What are the ways to reduce and optimize structure-borne and air-borne noise of electric machines?

September 22, 2022

The motion vibration generated by the motor is transmitted from the surface of the motor in the form of airborne noise, and is transmitted in the form of structure-borne noise at the shaft and the fixed base of the motor. To a certain extent, the structure-borne noise is converted into airborne noise and further spread to Environment. In order to reduce noise, the general approach is to interrupt the noise chain from the source to the transmission path to the ear, or reduce the noise generation directly at the noise source. If that's not possible, at least try to make the noise pleasant or less objectionable, an activity known as "noise optimization". The measures taken to change the noise being generated by optimizing the entire system must always take economical considerations into account.

Insulation and Soundproofing

The sound insulation barrier can be achieved by sound insulation and vibration isolation, the insulating layer must be clearly distinguished from the damping layer, and the vibration energy in the damping layer is converted into frictional heat. In solids, this frictional heat is caused by the mutual movement of molecules or relatively large particles in the body, and can also be caused by materials (such as foams, non-woven materials, elastomers) installed outside the device, and shows great large internal friction. In order for this material to also have a damping effect, it must be attached to the surface at the antinodes of the vibrations. In other words, attaching to the location where the vibration causes the most deformation of the material, this material is often referred to as an insulating material, acts as a damping even if it is not insulating.

For liquids, viscosity has a damping effect, but only in combination with the compressibility or significant deformation of the liquid in the container, e.g. water has a very low damping capacity because it has low internal friction and is almost incompressible. The oil is also almost incompressible, and its significantly higher viscosity produces a damping effect only when passing through narrow openings, in other words changing shape, such as in shock absorbers. Gases are compressible, but due to the large distance between their particles, they have low internal friction and therefore low damping capacity. However, if gas flows through e.g. screens, filters, foams, or if gas particles oscillate within these barriers, then friction, sound pressure and speed of sound increase, thereby reducing the volume of sound and the sound energy being "rubbed" into heat . Therefore, screens, filters and similar devices are all mufflers. In general, insulation and damping measures must be considered separately, and they are often mutually exclusive. However, in many cases it makes sense to employ insulation and damping measures, only if they are in the correct location.

reduce sound radiation

By encapsulating the entire motor, the radiation of airborne noise to the outside, where the propagation of airborne noise is limited and "blocked", can be significantly reduced. In this case, resonances caused by the package itself, as well as cavity resonances, must be considered. 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 an ideal mismatch of (sonic) resistance in relation to sound transmission and to avoid unpleasant reflections. Covering the cavity with soundproofing material helps prevent cavity resonance and helps dampen vibrations in the cavity itself. In the case of sound attenuation, as opposed to sound insulation, the sound energy is "destroyed" (transformed into friction). In the case of small motors, covering the capsule with insulating material is often not possible for space-related reasons, or for cost reasons.

Surfaces that radiate noise can be reduced by providing them with openings, which reduces the size of the radiating surface and also fundamentally changes the vibrational behavior of the surface. In this way, natural frequencies can be shifted, unpleasant vibration modes along with their nodes and antinodes can be rendered harmless, and additional stiffeners or supports can create an opening-like effect.

Reduce sound and vibration transmission

Measures taken to reduce acoustic radiation also apply to prevent motor vibrations from being transmitted through the shaft and the motor mounting system in the equipment (or environment), however, there are some general "recipes for success": mount as close as possible to the most annoying vibrational motion The node locations of the most important nodes are located near the bearing, and the vibration motion that still exists should reduce the force vibration as much as possible. In other words, mount 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 such as transmission shocks. If the vibration of the force is small, a component with additional small vibrational movement, i.e. low weight (light equipment) can be used. It is often advantageous for heavy equipment, especially in the area where the motor is installed. Of course, the vibrating mass of the motor, the elasticity of the mounting system, and the mass of the equipment near where the motor is mounted must match each other so that resonance with undesirable motion frequencies does not occur, and the system is tuned so that the resonance is lower than the operating point. Other measures, such as the use of active weight dampers or shock absorbers, are also theoretically possible.

Reduce acoustic and vibration excitation

Elimination of noise and vibrations is best reduced where they are generated, i.e. the source, in electric machines forces and torques are necessary and they often inevitably include unwanted components (oscillating torque, cogging torque, etc.), This cannot be completely avoided. There are many kinds, and in the motor concepts encountered so far, changes in their working principles can also lead to the excitation of various noises. Asynchronous motors behave differently from synchronous motors (including electronically commutated motors and stepper motors), and DC motors behave differently from piezoelectric motors, so noise and vibration excitation can often only be achieved by very careful selection of the right motor and suitable motor size to minimize.


Optimization is the effort to systematically affect sound and vibration excitation, and noise optimization is defined as the systematic change in the acoustic quality of noise to achieve the best possible value. Acoustic quality represents the degree to which needs are met in relation to the population of individual needs in an auditory event. Ideally, reducing the unpleasant sound field to the hearing threshold, which is often not technically or economically feasible, we can try to influence the noise and change it, thereby removing the unpleasant and unpleasant noise components.

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