PRODUCT NOISE AND VIBRATION REDUCTION

A quiet product will be well positioned to compete in the global marketplace. This is true for consumer appliances, office equipment, medical devices, power tools, and lawn and garden equipment. For certain classes of products, the European Union is imposing increasingly strict noise standards. For other products, consumers are demanding the same. Products can be quieted by adding sound absorbing materials (at an added cost/per unit) or, more efficiently, by using engineering principles to reduce the source of the noise or vibration.

The most effective way to ensure a quiet product is to focus on noise and vibration during the design process. RH Lyon Corp can assist by providing design guidelines, analyzing design choices and evaluating and/or specifying layouts and components. With an existing product, the "quieting" process begins with a method called the noise audit. During the noise audit, the sources, paths and radiators of noise (and/or vibration) are identified and quantified. Armed with this information, we can offer cost/benefit scenarios based on the relevant noise sources. With close client interaction, and with sensitivity to manufacturability and cost/per unit, we can then implement and test these solutions. Our engineering expertise extends to heat transfer, fluid flow, and electrodynamics, giving us the breadth to design mufflers and impellers, determine heat loss, or modify noise inducing electrical circuitry. Using engineering principles, rather than just adding absorbing material, we are able to provide expert consultation in the design of a product that is quieter, operates more efficiently, and is often less costly to produce.

PROJECT EXAMPLES

Consumer Products | Office Equipment | Medical Devices
Power Tools | Lawn and Garden Equipment | Other

Related Tech Briefs (Acrobat PDF)

Related Articles

Decibel by Decibel, Reducing the Din to a Very Dull Roar
by Richard Wolkomir. Smithsonian Magazine / February 1996 issue  

Related Books

Machinery Noise and Diagnostics, by R. H. Lyon

Consumer Products

Vacuum Cleaners
Among the consumer products most associated with noise are vacuum cleaners. The components that make up the vacuum, and the need to move air mandate that vacuums will be "noisy". However, with the careful design of the impeller, we have determined that this noise can be less tonal and more broadband, and thus less annoying.

Dishwashers
Impellers move air in vacuum cleaners and water in dishwashers. In one case, our noise audit indicated that the impeller needed to be redesigned if the sound were to be acceptable for a high end appliance.

Air Conditioners
Four sound quality components, namely compressor noise, low-frequency excitation of the partition in which the unit is installed, coolant flow noise, and air flow noise, can be competing sources of noise in window air conditioners. In one case, of these four, our analysis showed that the first two were indeed too high. Compressor noise was reduced through the application of a constrained-layer damping treatment in combination with a new acoustic blanket design. The low-frequency vibration was treated by a combination of improved vibration isolation for the compressor and dynamic absorbers.

Combination Convection/Microwave Oven
Ovens which cook by employing a combination of techniques also provide a combination of noise sources. Now air is moving not just to cool the microwave components, but to heat the food as well. We established that a detailed assessment of the fans, their placement and their ducting is necessary to keep the overall sound level within acceptable levels for the high end kitchens for which these ovens are targeted.

Electrically Powered Window Shades
The large flat panel of a window shade is an excellent radiator for sound transmitted from the motor. Transfer function measurements were used to determine the extent of the problem and the amount of reduction afforded by a properly designed isolator.

Mobile Home Furnaces
Mobile homes are very compact residences constructed from lightweight materials. Both these characteristics adversely affect the noise level of appliances. In addition, typical outdoor ambient sound levels in trailer parks are low requiring exterior sound emission to be low also. For one mobile home furnace brand we focused our efforts on isolating the motor for noise reduction so that the furnace housing could not be excited by vibration at rotational frequencies of the motor.

Office Equipment

Quiet Copiers
In a copier, paper handling mechanisms, fans and blowers all need noise reduction. A noise audit identifies where noise reduction efforts should be focused and which sources can be ignored.

Digital Projectors
A conflict exists between the need to cool the lamp of a digital projector by moving air across the bulb and the need for quiet in the vicinity of the projector. Other moving parts, such as a color wheel, add to the background noise. By performing a noise audit, we ranked ordered these noise sources so that noise reduction efforts were focused on only the important noise sources.

Computer Hard Drive
A systematic study of a particular hard drive indicated that the flat surfaces of the case of the DC brushless motor were the source of annoying sound. By perforating the case, we were successful in reducing the sound level to acceptable levels.

Medical Devices

Sleep Apnea Device
A particular challenge in noise reduction (and sound quality) is an air moving device which must operate in the bedroom when the occupants are sleeping and must be attached to the user. They require a design that quells anxiety and instills patient confidence. This necessitates that we address both the quality of the sound and its overall level . With proper outlet fan design, tonal components to the sound can be minimized, creating a soothing broadband noise.

