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issue: September 2009 APPLIANCE Magazine

Engineering Case Study: Dishwasher Sound Reduction
Silencing a Dishwasher in a Drawer


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Sound was the offender, and Fisher & Paykel needed a detective who could nab the villain quick.

Fisher& Paykel’s Tall DishDrawer.

Normal prototype testing cycles are often looked at as a somewhat inconvenient necessity during product development. But in late 2008, with intense competition and the global economy spiraling downward, engineers at New Zealand–based Fisher & Paykel found their engineering schedules were particularly tight for hitting the release-to-manufacturing design freeze dates on upcoming models.

At the top of the list was the new Tall DishDrawer, featuring a significantly higher tub for larger plates, a completely redesigned racking system, styling upgrades, and incredibly quiet operation.

With timelines cut short, Fisher & Paykel contacted LMS Engineering Services (Troy, MI, U.S.; www.lmsintl.com) to help lower the sound level of the new dishwasher model as much as possible before production began. The clock was ticking, and LMS had only a short time to identify noise sources and determine the best modifications to lower radiated acoustics. The project was a collaborative partnership between the appliance maker and the engineering firm, leveraging the expertise of both to arrive at optimal noise reduction.

Acoustic Survey Pinpoints Noise Sources

In the first phase, LMS engineers used its Test.Lab solution to perform an acoustic survey. Six microphones were positioned 50 cm from the dishwasher to measure sound pressure levels during the complete operating cycle. At the same time, arrays of accelerometers spaced at 10- to 15-cm intervals measured vibration throughout the machine’s interior. Using spectral analysis, a color map of vibration amplitudes versus frequencies for these areas was generated.

Next, speakers were placed around the dishwasher to generate broadband white noise, and reciprocal frequency response function (FRF) measurements were made inside the dishwasher. Transfer path analysis (TPA) and acoustic source quantification (ASQ) then determined the vibration paths through the dishwasher and pinpointed the greatest sources of noise.

“With these reciprocal measurements, TPA and ASQ works backward to determine where radiated noise came from at various frequencies during normal dishwasher operation,” explains Olivier Kirten, LMS Engineering Services project leader. “Two major noise sources were pinpointed. At higher frequencies from 300 Hz, the biggest contributor was the tub lid, excited by water jet spray during the wash and rinse portions of the cycle. At lower frequencies of 200 Hz and below, pump noise radiated through the front door.”

Design Modifications Lower Sound Levels

In the second phase of the project, drawing on experience in acoustics projects in industries such as automotive, construction equipment, aerospace, and recreational equipment, LMS engineers studied the effect of various noise-abatement modifications.

An extra layer of bitumen sound damping material added to the tub lid decreased noise levels by more than 35%. Rubber strips placed between the pump and a filter on the underside of the tub eliminated virtually all noise spikes produced by the five pump impellers. Modifications to the ventilation hole at the rear of the unit showed that adding a one-way flapper valve could potentially reduce noise levels by 0.5 dB. Silicon sealant added between the frame and door seal eliminated acoustic leaks from the front of the unit. Averaged sound pressure measurements taken around the machine after all modifications were made showed a decrease of 2–3 dB overall.

Impressive Business Value

Fisher & Paykel senior noise and vibration engineer Ricky Kim says the appliance maker was “extremely pleased” with the engineering firm and the project’s fast turnaround. “LMS Engineering Services completed the noise reduction project in just two months—a task that would have taken us almost a year and several rounds of prototype tests,” Kim said. “They obviously know their stuff and have the advanced tools to zero in on problems and quickly find solutions. They met with us extensively and familiarized themselves with our product, inside and out.”

“Our partnership… let us implement noise-reduction modifications much sooner in new models than would otherwise be possible with our resources alone,” says Fisher & Paykel functional manager of wash systems Steven Black. “The result is that our ability to develop quiet machines has been stepped up considerably, putting us in the same league as the largest global players in the industry. With quietness now a major factor in today’s appliance market, the lower sound levels of the new models now in production and those still in development for future release will undoubtedly boost sales and strengthen our position as a force to be reckoned with in the growing worldwide appliance industry.”

 

LMS’ acoustic survey used six microphones to measure sound pressure levels while additional microphones localized background noise levels.

Acoustic Survey Pinpoints Noise Sources

In the first phase, LMS engineers used its Test.Lab solution to perform an acoustic survey. Six microphones were positioned 50 cm from the dishwasher to measure sound pressure levels during the complete operating cycle. At the same time, arrays of accelerometers spaced at 10- to 15-cm intervals measured vibration throughout the machine’s interior. Using spectral analysis, a color map of vibration amplitudes versus frequencies for these areas was generated.

Next, speakers were placed around the dishwasher to generate broadband white noise, and reciprocal frequency response function (FRF) measurements were made inside the dishwasher. Transfer path analysis (TPA) and acoustic source quantification (ASQ) then determined the vibration paths through the dishwasher and pinpointed the greatest sources of noise.

“With these reciprocal measurements, TPA and ASQ works backward to determine where radiated noise came from at various frequencies during normal dishwasher operation,” explains Olivier Kirten, LMS Engineering Services project leader. “Two major noise sources were pinpointed. At higher frequencies from 300 Hz, the biggest contributor was the tub lid, excited by water jet spray during the wash and rinse portions of the cycle. At lower frequencies of 200 Hz and below, pump noise radiated through the front door.”

 

 Design Modifications Lower Sound Levels

In the second phase of the project, drawing on experience in acoustics projects in industries such as automotive, construction equipment, aerospace, and recreational equipment, LMS engineers studied the effect of various noise-abatement modifications.

An extra layer of bitumen sound damping material added to the tub lid decreased noise levels by more than 35%. Rubber strips placed between the pump and a filter on the underside of the tub eliminated virtually all noise spikes produced by the five pump impellers. Modifications to the ventilation hole at the rear of the unit showed that adding a one-way flapper valve could potentially reduce noise levels by 0.5 dB. Silicon sealant added between the frame and door seal eliminated acoustic leaks from the front of the unit. Averaged sound pressure measurements taken around the machine after all modifications were made showed a decrease of 2–3 dB overall.

Impressive Business Value

Fisher & Paykel senior noise and vibration engineer Ricky Kim says the appliance maker was “extremely pleased” with the engineering firm and the project’s fast turnaround. “LMS Engineering Services completed the noise reduction project in just two months—a task that would have taken us almost a year and several rounds of prototype tests,” Kim said. “They obviously know their stuff and have the advanced tools to zero in on problems and quickly find solutions. They met with us extensively and familiarized themselves with our product, inside and out.”

“Our partnership… let us implement noise-reduction modifications much sooner in new models than would otherwise be possible with our resources alone,” says Fisher & Paykel functional manager of wash systems Steven Black. “The result is that our ability to develop quiet machines has been stepped up considerably, putting us in the same league as the largest global players in the industry. With quietness now a major factor in today’s appliance market, the lower sound levels of the new models now in production and those still in development for future release will undoubtedly boost sales and strengthen our position as a force to be reckoned with in the growing worldwide appliance industry.”

 

More Sound Reduction:

A Stealthy Pump

ApplianceMagazine.com/content/2233

Low-Noise Pumping

ApplianceMagazine.com/content/2152


 

 

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