Do you want to see the sound?
As I introduced in my previous notes on aerodynamics, the acoustic optimization of external elements and sharp shapes of an automobile is an important field of the aerodynamic development activity. The acoustic camera is one testing tool employed in this optimization effort.
How does it work.
It typically consists of an array of microphones with a camera attached in the center of the array. The pressure/sound measurements by each microphone are computed in an algorythm taking into account the position of the microphone in the array and in relation to the testing object. The result of this computation is overlaid on the video recording of the camera.
In the case of wind tunnel testing, the algorythm may in addition take into account the offset effect between the location of the noise source on the car surface and the position of the array produced by the airflow blowing in between. See here a picture of the setup of an acoustic camera:
How is it applied.
By locating the camera setup some meters to the side of the car (out from the airflow and boundary layer) and letting the airflow blow around the car, the computed result of the noise measurement laid over the real image helps to see and to locate where the noise sources really are.
Find below a picture illustrating the typical arrangement of the camera to the side of the test vehicle. Note the position of the camera far from the airflow and the detached boundary layer coming out of the wind tunnel nozzle in front of the car:
If the computing power of the system allows for delivery of results on site, the tool provides flexibility to test different shapes and configurations with results at hand. Otherwise, a the test program must be completed and afterwards the results will be analyzed.
This location identification is complemented by further analysis with microphones in the interior of the car and other types of pressure sensors located closer to the noise source. The nature and relevant parameters of the noise can in this way be better understood and design decissions be taken.
Examples and a personal note.
In automobile aerodynamics, a typical example of use of the acoustic camera is the location of noise sources around the so called A-Pillar of the car. This is the pillar formed by the sides of the front windscreen and the front door windows, where several sharp edges meet and external elements such as the side mirrors are located. Is therefore necessary to determine if the noise source is one of these edges or the side mirror. The acoustic camera provides a very useful image to understand this.
In my year as subcontractor for Audi, I had the chance to actually test elements with this technique and witness how the technology of these tools evolved. I am person who relies heavily on visual inputs, therefore it was very helpful in my work to have this kind of test results.
Anyway, this technique is not limited to car aerodynamics (not even to car development) and can be found in applications for other vehicles or machinery, or in any research effort where a noise source must be located.