What is the difference between airborne sound and structure-borne sound in ultrasonic testing?

In ultrasonic testing, a distinction is made according to how the sound signal propagates.

Airborne sound propagates through the air. It is generated, for example:

• at leaks in compressed air, gas, or vacuum systems,
• by electrical partial discharges and insulation defects,
• at leaks in non-pressurized components when used in combination with an ultrasonic transmitter.

Airborne sound measurements are carried out using airborne sound sensors or acoustic cameras.

Structure-borne sound propagates through solid materials. It is generated, for example:

• in rolling and plain bearings,
• by friction and insufficient lubrication,
• in valves and steam traps,
• by cavitation in pumps and compressors.

For structure-borne sound measurements, the sensor is coupled directly to the measurement point or via a waveguide.

The appropriate measurement method depends on whether the relevant ultrasonic signal propagates through the air or through the component.