Noise: Quieting Appliances
May 1, 2010
Test method identifies noise sources and optimal reduction approach.Consumers are now demanding quieter operation for most major household appliances. In addition, manufacturers have identified the acoustic performance of an appliance as a means to improve their model over competitors. Today, it is common to find claims of noise reduction on many major household appliances. However, the extent of noise control products that are used, and the resulting acoustic performance of the appliance, can vary widely.
There are standardized test methods to determine the noise radiated from an appliance. One of these methods is the International Electrotechnical Commission (IEC) 60704 test to measure the airborne acoustical noise radiating from an appliance. To conduct this test, microphones are placed in a rectangular or hemispherical array, surrounding the appliance, and the acoustic sound pressure is measured while the appliance is operated. Typically, these measurements are conducted in a semi-anechoic or quiet room to avoid background noise or reflections of the noise generated from the appliance. To investigate various noise control products and their effectiveness, the IEC 60704 test method was performed on a front-loading clothes washer.
Nine microphones were placed in a rectangular array surrounding the front-loading washing machine to measure the radiated sound. Once the noise level was recorded from the microphones, the information was analyzed in various ways. Spectrum analysis of the recorded microphone signals was completed to show what frequencies, or tones, of noise were being generated by the clothes washer. Fig. 1 shows the average sound pressure level spectrum for the noise when the machine was operated at three different RPM speeds during the spin cycle. As shown, the noise radiated is very broadband (contains a wide range of frequencies). Consequently, any identified solution for mitigating noise must be able to perform well over a wide range of frequencies.
Another method to analyze the radiated noise is to compute the overall sound power level emitted by the clothes washer. The sound power level is the product of the area of the enclosure, formed by the microphones, and the sound intensity, calculated from the microphone signals. Using a summation process, all of the noise radiated from the washer, over the entire frequency range, can be reduced to using a single sound power level value. Fig. 2 shows the sound power level for the clothes washer under four different operation modes. It is clear that the noise level peaks during the spin cycles versus the wash cycle, and that it increases with RPM speed.
A six-sigma methodology, as outlined in Fig. 3, was adopted to identify potential root causes for high noise-levels. Various testing techniques were utilized to eliminate possible causes in order to identify the main cause. For example, the clothes washer was operated without the belt in place to evaluate only the motor noise. Another technique utilized, was to measure the vibration level to determine the amount radiated noise from the side panels. Finally, the root cause of the high sound power level was found to be the motor noise during the spin cycles. Once this root cause was found, possible solutions to reduce the noise level were then investigated.
The following noise control products were installed to determine their effectiveness:
1. Acoustic foam.
2. Acoustic barriers.
3. Viscoelastic damping materials.
The products were tested separately and in combination to determine their performance. The object of these tests was to determine which material reduced the sound power level most, and the best method of installation for that material. Acoustic foam reduces noise by dissipating the airborne sound that hits the surface. When a sound wave hits the foam surface, it causes an oscillating flow of air to penetrate into the cells of the foam. This air flow is then dissipated by resistive effects caused by the foam’s cell structure. The sound absorption of the foam is greatly determined by the foam’s thickness, surface characteristics, and cellular properties.
For these tests, the acoustic foam was installed on the inside surface of the rear access panel and in the open space at the bottom of the washing machine. The foam was contoured to fit well into the space that was available, so that no modification to the clothes washer was needed to accommodate the foam.
Acoustic barriers are typically used to form an enclosure which blocks the transmission of noise to the outside. Often times, barriers are made of heavy layers to maximize the noise blocking capability, called the transmission loss. In addition, barriers commonly have fibrous or porous material attached on the inside of the enclosure to eliminate the reflected sound through absorption. During this clothes washer machine evaluation, an acoustic barrier was formed from a 3 mm thick layer of thermoplastic olefin (TPO) material with a 12 mm thick layer of flexible acoustic foam on the inside. This barrier was used to form a custom-fit enclosure around the clothes washer’s motor.
Damping materials are viscoelastic layers that are designed to absorb the vibration energy of a surface to which they are attached. During vibration, a component undergoes small amounts of deformation which can cause radiated noise. When damping material is applied to this vibrating surface, it also experiences the deformation, but because it absorbs some of the energy in the process, the radiated noise is reduced. For these tests, the damping material was applied to the inside of the clothes washer’s metal cabinet.
These materials were evaluated by installing them on the clothes washer and operating at the highest rpm spin cycle. Since this was the worst-case scenario for the noise radiation, it would likely provide the maximum benefit from the noise reduction materials. The noise was recorded using the same IEC test setup as in the original condition and the sound power level was calculated. Fig. 4 shows the test results with the various combinations of the noise control materials installed.
As shown in Fig. 4, a 5 dB reduction in the sound power level was measured with only the addition of the acoustic foam in the lower and back panel portion of the machine. Meanwhile, the motor barrier was only able to reduce the sound power by 2.9 dB, and both materials combined to provide a total reduction of 5.6 dB. Since both materials were located in close proximity to each other, they were likely addressing much of the same noise. Therefore, when both materials were being used, the cumulative effect of each material was not fully realized.
The addition of damping material on the side panels provided only a 0.5 dB decrease over the combination of the acoustic foam and barrier test case. Since the damping material had little effect in the sound power level, it can be concluded that noise radiating from the side panels of the machine is low. Separate vibration measurements on the side panels, performed earlier, predicted this conclusion.
The six-sigma methodology demonstrated that much of the highest noise level was coming from the motor in the lower portion of the machine. The addition of a sound absorbing material, like the acoustic foam, had a significant effect on the total radiated noise. The reduction of 5 dB was clearly noticeable in the recorded noise. The barrier and the acoustic foam together, provided a significant reduction, however the total added cost to the appliance would be significantly higher than the acoustic foam by itself. The acoustic foam part provided acoustic benefit at low cost, with little added weight, and with no change to any of the existing clothes washer components.
This study showed that various materials can be used to reduce the noise emitted from washing machines or other household appliances. However, up-front testing is necessary to determine the source of the noise, to help guide in the selection of the proper material, and to assist in the design of the part. Installing materials in areas that are not a significant contributor to the total radiated noise do not reduce the overall level much and are not cost effective. Proper material selection and part design can lead to a quiet appliance with a minimum of added cost in noise control products.
For more information, visit: Dow.com.