An introduction to acoustic imaging
What is acoustic imaging?
The name says it all! In the same way an ordinary camera uses light from a scene to produce a picture, an acoustic imager uses sound. The imager converts the sounds it picks up into a picture on a display screen where, for example, loud sound sources are displayed in a different colour from quieter ones. To make it easy to see exactly where sounds are coming from, acoustic imagers usually incorporate an ordinary optical camera, and on the display screen, they superimpose the acoustic image (the sound picture) on top of the picture captured by the optical camera.
How acoustic imaging works
In place of the lens an ordinary camera uses to collect light, an acoustic imager uses an array of microphones – typically 100 or more – to collect sound. The signal from each of the microphones is passed to a sophisticated processing system that works out where the sound is coming from, how loud it is, and the frequencies it is made up of. The processing system uses this information to produce the acoustic image. Two points are worth noting. The first is that the sounds picked up and processed by the acoustic imager are typically at ultrasonic frequencies, that is, at frequencies higher than can be directly heard by the human ear. The second is that because of the large number of microphones used in the array, their special arrangement and the advanced processing techniques used in the imager, the array can, in effect, be focussed rather like a lens, which makes it possible to locate sound sources accurately.
Why acoustic imaging is useful
Many phenomena of interest to engineers and technicians produce ultrasonic sound, but among the most important are gas leaks and partial discharge (PD) in medium- and high-voltage electrical systems. Depending on the substance leaking, gas leaks can be extremely dangerous and almost always lead to costly waste of resources. Partial discharge in electrical systems often indicates a fault in its early stages, which, if not given prompt attention, may progress to a complete and very costly breakdown.
In other words, whether a sound source results from a gas leak or partial discharge, locating it quickly and accurately so that prompt remedial action can be taken to minimise hazards and save money.
It is possible, of course, to locate gas leaks - and sometimes partial discharge - by inspection or even by using simple listening devices that convert ultrasonic sounds to frequencies that can be heard. In cases, surveying all but the smallest plant to find the problems is a tedious and time-consuming process that often involves hazards such as using ladders to get close to potential fault locations and, for partial discharge, working in close proximity to energised high voltage equipment.
Acoustic imaging offers a safer, faster and much more convenient solution.
Acoustic imaging in practice
Modern acoustic imagers, such as those in the Megger MPAC range, are compact handheld devices with internal batteries and so are as comfortable and convenient to use as a digital camera. For basic operation, it’s only necessary to switch the imager on and set two parameters: the frequency range of the sounds to be detected and their dynamic range, which determines the quietest and loudest sounds the imager will detect. A few minutes of experimentation is all that’s needed to become familiar with these settings.
Once the frequency and dynamic ranges have been set, the acoustic imager is ready for use. It can be scanned slowly across the plant or equipment it is required to investigate, and sound sources will be clearly shown on the screen, superimposed on an optical image. Depending on the instrument, sound sources at distances from less than a metre to over 100 metres can be imaged. This is a big advantage because large areas can be inspected from a single vantage point and make it unnecessary to approach the plant or equipment. For instance, a safe working distance can be maintained from energised high voltage equipment or hot pipework, and the need for working at height is minimised.
Acoustic and optical images captured by the acoustic imager can be stored for further analysis along with other data such as leakage rates and partial discharge type. Video storage is also possible with many devices, and some also offer the ability to connect headphones to listen to the sounds being captured, which are modulated so that they are reproduced in the audible range. Another useful feature provided by some models is the manual focus of the microphone array, helping the user pinpoint an issue where multiple sound sources are occurring or they are working in an area with a high level of background noise.
Benefits of acoustic imaging
Acoustic imaging offers many benefits, some of which have already been mentioned. Among the most important are:
- Easy surveying of large areas of plant quickly
- Fast location of gas leaks and PD
- No need to get close to potentially dangerous equipment
- The need for working at height is often eliminated
- Acoustic imagers are simple to set up and use
- Instant results are provided, which are easily recorded for future reference
- The results are clear and easy to understand
- Acoustic imagers detect leakage of any gas and of partial discharge in almost all types of equipment
These benefits are difficult, if not impossible, to match using any other technique.
Choosing an acoustic imager
Acoustic imaging is a relatively new technology, and, as a result, there can be significant differences between products in terms of convenience, versatility and performance. These need to be investigated and taken into account when choosing an instrument. Some of the most important things to consider are:
- Where it will be used. If the detection of gas leaks on process plant is the main application, the imager will likely need to be used in hazardous areas where explosive gases and vapours could be present. For use in these areas, the imager must have ATEX approval. However, it is worth remembering that an ATEX-approved imager is perfectly suitable for use in non-hazardous areas, making it a good all-round choice. In addition, the ingress protection rating should be considered as acoustic imagers are frequently used outdoors and in dusty environments. The IP rating should reflect this; IP54 or better is recommended.
- Detection range. While the best instruments can image sound sources like leaks and partial discharge at distances of more than 100 metres from the instrument, this sort of range is not achieved by all types. A good detection range is an important benefit as it enables equipment and plant to be surveyed more quickly and easily from fewer locations.
- Number of microphones. Generally, the larger the number of microphones in the imager’s microphone array, the more detailed the acoustic images it can produce. However, the law of diminishing returns applies, and beyond a certain number of microphones, the potential benefits are outweighed by increased cost and complexity. Around 100 microphones is generally found to be the optimum number.
- Acoustic focusing function. In most environments, a good acoustic imager will operate reliably with only the most basic settings. In very noisy locations or situations where there is a need to sample multiple sound sources, a manual focusing function is invaluable, allowing the user to target specific sound sources and exclude local background noise.
- Optical camera resolution. The optical images – photographs or videos – produced by the acoustic imager are an important part of the data captured and it is often useful to be able to zoom in on these to examine in detail a location that is associated with a particular sound source. This is only possible if the imager captures high resolution optical images. A camera resolution of 8 megapixels or more is desirable.
- Built-in analytical functionality. An important benefit of acoustic imaging is that it provides instant on-the-spot results. However, the value of these results is further enhanced if the imager has built-in analytical capabilities. In the case of leaks, for example, some instruments can estimate the leakage volume in real-time, allowing the ongoing cost and, therefore, the urgency of the repair to be estimated. For partial discharge, the best instruments use phase resolved methods to discriminate between surface discharge, suspension discharge and corona discharge, which is a big help in deciding the best maintenance strategy to adopt.
- Supplier reputation and capabilities. While acoustic imagers are, in the main, reliable instruments that are easy to use, it is always a benefit to buy from a supplier that can and will provide expert support and advice should it be needed.
Acoustic imaging is a revolutionary technology that has the potential to provide invaluable input into preventative maintenance programmes across a wide range of industries, as well as aiding the location and identification of existing and developing faults of many kinds. It is easy and convenient to operate, and because it works effectively over large distances, it helps users avoid working in hazardous locations. To learn more about this technology and Megger's MPAC128 and MPAC128-ATEX acoustic imagers, click here.
