Sound is formed by vibrations that create an audible mechanical wave of pressure through a medium, usually air or water. In hydraulics, noise can be grouped into three categories: airborne noise, which travels from the air to the ear; fluid-borne noise, which is transmitted through the hydraulic system; and structure-borne noise, which is created when one component of a system propagates vibration through another component.
The factors that influence noise generation are summarized in Figure 1. Unfortunately, people often reference only input excitation and sound pressure or sound power. They tend to avoid the other factors that make up the physics of noise generation. Sometimes one part is dominant while others are not. Therefore, one must consider all of these factors when designing for low noise. Furthermore, the process applies separately to airborne, fluid-, and structure-borne, noise. Each application is unique, so you can’t assume that what works in one system or assembly will work in another.
A Closer Look at Noise Sources
Simply put, noise is any unwanted sound. More technically, it is the unwanted byproduct of fluctuating forces in a component or system. As mentioned, noise can be transmitted in three ways: through the air, through the fluid, and/or through the system’s physical structure.
Airborne
We generally think of noise as traveling only through the medium of the air, going directly from its source to some receiver—our ears. This is airborne noise. Airborne noise, however, must have a source within some component of the system or application. That component can be—but is not always—the pump.
All noise heard by the operator is technically airborne noise. From the perspective of the noise, vibration, and harshness (NVH) engineer, airborne noise refers to noise that came directly from the surface of the source.
Fluid-borne
Whether it’s a piston, vane, or gear pump, these positive-displacement pumps all have some level of pressure ripple. As a result, uneven flow characteristics and pressure pulsations are created and transmitted through the fluid. This is known as fluid-borne excitation. The fluid-borne excitation generates vibration at the surface of the hose, which can be transferred into adjacent structures via the hose clamps/supports, or due to direct contact of the hose to the structure when under pressure.
The pressure pulsations of fluid-borne excitation, in turn, create corresponding force fluctuations. The vibrations in the hydraulic hoses are known as fluid-borne excitation. These result in vibrations that create fluid-borne noise.
Proper hydraulic-line configuration can be used to maintain vibration isolation when pumps and electric motors are mounted on isolators. A proper combination of rigid and flexible conduit can provide a more stable configuration, providing reduced vibration and noise.
Isolation of hydraulic lines and hoses from the application structure (i.e., frame, supports, or panels) offers another opportunity to reduce noise in the design of the machine. Panels and shields can often act as speakers and amplify relatively low vibration levels into high noise sources.
Hydraulic hoses and tubing can be transmitters of fluid-borne vibration in hydraulic hoses and tubing, turning structural components into “speakers.” It’s important to address the position of hoses or tubes when designing quiet hydraulic equipment in order to achieve maximum noise reduction.
Structure-borne
Structure-borne noise is the result of vibration transmitted only through the structure of the application. The vibration, as shown in Figure 1, is the combination of the force and the response of the component, and the radiation efficiency of the component. These structures then emit an audible sound, or airborne noise, which is what hydraulic equipment operators physically notice.
As an example of structure-borne noise, refer to Figure 3. Structure-borne noise starts with vibration from an external source or component and is transferred directly into the electric motor, structure or frame of an application.
Once the vibration enters the structure, it propagates through the structure at the speed of sound of the structure (most likely steel), which can excite other components and cause them to become effective radiators of noise; i.e. speakers. Components on the application, such as panels, shields, supports, and reservoirs, can radiate noise at pumping frequencies, and multiples of pumping frequencies, very effectively (Fig. 4). That’s because these types of components have many resonant frequencies. Components such as these are known as high modal density components.
Vibration control can be used to minimize transmission of vibration from pumps and drives to machine structures and equipment. This can be achieved by isolating the pump and/or motor from rigid foundations by using subplates or other base isolators.
Large areas of thin metal in systems can also radiate noise effectively. This noise can be reduced by strategically placing engineered stiffening ribs or damping treatment to the metal surfaces.
Understanding Noise Parameters
Evaluating noise can become confusing, because multiple vibration paths can exist at the same time. One must understand the source ranking of the noise to properly evaluate the system transmission paths and effectiveness of each at any and all operating conditions.
Quite often a noise source is surrounded by a box-like enclosure to provide a physical barrier between the noise sources, which can be caused by hydraulic-power units, valves, hydraulic manifolds, motors, cylinders, hoses/tubing, and additional machine equipment. These barriers are designed to reduce the sound generated by hydraulic equipment at the operator or bystander locations.
Acoustic leakage around door seals, etc., can also greatly affect the ability of an enclosure to reduce generated sound. In general, a 1% “hole” in an acoustic enclosure will permit 50% of the noise measured in it to leak out. When enclosed, amplitude of the noise within the enclosure actually increases; the noise reflects within the enclosure, rather than projecting out.
