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Author: Richard H. Lyon and David L. Bowen
New technologies can lead to quieter mechanisms and materials or unexpected, unpleasant sounds.
Sounds made by products can be either pleasing or annoying. Sounds are pleasing when expectations are met and the decision to acquire the product is reinforced (e.g., the familiar sound of a car door closing). Product sounds are annoying when they are unexpected, intrusive, or raise concerns about the function or quality of the product (e.g., a knocking sound in the engine). We tend to refer to sounds in the latter category as “product noise.”
This article describes some trends in product design and engineering that have made some products noisier and some quieter. In some cases, quieter mechanisms and materials have replaced noisier ones, but in other cases new technologies have led to unexpected and unpleasant sounds. Psychometric methods that correlate design alternatives and user perceptions are helpful in designing products with sounds that are pleasing to consumers.
Definition of “Quiet”
These days, most product managers understand that the descriptor “quiet” has a broader meaning than “less loud” as measured on a sound-level meter. The sound made by a product must be appropriate to the nature and use of that product, and even small sounds that are inappropriate or distracting should be avoided if possible. Designers also realize, however, that appearance, performance, and functionality are also (maybe even more) important than “quietness” to customer satisfaction. Designers must balance the latter attributes, which they understand very well, with the product sound, which is harder to nail down. Unfortunately, some current design trends that work toward satisfying other requirements make designing “quiet” products more difficult.
Quantifying the correspondences between design choices (e.g., materials, geometry, mechanisms, parts layout, and assembly sequence) and the perceptual reactions of people to the sound of that product can seem like an insurmountable task. The complexity of sound-generation processes and the inherent variability in people’s perceptions to those sounds creates much “scatter” in predicting user reactions. Nevertheless, through careful experimental design and testing, consistent relationships between design choices and perceptions of acceptability (so-called product sound quality) can be established.
Trends in the development
of electric motors have
led to noisier products.
Trends That Make Products Noisier
Fifty years ago, an absorption refrigerator was absolutely silent except for the hiss of a small gas flame. Some years later, the hermetically sealed compressor in an electric refrigerator was fairly unobtrusive, although not silent. However, the condenser coils on the back of the refrigerator and the 2 to 3 inches of wall thickness required to accommodate thermal insulation reduced the interior space of the refrigerator; therefore, the condenser coils were moved to the base of the refrigerator, and the wall thickness was reduced to less than 2 inches. As a result, the compressor had to work more often to counteract heat conduction through the walls. In addition, heater coils were placed in the walls of some refrigerators to reduce sweating in humid weather. Refrigerators, which once had no fans, now have two—one for the condenser coils1 and one for the freezer compartment. As a result, refrigerators are now much noisier than they used to be.
In addition, customer demand for lighter weight, more color options, and interesting shapes and configurations has led to housings for many products made of rigid, low-density plastics. Because these housings are light weight, it is more difficult to isolate them from the forces of the motors and air-handling devices, and, because of their reduced mass, they vibrate more.
Another reason for more product noise is that sound can escape more easily through stiff, lightweight walls. At the same time, the resonance frequencies of the structural modes of the housing increase as the result of the combination of stiffness and reduced mass, thus increasing the radiation efficiency of the structure and moving the sound frequencies into the “loudness and annoyance” range.
Trends in the development of electric motors have also led to noisier products. Induction, universal, and DC-commutator designs are being supplanted by, for example, variable frequency induction, brushless DC, and hysteresis motors, all of which convert a supply voltage to DC, then convert the DC voltage to an AC voltage (single or multiple phase) to control speed and torque, which makes the motor more programmable and can improve energy efficiency and increase performance flexibility. Generally, the AC voltage produced is far from sinusoidal, leading to vibrations at frequencies well above the basic excitation level. Although these small vibrations may not affect the operation of the motor, they do produce audible noise that may be objectionable.
Hysteresis motors are attractive for some applications because of their high starting torque, but this can be accompanied by fluctuating forces that produce vibrations that radiate sound. The forces between rotor and stator can be computed fairly accurately, but the generation of vibrations and the transmission of those vibrations to surfaces that radiate sound can be very difficult to predict.
Even in conventional motors, design trends have increased noise. To improve performance and reduce the size of the overall package, the power density in motors has increased. Current designs for universal motors are a fraction of the weight and size of the electric motors of 50 years ago. Some of the change is attributable to better magnetic materials, but sometimes the motor is excited to the point that the magnetic circuit is driven to saturation, which increases the harmonic content of the excitation and causes the magnetic field to balloon around the motor and attract nearby ferrous material. Garage door lift motors, for example, attract the sheet-metal steel frames that support them and produce this kind of product noise.
Another less obvious cause of increased product noise is changes in manufacturing and assembly methods. “Design for manufacture” is a popular theme, but when parts are assembled by a sequence of layering operations, assemblies may have tolerance stack-up problems that lead to more noise (and other problems as well). For example, a popular food-preparation appliance was beset with such problems when new assembly procedures were introduced. Because of the lay-up of shafts and gears in the drive system, the motor could not maintain a constant speed ratio, which led to excessive gear noise (so-called “transmission error”).
Electrical and electronic equipment is cooled by passing air over the elements. This airflow is normally turbulent for higher cooling efficiency, and correlations have been developed between airflow parameters (e.g., velocity, turbulence length scales, temperature rise) and passage parameters (e.g., geometry, dimensions, heat release). Balancing heat transport with acceptable temperature rise and pressure drop is a design issue for laptop computers and digital projectors, which are being produced in smaller and smaller packages with the same, or even higher, performance levels.
Turbulent heat transfer inevitably creates a certain amount of noise, which we can label irreducible, because heat-transfer processes and noise generation by turbulent airflow in restricted passages are inextricably linked. As the air flows, the turbulent eddies decay and new ones are created, which produces forces on the components that generate sound and, at the same time, carry heat away from the surface. Thus noise generation is inevitable with turbulent heat transfer. Figures 1 and 2 (see PDF version for figures) show the calculation of heat transfer to sound for airflow between banks of circuit boards.