Unwanted noise can affect the user experience, perception of quality and is a symptom of inefficiency within products. Noise can originate from a variety of components and is often a difficult problem to troubleshoot. Understanding the factors that influence sound levels can allow design engineers to create more efficient products.
Troubleshooting excessive noise in the development and prototyping stage can be problematic, especially when there are multiple sources of sound. It can often be the case that the engineer resolves the dominant sound issue only to uncover a quieter but more unpleasant secondary source of noise. Reducing sound can be a complex issue. It’s not necessarily just the sound level that requires consideration but also the characteristics of the sound. In some scenarios, a louder noise might be desired.
Moving parts will always create some noise, whether from motors, sliding contact, or vibration. In applications where gearing is required, the choice of gear type, gear spacing, surface finish and lubrication regime all have an impact on the noise of the gear arrangement. IDC recently reduced the sound level of a worm and wheel gearset by adjusting the mechanical design of the product to achieve tighter tolerances on the worm gear position relative to the wheel.
Another common example of excessive gear noise is the whine created by straight cut teeth on spur gears - where the large contact area of the teeth can lead to a loud, high pitched whining noise at high speed. You may have heard this noise when driving older cars in first gear or reverse. The solution to reducing the sound level is to use spur gears with helical cut teeth, giving less contact area and therefore dramatically reduced noise.
The use of bushings to support rotating shafts is a great example of how surface finish of components in relative motion can have a drastic effect on sound levels. If you installed an Oilite bush on a roughly turned steel shaft and rotated it, you would notice an unpleasant, almost scratching like sound, which with increasing speed would reach an uncomfortable volume. The noise is exactly what it seems, the asperities on the shaft are gouging the surface of the bushing, leading to excess noise, heat generation and rapid wear. If you were to surface grind or polish the shaft to a suitably fine finish and repeat the test, the result would be near silent operation because the gouging of the bushing surface would be eliminated. This would also encourage the hydrodynamic lubrication regime to take effect - where the shaft would be floating on a thin fluid film of lubricant drawn out of the bushing. A similar problem was solved by IDC on a recent project involving a motorised carriage which travelled inside an extrusion.
Noise due to vibration is a common problem in motion systems which can often be challenging to troubleshoot. For example, a motor may be almost silent when tested on the bench, but assembled in to a motion system the vibrations caused by the motor could generate audible noise through the frame of the product. The noise due to vibrations will vary depending on motor speed; under no load the system may operate at acceptable noise levels but at other loads and speeds, unwanted noise from resonance can become a problem.
Air flow is required for cooling in many products, and there are multiple ways in which air flow can cause noise issues if it is not fully considered in the design stage or when specifying components. It is common for enclosures to need some form of cooling, such as exhaust fan to remove hot air. The obvious source of noise in this example is the fan itself, either from its motor and bearings or the sound of the air flow over the fan blades. In some cases it is possible to simply use a larger fan which rotates slower to deliver the same flow rate at lower air velocity, but often it isn’t possible to package a large fan and subsequently smaller, high speed fans must be used where optimized blade design and quiet bearings can bring the fan noise to acceptable levels.
The issue with small, high flow rate fans is how to manage the fast-moving air without causing additional noise; any disruptions to a high velocity flow, such as fan grills, PCB components or changes in direction through ducts will generate sound. Another consideration with fan noise is flow restrictions. If the intake restricts the flow, this can make the fan work harder, draw more current and produce more noise.
Acceptable sound levels of a product should be understood at the beginning of the design process so that solutions for reducing and managing noise can be incorporated from the beginning. Products involving pumps can be particularly challenging from a sound perspective because they include many of the worst offenders for excessive noise; motors, rotating or sliding contact and fluid flow.
There are various styles of pumps, each having different pressure and flow rate characteristics, and as such different noise characteristics. Noise problems tend to arise when the product requirements lead to pump specification at either pressure extremes, or flow velocity extremes. High pressure – low flow rate applications are well suited to reciprocating pumps which can be extremely loud. Low pressure – high flow rate applications inherently have high velocity air flow and high-speed motors, a vacuum cleaner being the obvious example. IDC recently carried out a project to reduce the sound level of a high-pressure system with a large reciprocating liquid pump, generating upwards of 900bar.
A key part in all of this is that appropriate components are selected. Not components that are delivering far more than what is actually required. This comes from a total understanding of the user needs and requirements specification.
In the cases described, excessive noise levels are often the most noticeable symptom of inefficiency;
• Noisy bearings will wear faster and have greater/sliding rolling resistance.
• Scratching and scraping sounds from bushings and sliding linear bearings indicate poor surface finish and alignment, causing accelerated wear and increased friction.
• Loud gears may be incorrectly spaced, have poor surface finish or be incorrectly specified.
• Loud airflow can be a result of flow disruption and frictional losses, leading to increases in pressure causing cooling fans to work harder.
It is not always possible to reduce the sound to an acceptable level at the source, all components will have a minimum sound output and, in some cases, this may still be too loud. In these situations, additional damping and sound insulation are the most common methods in reducing sound levels.
Feel free to ask any questions surrounding this area. It’s a significant topic with many factors where each case has to be handled individually, but we hope this article may help to shed some light on potential considerations. If your business is experiencing problems with unwanted noise in your products and this is something you would like support with to reach a solution, feel free to get in touch at firstname.lastname@example.org
Nick Brown is a Mechanical Design Engineer at IDC. With a Master's Degree in Motorsport Engineering, Nick applies his in-depth understanding of mechanical engineering principles and complex motion systems to solve a range of technical problems across a broad spectrum of projects including robotics, medical devices, consumer products and industrial equipment.