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The painting of sound

05. 09. 2017 - You have certainly heard about temperature maps. Infrared thermography records the temperature of an object or area - from blue to green and yellow to red, showing the temperature distribution map from the coldest to the hottest.Sound can be depicted in a similar manner. That’s what the acoustic camera is used for. It is a very interesting device that is also now being used by our development laboratory.

Microphones ... lots of microphones

The acoustic camera consists of a large number of microphones - usually from about 30 to 120, which are variously arranged in a space. At the centre of this microphone field is a high resolution video camera that optically records the scene, which allows the result of an acoustic calculation, that is, an acoustic map, to be projected into the recording.

In addition, the unit includes a converter that "picks up" the signals from the individual microphones and converts them into a format that the computer and appropriate software equipment will then be able to handle. A variety of different algorithms are used for various purposes and for the deployment of the acoustic camera.

How it works

In practice, the actual use of an acoustic camera is very easy. You simply set it up in front of the measured device - in the case of buildings or business premises, in a manner ensuring that all sources of noise are visible in the field of view. This can often reach even eight hundred meters. If machinery is being measured, then the camera, on the contrary, is placed as close as possible.

The acoustic camera is started up, and usually within a short period of time both the sound from all the microphones and the video from the video camera are recorded. The advantage, of course, apart from the possibility of capturing and depicting different levels of noise, is also the ability to analyse mobile sources as well as create a time recording of sounds for very fast processes. For a whole range of measurements, not only the level of noise but also how it changes in time and space is essential.

In addition, you can attach recordings from a variety of other sensors to the sound recording. For example, they can record the angle of rotation in the space, the voltage and current level, or the engine speed. Which is precisely the case that we use at Swell the most. According to the head of the expert group of the test laboratory Lukáš Kovárník, the test lab uses the acoustic camera for the noise analysis of the tested components "In principle, we use it for two basic tasks. The first is the evaluation of components that have a kind of internal kinematics, and it must be ascertained whether there is unwanted noise (creaking, squeaking or knocking) during its operation. A similar example of this is with components that contain any electrical noise sources – for example, drives (electric motors or solenoids). Here again, it is often necessary to analyse the occurrence of unwanted noise during operation. It is then typically necessary to move the tested piece in a controlled manner (mechanically or electrically) while conducting an analysis," he explains.

The second task is to measure acoustic properties under a vibrational load. "Occasionally, the parts function properly, but when mounted on a vehicle and excited by external sources such as driving vibrations from the chassis (often a random spectrum), or from aggregates (engine, gearbox) unwanted noises occur as in the first case (resonance, rattling, jolting) in the usually harmonic spectrum. The test piece is usually placed on a vibration testing device, the anchor platform of which is located in a silent (anechoic) chamber. Subsequently, the test piece is loaded with a random or harmonic vibration profile in the desired frequency range, and its behaviour is meanwhile analysed by the acoustic camera," adds Kovárník.

There may, of course, be several measurements, and the resulting data can then be averaged, grouped or otherwise analysed. The core of the analysis is the appropriate selection and evaluation of time and frequency domains. It is therefore a matter of selecting the measured signals in time that are of interest to the customer, as well as selecting the desired frequencies. Then the source distance to the acoustic camera is set and the appropriate algorithm is selected.

It is not able to handle deep tones

Once all the calculations have been completed, individual sound pressure levels, i.e. the noise level, can be displayed in the video from the video camera. It can then be seen how the noise scene changes over time and in individual areas.

The acoustic camera is truly versatile, with a very broad range of application. It is the ideal partner for detecting noise sources, for example, in business premises (we’ll find out where the sources of noise are, such as air-handling equipment, conveyors or pipes), in noisy machines, where we do not know where the noise is coming from, or in identifying parts of machines that may be working erroneously .

The acoustic camera, on the other hand, is not suitable for capturing deep tones, meaning low frequencies. Depending on the size of the microphone field and the analysis of thee algorithm used, the device can distinguish frequencies from about 400 hertz. The acoustic camera will not survive even where the measured signal blends with the acoustic background or when there are no dominant frequencies in the measured signal and their levels are similar to the background.

This is especially true for measurements in a free environment with loaded and surrounding background noise. In some cases, however, it is possible to place the test objects in a separate silent chamber (ideally anechoic), including an acoustic camera, and then take measurements there. However, the introduction of real load conditions (drives, adjacent components, etc.) is often the problem.

Interiors and engines are measured in the test room

Acoustic cameras can be used, for example, when monitoring noise in a free environment, such as the noise mapping of wind power plants and how the different shapes of their “propeller blades” affects it.

They are of course used in industry, transport and services where the proper function of various devices is associated with a particular sound accompaniment whose change may indicate a malfunction. They can also be used to detect acoustically weak spots on the facades of buildings.

Of course, we are most interested in the automotive industry. There, apart from interior noise measurement using acoustic cameras, for example the combustion cycle in the engine is monitored - the moment of ignition or the release of exhaust fumes from the cylinder can be displayed. Another use is, for example, to monitor how tire noise arises from different types of surfaces and at different speeds. Thanks to the acoustic camera, engineers, for example, have found that the rolling noise from the wheels exceeds engine noise at speeds above 30 km / h.

Jaromir Kejval, Head of the Testing and Simulation Department, adds some practical observations and experience directly from the test room: "This innovative test facility also greatly facilitates and accelerates the work of us and our customers when it's not clear where the cause of the unwanted sounds is. Due to the ever-increasing comfort requirements, especially in interiors (in practice, this area is called NVH - Noise, Vibration, Harshness), it is also increasingly important in the perception of users," he explains.

Compared to common variables such as stiffness, strength, temperature, colour, and other well-defined and measurable parameters, noise assessment is often subjective.

"That's why this type of device is another positive step forward towards better understanding and explanation for non-professionals as well in the field of acoustics. The graphic output and clear visualization of noise sources are often positively evaluated by customers. Sometimes the search for the sources and causes of noise can be a bit reminiscent of detective work, but that makes it even more interesting to us, and now we have another strong helper in the lab, thanks to the acoustic camera," concludes Kejval.

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