Here at The Snell Group, we employ airborne ultrasonic detection as an inspection methodology for detecting two types of failure modes in equipment: leak detection and electrical discharge detection.  While at first glance these two types of tests may seem like they’re miles apart, they actually aren’t.  Before we dive into that, let’s discuss what airborne ultrasonic detection is.

Human hearing can detect acoustic signatures in the frequency range between 2,000 and 20,000 hertz.  Modern airborne ultrasonic detection systems detect in the frequency range between 20,000 and 100,000 hertz, which obviously is outside of what the human ear can hear.  These test instruments then use a process known as “heterodyning” to make the acoustic data they detect usable for a person utilizing the system. That process works very similar to how an ordinary broadcast radio functions.  Sound waves, like the music of voices, can’t travel at the speed of light, but radio waves can. In a broadcast station, the information being transmitted is embedded on a radio wave signal and broadcast, and then at the receiving end (your home or car, etc.) it’s pulled back off the radio wave signal.  Heterodyning in an airborne ultrasonic detection system is akin to that process. It takes the acoustic signal that the human ear can’t hear and puts it onto a signal it can hear, so we can then analyze the sound.

In leak detection, the acoustic signature that is produced is the result of turbulence in the flow of the material we’re inspecting. Compressed gas, vacuum systems, liquid transfer systems all have fluid flow through them, and when a leak develops the flow moves through the unintentional opening, typically in a turbulent fashion. That turbulent flow produces an acoustic signature (or noise, if you prefer) that the instrument can detect. Those acoustics signatures are generally in the 20,000-30,000 hertz range.

In the case of electrical discharge, we’re sensing acoustic signatures in a different frequency range, but the basic principle is the same. In medium and high voltage systems (generally defined as systems operating above 1,000 volts AC) there often occurs discharge events, which is where the electrical energy in the system will breach the system components in an attempt to equalize back to Earth ground. Generally speaking, the ability to breach air gaps increases as the system voltage increases. As a result, we see more of these types of events in systems operating at higher voltages.  

When the electrical energy tries to “escape” and reach Earth ground, there is an acoustic signature associated with the electrical event. As the discharge amplitude increases, so does the intensity of the acoustic signature. So basically, is gets louder.  Even as it’s amplitude increases, the frequency range at which the discharge occurs is still outside of the human ear’s ability to detect, so the airborne ultrasonic instrument helps us discover these conditions. Usually the signal strength (or loudness) is an indicator of how severe the discharge is. As these types of failure modes progress, they can eventually reach a point where they actually can be heard by the human ear.  However, by that point the equipment condition has severely degraded. We obviously want to find these events before they reach this stage.

Airborne ultrasonic detection is considered one of the more valuable Condition Based Monitoring (CBM) technologies available today. It can be used for other purposes beyond what we’ve described here, and is an affordable option for implementation into reliability centered maintenance programs of any maturity level.