Silencing the dishwasher in a drawer

With this first-class brand image on the line with each new model, noise and vibration engineers at Fisher & Paykel are sticklers constantly striving for lower appliance sound levels with each new model. To test noise levels, Fisher & Paykel has an anechoic chamber and extensive systems of microphones and accelerometers to measure and record sound and vibration amplitude and frequencies. During several prototype-testing cycles, engineers count on their decades of experience in modifying the design and strategically positioning acoustic damping materials until sound levels are acceptable.

Due to time constraints, the normal prototype testing cycles were considered a somewhat inconvenient necessity. But in late 2008 – with intense competition and the global economy spiraling downward – engineering schedules were particularly tight for hitting the release-to-manufacturing design freeze dates for upcoming new 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 to help lower the sound level of the new dishwasher model as much as possible before production began. With the clock ticking, 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 Fisher & Paykel and LMS, leveraging the expertise of both to arrive at optimal noise reduction.

In the first phase, LMS engineers used LMS Test.Lab to perform an acoustic survey in which 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, LMS Test.Lab then generated a color map of vibration amplitudes vs. frequencies for these areas.

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) capabilities of LMS Test.Lab 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 LMS Engineering Services Project Leader Olivier Kirten. “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.”

In the second phase of the project, LMS engineers studied the effect of various noise abatement modifications. These methods were based on their experience and background with a wide range of acoustics projects in various industries including automotive, construction equipment, aerospace, industrial machinery, and recreational equipment to name a few.

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 two to three dB overall.

“We are extremely pleased with LMS Engineering Services and with their fast turnaround on the noise and vibration project,” explained Fisher & Paykel Senior Noise and Vibration Engineer Ricky Kim. “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. 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.”

Fisher & Paykel Functional Manager of Wash System, Steven Black notes the business value of working with LMS. “Our partnership with LMS Engineering Services let us implement noise-reduction modifications much sooner in new models than would otherwise be possible with our resources alone. 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.”