What are the ways to optimize structure-borne and air-borne noise in motors?

Motion vibrations occurring in a motor are propagated from the motor surface as airborne noise, and at the shaft and the motor’s fixed base as structure-borne noise, which to some extent is transformed into airborne noise for further propagation into the environment. In order to reduce the noise, a general approach is to interrupt the noise chain from the source to the transmission path to the ear, or to reduce the noise generation directly at the source. If this is not possible, one can at least try to make the noise pleasant or less objectionable.
Insulation and soundproofing
Sound barriers can be realized by sound insulation and vibration isolation. The insulation must be clearly distinguished from the damping layer, in which the vibration energy is converted into frictional heat. In solids, this frictional heat is caused by molecules or relatively large particles moving against each other in the body, but can also be caused by materials mounted on the outside of the device (e.g. foams, non-woven materials, elastomers) and exhibit a great deal of internal friction. In order for this material to also have a damping effect, it must be attached to the surface at the belly of the vibration wave. In other words, attached at the point where the vibration causes the greatest deformation of the material, this material is often referred to as an insulating material, and even if it is not insulating, it also acts as a damping agent.

In the case of liquids, viscosity has a damping effect, but only in combination with compressibility or significant deformation of the liquid in the container, e.g., water has a very low damping capacity because it has low internal dissipation and is virtually incompressible. Oil is also almost incompressible and its significantly higher viscosity only produces a damping effect, in other words a change of shape, when passing through narrow openings, e.g. in shock absorbers. Gases are compressible, but because of the large distances between their particles, they have low internal dissipation and therefore low damping capacity. However, if gases flow through e.g. screens, filters, foams, or if gas particles oscillate within these barriers, then friction, sound pressure and sound velocity increase, thus reducing the volume of sound and “rubbing” sound energy into heat. Screens, filters and similar devices are therefore mufflers. In general, insulation and damping measures must be considered separately and are often mutually exclusive. However, in many cases it makes sense to use insulation and damping measures, which are only feasible if they are in the right place.

Reduction of sound radiation
The radiation of airborne noise to the outside can be significantly reduced by encapsulating the entire motor, in which case the airborne noise propagation is limited and “blocked”. In this case, resonances caused by the encapsulation itself, as well as cavity resonances, must be taken into account. Often, the entire motor cannot be completely sealed due to the connection to the drive or the environment. In the case of openings, care must be taken to achieve the desired mismatch of (acoustic) resistance related to sound transmission and to avoid unpleasant reflections. Covering the cavity with soundproofing material helps to prevent cavity resonance and helps to dampen vibrations in the cavity itself. In the case of anechoic insulation, sound energy is “destroyed” (converted into friction) as opposed to sound insulation. In the case of small motors, covering the sealed compartment with insulating material is often not possible for space-related reasons, or for cost reasons.
Noise-radiating surfaces can be reduced by providing them with openings, which will reduce the size of the radiating surface and also radically change the vibration behavior of the surface. In this way, the intrinsic frequency can be shifted, unpleasant vibration modes along with their nodes and wave bellies can be rendered harmless, and additional reinforcement or bracing can have an effect similar to that of an opening.
Reduction of sound vibration transmission
The measures taken to reduce acoustic radiation are also applicable to prevent the transmission of motor vibrations through the shaft and the motor mounting system in the equipment (or environment), however, there are some general “tips for success”: mounting as close as possible to the nodes of the most unpleasant vibration motions, with the most important nodes being located in the vicinity of the bearings, and the vibration motions that are still present should be Force vibrations should be minimized as much as possible. In other words, install the system as flexibly as possible in the direction of vibration and with as little damping as possible for a given motor application and other conditions (e.g. transmission shocks). If the force vibrations are small, components with additional small vibration motions, i.e. very low weight (light equipment), can be used. This is often advantageous for equipment with a high weight, especially in the area where the motor is mounted. Of course, the vibration mass of the motor, the elasticity of the mounting system and the mass of the equipment in the vicinity of the motor mounting must be matched to each other so that resonances with undesirable motion frequencies do not occur and the system is adjusted so that the resonances are below the operating point. Other measures, such as the use of active weight dampers or vibration dampers, are also theoretically possible.
Reducing sound and vibration excitation
Eliminating noise and vibration is best done by reducing them where they arise, i.e., at the source. In motors, where forces and torques are necessary, they often unavoidably include unwanted components (oscillating torques, cogging torques, etc.), which can’t be avoided altogether. There is a wide variety of motor concepts encountered, and variations in their operating principles can lead to various noise excitations. Asynchronous motors perform differently from synchronous motors (including electronically commutated motors and stepper motors), and DC motors perform differently from piezoelectric motors, so that noise and vibration excitation can usually only be minimized by very careful selection of the right motors and proper motor sizing.
Optimization is the effort to systematically influence the sound and vibration excitation. Noise optimization is defined as the systematic alteration of the acoustic quality of the noise to achieve the best possible value. Acoustic quality indicates the degree to which the demands associated with the totality of individual demands in a listening event are satisfied. Ideally, it is usually not technically or economically feasible to reduce an unpleasant sound field to the hearing threshold, and we can attempt to influence the noise and alter it so as to eliminate the unpleasant and unpleasant noise components.