Brüel & Kjær has long since been a pioneer in the field of acoustic holography offering solutions both for stationary and non-stationary sound fields. The techniques have been continually developed right up to the present day, from the original STSF, to SONAH, to ESM, to Wideband holography and Proximal Holography.
Spatial Transformation of Sound Fields (STSF)
The name 'Spatial Transformation of Sound Fields (STSF)' gives a good indication of the capabilities of the technique. Based on a measurement over a 'scan plane' using a regular microphone array, or a robot positioned row of microphones, close to the sound source, the sound field can be calculated over parallel planes in the near-field region. The measured 2D sound field data can be 'transformed' to other surfaces enabling a complete 3D description of the sound field. STSF can calculate any sound field descriptor such as sound pressure, sound intensity, or particle velocity as a function of position. Contribution analysis can be performed so that you can investigate the effects in the far field of changes to the sound source.
Non-stationary sound fields
By combining acoustic holography with quasi-stationary or transient calculations, any sound field descriptor such as sound pressure, sound intensity or particle velocity as a function of position and time can be obtained. Results are typically displayed as animated maps to illustrate how a specific property changes as a function of time.
Statistically Optimised Near-field Acoustic Holography (SONAH)
The spatial FFT processing used in Near-field Acoustical Holography (NAH) makes the method computationally efficient, but it introduces severe spatial windowing effects, unless the measurement area is significantly larger than the source. The Statistically Optimal NAH (SONAH) method performs the plane-to-plane calculations directly in the spatial domain. Therefore, the need for a representation in the spatial frequency domain and for zero padding is avoided, significantly reducing the spatial windowing effects.
Equivalent Source Method (ESM)
The SONAH method should be chosen when measuring close to planar surfaces. However for curved surfaces, the Equivalent Source Method gives more precise results for a slightly longer calculation time. When the ESM algorithm is selected, a Spatial Smoothing parameter can be used to control the distance between the calculation surface and the surface of the equivalent sources (which are placed behind the target surface). ESM can be used on both planar and curved surfaces.
Wideband Holography (WBH)
Acoustic holography methods such as SONAH and ESM are restricted in uses to arrays with less than half wavelength average, inter-microphone spacing. For a given array, this restriction defines an upper limit on the supported frequency range. To extend the frequency range, irregular 'combo array' geometries are used for SONAH at low frequencies and for beamforming above the previously mentioned upper limiting frequency. A drawback here is the need for two methods to cover the full frequency range: a low-frequency measurement at close range with SONAH and a high-frequency measurement at a further distance with beamforming. The patented Wideband Holography (WBH) method can cover the combined frequency ranges of SONAH and beamforming based on a single measurement at an intermediate distance.
You can find a wealth of information about acoustic holography and other array techniques in our Technical Reviews.