What is the difference between air-borne noise and structure-borne noise?


When an acoustic source is installed in a receiving structure, noise can be transmitted to a receiver through two different paths.

Noise can be radiated in the air and transmitted to the different area of the receiving structure, depending on the acoustic transparence of the partition walls: it is the air-borne path (AB). But most acoustic sources also generate vibrations, and those vibrations can be transmitted to the receiving structure through the attachement points. Receiving structure vibrations become an additional source of noise: it is the structure-borne path (SB).

Application to e-NVH

Electric vehicle level

At electric vehicle level, the vibro-acoustic source is the Electric Drive Unit, the radiating structure is the vehicle itself and the receiver is the driver. On one hand, motor noise is radiated in the engine compartment and transmitted to the drivers ears through the partition wall separating passenger compartment from engine one. This results in Air-Borne Noise (ABN), in orange in the image below. On the other hand, motor vibrations are transmitted to vehicle through the motor/vehicle connection points, resulting in structural vibrations which can themselves generate noise at driver’s ear. This results in Structure-Borne Noise (SBN), in pink on the image below. Electromagnetically-excited vibrations of electric machines can be transmitted by both paths, depending on their origin.

Air-borne and structure-borne transmission in EV
Electric powertrain level

An electrical motor can be very schematically represented as follows:

Electric powertrain components for e-NVH Transfer Path Analysis

The stator and casings (usually rigidly coupled) are considered as the radiating structure. Mechanical or electromagnetical forces are generated at the interface (stator tooth tips and bearings), these are considered as internal excitation sources (active part) of the electric drive unit. In this framework, the question of the transfer path of the noise sources to a receiver that is located close to the motor can be raised.

Subjected to the excitation of the active part, the radiating structure vibrates and generates noise by air-borne path. Rotor excitations can also be transferred to the casing by structure-borne path, resulting in radiating structure vibrations and noise.

When facing a noise issue on an electrical motor, it can be useful to know the origin of the noise (AB from the stator or SB from the rotor) in order to choose the most appropriate mitigation technique.

NVH control techniques

It is important to know the air-borne or structure-borne origin of NVH issues because noise reduction strategies highly depend on transmission path.

To solve air-borne noise issues, one must reduce the sound power level emitted by the source and/or enhance the acoustic transmission losses of the partition walls separating the source from the receiver. To solve structure-borne noise issues, one must reduce vibration level at mounting brackets when the source is operating and/or reduce the transmission of the vibrations (by using silent blocks or by stiffening receiving structure at connection points, when silent-block cannot be installed).

The quantification of sound and vibration transfer paths can be carried using Transfer Path Analysis (TPA). EOMYS consulting and testing services include TPA to solve noise and vibration issues coming from electric motor excitations.

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