Noise amplitude within the enclosure depends on the distance away from the dominant source that the noise is measured. As a general rule, the amplitude of a noise source when placed inside of an enclosure can increase noise inside the enclosure by five to eight decibels, or 45% to 60% greater than the source without an enclosure (Fig. 5).
Another important factor in terms of enclosures is absorption coefficient. All enclosures have some level of internal absorption, but adding additional absorption material will help to reduce noise level. Larger enclosures will have a lower amplification factor than smaller enclosures. Gaps or holes in the enclosure will reduce the effectiveness of noise reduction outside of the enclosure. Even a tiny hole or gap in an enclosure can significantly reduce its effectiveness in curbing sound.
Quieter Products and Systems by Design
A successful noise-control program requires a team effort by individuals in several areas of expertise. A quiet hydraulic pump does not guarantee a quiet system. Choosing a quiet pump should be only one part of a multifaceted program that calls upon the talents of the system designer, fabricator, installer, and maintenance technicians. A breakdown in any of these areas can unravel the entire noise control program.
Integrated motor pumps help simplify hydraulic power solutions, even those within harsh environments. Housed in compact, sound-reducing enclosures, integrated motor pumps are often prominent factors in the design of quieter hydraulic equipment.
Moreover, pump and system designers play a key role in achieving successful noise control. They must evaluate every noise-control technique available from the standpoints of effectiveness, cost, and practicality.
At the onset of developing a noise-control program, it’s best to start at the source: the pump. Of course, the pump manufacturer is responsible for delivering a quiet pump. Subsequently, the most common strategy is to use a porting design to minimize the pressure pulsations at the pump’s rated speed and pressure.
At the component level, designers may want to start off with variable-speed pumps. In variable-speed drive (VSD) systems, the pump speed varies to match the duty-cycle requirement. This will lower noise, because speeds are reduced when not needed by the system.
Although quieter individual components may contribute greatly to noise reduction, additional gains can be achieved by reviewing the overall system design for opportunities to reduce noise. Vibration control works to minimize transmission of vibration from pumps and electric motors to machine structures. This can be achieved by isolating the pump and/or electric motor from rigid supports via sub-plates or other base isolators.
System testing and evaluation can provide further insight into noise reduction. In properly designed testing areas, isolating components from background noise makes it possible to focus on noise sources, transmission paths and opportunities for reduction. For example, Eaton’s Eden Prairie, Minn., facility tests components in a hemi-anechoic chamber equipped with data-acquisition equipment to evaluate noise. Sound data are recorded from the operating equipment for further analysis and refinement.
The potential for successfully reducing noise becomes greater when evaluating noise with a systematic approach rather than simply selecting individual components. An informed team, cognizant of the various components and roles in the overall system, can help identify noise sources and design for low noise.
Sound Quality in Hydraulic Systems
Hydraulics is not always the source of a noise problem, but hydraulics frequently gets the blame. The reason has more to do with the quality of the sound produced than with its volume or pressure. Most readers are familiar with the annoying quality of a hydraulic whine. Measured objectively, that whine typically doesn’t have a lot of sound power. However, it is unpleasant and tonal, and that makes the actual sound seem even louder.
Therefore, in addition to the objective issue of how much the hydraulic system contributes to overall sound levels, OEMs also have to address the subjective issue of how the quality of their application’s sound affects overall customer perception of its quality. It’s similar to judging the quality of an automobile by the sound of a door being shut.
Pleasant sounds can actually be much louder than unpleasant ones and still generate positive reactions. The rumbling of an engine is typically much louder than hydraulic whine, but the perception is one of power and strength.
The good news is that both regulatory and customer-driven mandates can addressed with essentially the same technologies. The even better news is that the net result is usually a more efficient hydraulic system.
Low-Noise Power Unit—A NVH Analysis
Eaton’s Eden Prairie facility tested these sound-reducing suggestions, as engineers explored ways to prove the appropriate modifications for building a low-noise power unit. The test chamber, Figure 6, allowed the team to effectively evaluate the noise, providing the necessary data to analyze noise sources and refine strategies for mitigating the noise.
The test reviewed the reservoir as the noise source, as the top, bottom, and sides of a reservoir can effectively generate noise if the panel resonances are excited by external vibration sources. Component vibration isolation and damping treatment were incorporated to minimize this effect.
The results of this testing are illustrated in (Fig. 7), which shows that when compared to the baseline, the low-noise power unit displayed a drop of 3.8 dBA—a 35% reduction in average sound pressure level.
