A vacuum cleaner manufacturer hired Acentech to reduce the noise in a product that was under development. The company provided a goal in terms of an overall sound pressure level. Acentech’s product sound consultants approached this project systematically, by first determining the noise sources and how much each source contributed to the overall sound pressure level of the vacuum.
We found that the noise reached the listener through the inlet and outlet air flow paths and by radiation from the housing (structure-borne noise). In addition, some noise “leaked” out through the edges of a filter access door. The noise itself consisted of turbulent flow noise, and tones produced by the rotation of the motor/impeller, especially at the fundamental rotation rate and the blade passage rate. Overall sound pressure level was governed by the amount of flow noise, so reducing this source would allow the manufacturer to meet the overall sound pressure level goal. However, our researchers cautioned that reducing the broadband flow noise without reducing the tones, would make the tones more prominent and the vacuum more unpleasant to the consumer. We suggested that the goal should be reconsidered with sound quality in mind.
The tone at the fundamental rotation rate was almost entirely a result of structureborne radiation while the blade passage tone was generated aerodynamically and was part of the air flow noise. By placing a small accelerometer on the motor we measured the tones from the motor casing and quantified the variability in level among 12 motors. Potential solutions for reducing the tone at the fundamental frequency included sourcing a different motor/impeller, pre-qualifying the existing components by measuring their vibration, and isolating the motor/impeller from the vacuum housing. The level of the blade passage tone would be reduced potentially by adopting any of these approaches. In addition, redesigning the impeller or adding absorption in the flow path would surely reduce the blade passage tone. With the tones reduced, the manufacturer could return to the original goal of meeting a specified overall sound pressure level.