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Sound source localization

Sound source localization
Jeroen Lanslots, Filip Deblauwe and Karl Janssens of LMS International discuss the various sound localisation techniques.

The toughest challenge facing any acoustics engineer is figuring out where the sound originates - especially when there is a good portion of interference and reverb flying around. Since the early nineties, a number of rather standard and highly functional methods, based on microphone arrays, have matured, but the problem remains that there is no one 'magical' single technique that prevails.

Near-field Acoustic Holography (NAH) is a technique where the microphone array is placed relatively close to the sound source. The near field can be described as the area that is closer to the sound source than one or two wavelengths of the highest frequency. NAH measures sound pressure by arranging several microphones in a rectangular planar array. Microphones are regularly spaced both horizontally and vertically. The sound pressure in the plane is then back-propagated to the actual surface of the object. The spacing between the microphones determines the half-wavelength of the maximum frequency, and the size of the array determines the half-wavelength of the minimum frequency. The spacing also determines the spatial resolution: a coarsely spaced array cannot accurately localize sources on the fine mechanics of a small object. 

The NAH method is a very precise engineering tool for source localisation, with spatial resolution independent of frequency. However, it has some disadvantages. It can only propagate sound pressure to a surface that is parallel to the measured surface. The size of the propagation plane has to be identical to the measured plane. To localise a source on a complete vehicle, the measurement plane has to span the complete vehicle. A further disadvantage is that the higher the required maximum frequency, the closer the microphones spacing. Practically speaking, NAH can be inconvenient for higher frequencies due to the large amount of data required to achieve a good analysis.

An alternative technique is beamforming, where the microphone array is placed in the far field. As a rule of thumb, the far field is defined as being further away from the source than the array dimensions or diameter. Numerous microphone configurations are possible in beamforming arrays. In general, the configuration is usually a trade-off between dynamic range and source localisation accuracy. To get the best of both worlds, it is best to select a circular array with a pseudo-random microphone distribution.

In the far field, it is possible to propagate the measured sound field directly to the test object. All microphone signals measured by the beamforming array are added together, taking into account the delay corresponding to the propagation distance. The pressure can be calculated at any point in front of the array, allowing propagation to any kind of surface. Beamforming requires that all data is measured simultaneously. It is typically done with a measurement system of 40 channels or more. 

Beamforming has a number of advantages. Propagation does not relate to the size of the measurement array. The test object can be larger than the array. With an array with a 0.5m diameter, it is possible to propagate pressure to an entire car. Further, because of the relatively fast acquisition and analysis speed, beamforming lets engineers evaluate several configurations in a limited amount of time. However, this flexibility has some negative aspects. The spatial resolution is proportional to the wavelength, so beamforming, in general, is only usable at frequencies above 1000Hz. And beamforming cannot be used to calculate sound power.

Recent advances
Near-field focalisation (NFF) is a beamforming technique based on near-field measurements. It either reprocesses already acquired NAH data or allows the beamforming array to be moved closer to the sound source. The original beamforming back propagation is reformulated to deal with the spherical near-field waves. NFF improves the spatial resolution by a factor of two over the entire frequency range. This technique is very useful for wide-angle beamforming acoustic cameras that can zoom into a sound hot spot, providing high-definition source localisation. It lowers the threshold of the minimum frequency, at which beamforming can be employed.

Another recent development in sound source localisation is a technique that can be applied in complex, reflective sound fields, such as the interior of a vehicle or the cabin areas in trains and airplanes. Spherical beamforming uses a far-field beamforming technique in an interior cavity, which is a reflective sound field. Spherical beamforming employs a closed spherical array. Therefore, the technique helps identify the exact position of a sound source in the surrounding space. Further, spherical beamforming allows back propagation of the measured sound on the geometry. It is shown that back propagating sound to a location that is further away or closer to the spherical array than the actual sound source leads to errors and lower dynamic range. Using the actual geometry of the interior has therefore clear advantages over using a virtual sphere around the acoustic antenna. 

As beamforming techniques show poor results in low frequencies, an exciting recent improvement adapts an inverse method: the Equivalent Source Method (ESM). A principal component analysis (PCA) is applied before the ESM to deal with the uncorrelated characteristic of noise source distributions. The propagation matrix to inverse through ESM includes the sphere diffraction. The noise source distribution takes the geometry of any cavity instead of the usual two dimensional map. The results of ESM in low frequency not only show a far better spatial resolution, they also provide information about the sound power level. At higher frequencies, localisation performances using ESM are similar to standard spherical beamforming, but has the advantage of combining noise mapping and sound quantification together in one single shot.

Near-field acoustic holography with irregular arrays is another recent development. NAH requires a rectangular array with evenly spaced microphones. NAH cannot be performed with a beamforming array as it is not rectangular and has a pseudo-random microphone distribution. To overcome this, the problem is rewritten as an inverse method. The transfer function includes both propagated and evanescent wave functions, and needs an optimal and stable PCA-based regularisation, which includes evanescent wave filtering.

Inverse numerical acoustics (INA) is a method which reconstructs the surface normal velocities on a vibrating structure from the sound measured in the near field around the structure. This is of particular interest when the structure is rotating or moving, or too light or too hot to be instrumented by accelerometers. INA is a unique and hybrid solution as it combines experimental sound pressure data in the near field, measured by a microphone array, with Acoustic Transfer Vectors (ATVs) obtained from simulation.

So how do you decide which technique to use? As a general rule, near field techniques should be preferred, when possible, as they often provide the best results in terms of dynamic range and spatial resolution. Combined NAH-NFF delivers optimal results, as NAH is the most suitable technique in the low and mid frequency range, and NFF most appropriate for higher frequencies. 

There are some cases where combined NAH-NFF is not practical. Perhaps it is not possible to measure in the near field, or the array size becomes too big, or it is not possible to measure in patches due to rapidly changing operational conditions. In these cases, a beamforming solution will be chosen. A good strategy is to first use classical beamforming like a far field wide angle acoustic camera to get a global view on where the sound sources are located. As a next step, the acoustic camera is moved into the near field for a zoomed view of these sound sources using focalization. Finally, further refinement in low and mid frequency can be obtained using NAH for irregular arrays with the same data.
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