The Sound Beneath the Floors

At its core, sound is a relatively simple natural phenomenon caused by pressure pulsations or vibrations propagating through various mediums in the world around us. We are constantly immersed in this phenomenon to the point that most sound becomes white noise that we tend to ignore.

Studies have shown that the complete absence of sound can drive a person insane, causing them to experience hallucinations. Likewise, loud and overwhelming sound can have the same effect. This especially holds true in manufacturing and plant environments where loud noises are the norm.

A New Jersey-based food processing plant began experiencing this issue firsthand after building a new manufacturing space. Some of the plant's equipment is powered by a series of hydraulic systems in a central pump room that contains a number of dual-stage hydraulic pumps. These 3,000 psi to 5,000 psi pumps push fluid through steel pipes that run under a conference room and up the side of a wall next to offices before moving out into the plant.

Unfortunately, out of sight out of mind isn't always the case with hydraulic machinery. The pipes connected to the pump were hard-clamped to the building structure to prevent them from moving around. This setup created a severe noise issue as the hydraulic pump went on stroke and pushed hydraulic fluid through the pipes, rendering the office space and a conference room almost unusable.

The unbearable and disorienting noise wasn't coming from the pump itself. The pulsations, produced by the pump, created vibrations that radiated noise through the pipes and off of machine surfaces.

Norm Dotti, consulting acoustical engineer, Russell Acoustics, diagnosed the source of the noise problem. "The pipes were singing as fluid was pumped through them," he said. "They would shake the steel walls and cause them to radiate sound. The entire building structure around the office space was essentially acting as a loudspeaker to convey that sound."

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A simple house fan can be used to provide a layman's view of the pump sound effect. If you take the number of blades on a fan (or the number of pistons in a pump) times its rotational speed, that fan (or pump) is going to put out a tone at the rotational speed called blade passage frequency. A helicopter does the same thing. Take the number of blades times the speed at which the rotor turns and you get a component sound or vibration at that frequency.

In our food processing plant scenario, even detaching the piping from the structure wouldn't completely resolve the issue. The force and sound was coming from pulsations in the hydraulic fluid running down the pipeline. These pressure pulsations flowing through the lines are difficult to absorb since hydraulic fluid is hard to compress. Dotti determined that the best, most efficient solution would be to utilize a muffler.

The muffler needed for this application is very different from the commonly known car muffler used in exhaust systems. Rather than stifling the force of airborne sound, the muffler needed to reduce the pressure pulsations in the hydraulic fluid. Sound can travel through air, water, or even hydraulic fluid, but properties such as viscosity and speed at which sound travels varies for each medium.