Oxygen Concentrator
Again because of the proximity to the user, noise is an especially sensitive issue in oxygen concentrators. Noise sources which can be important are valve flutter, lightweight tubing and gaps in the housing (both of which transmit sound), voltage waveforms which amplify valve impacts, and improperly designed mufflers.

Centrifuge
In one product the tonal component of the sound has been related to imbalance in the unit. We associated this imbalance with the motor rotor, the spindle and the cover for the tube holder. The question of why some motors are better able to "handle" imbalance required looking into the electrodynamics of the motor itself.

Power Tools

Power Hand Tools
Drills, sanders, and saws pack more power these days. More power usually means a faster motor and more gearing for better working torque. In addition, there is a competitive need for tools to be lighter, quieter and less expensive. In a variety of hand tools, we have identified the primary noise sources and created new ways to control and reduce sound levels.

ISO standards for tools
Circular Hand Saws are qualified according to ISO 5349: "Mechanical Vibration — Guidelines for the Measurement and The Assessment of Human Exposure to Hand-Transmitted Vibration". A vibration meter, set to the hand/arm mode, can determine whether or not a tool meets the level of the standard, but it cannot reveal the source of the vibration. Analysis using an accelerometer and a narrow band spectrum analyzer can relate the level of vibration to the underlying source of vibration permitting remediation.

Vane-popping in Air Powered Tools
Exhaust pulsations, or "vane-popping," was found to dominate the overall sound in certain air powered tools. An analytical expression for the sound pressure waveform based on cell pressure was developed, which showed the amplitude of the radiated sound to be highly sensitive to sudden changes in the cell pressure. Based on this, the discharge porting of the tool was modified to allow a more gradual discharge flow from the motor cells. The result was a reduction of 4 to 5 dB in the overall A-weighted sound level and a 6% increase in the measured power output of the tool.

Airless Paint Sprayer
A noise audit of an airless paint sprayer determined that substantial noise reduction could be achieved by modifying the housing and isolating the unit from its stand. These modifications reduced the structural excitation which resulted from force fluctuations in the pump which in turn caused the gears to impulsively disengage.

Lawn and Garden Equipment

EU Noise Regulations
The European Parliament has stated that within the framework of the internal market, requirements for the noise emission by equipment for use outdoors have to be harmonised (the EU’s word) in order to prevent obstacles to the free movement of such equipment. While intended for goods sold only within EU countries, the reality is that these regulations become mandatory for products competing in the global marketplace. The phasing-in of increasing stricter regulations requires manufacturers to carefully analyze the noise sources in their products and tackle noise problems at their source.

Riding Lawn Mower
A riding lawn mower has noise sources that are distributed throughout. The engine is an obvious noise source, but additional sources in one particular model were the hydraulic motors and valves, the pulleys and belts, and noise from the blades due to both aerodynamic forces and induced structural vibration. Also, the speed of operation is sufficiently high to create significant imbalance forces in any of the rotating parts. These forces are high enough to cause impact and rattle, which act as another distributed noise source throughout the machine. The relative contribution of these various sources will differ based on the design of the machine. A noise audit will sort out the dominant sources in a particular model of mower.

String Trimmers
Candidate noise quieting solutions for a particular string trimmer included increasing the muffler performance, redesigning the engine housing along with evaluating alternative housing materials, and redesigning the string guard to reduce turbulent air flow noise. The noise audit is a necessary precedent to determine the potential benefit of each approach.

Other

Electric Motor Squeaks and Squeals from Stick/Slip Action
If a motor brush vibrates at a velocity equal to the surface velocity of the commutator, it is possible that static friction will cause the brush to "stick" to the commutator for a brief moment. On reversal of vibration, "slip" occurs. This stick/slip action can result in unacceptable squeaks or squeals, especially when the stick/slip frequency couples into an armature torsional vibration mode.

Electric Motor Vibration
There are competing sources of vibration excitation force in electric motors. For noise and/or vibration reduction the relative magnitudes of these sources must be determined. For one manufacturer, test fixtures were devised to measure the rotor mechanical imbalance, torque fluctuations, winding imbalance, and fringing flux. The results were applicable both to motor design and assembly line qualification.

Split Blade Fan
A split blade arrangement effectively eliminates an often annoying tonal component of fan sound at the blade passage frequency. In one 48 blade fan that we analyzed, the rotation rate was 1000 rpm, or 16.7 Hz. This makes the blade passage frequency 800 Hz, which was very weak in the sound spectrum. However, there was a fairly strong tone at 1600 Hz (twice the blade frequency), because the blades were arranged in the split (or double) row. This tone was reduced by modification of the blade design.