is a very good place to start the process.”
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