Microphone Array Beamforming and Acoustic Imaging for Noise Source Identification

Microphone arrays enable spatial acoustic measurement and noise source localisation — the ability to identify where sound is coming from in a complex source environment. This capability is invaluable in automotive NVH development, aircraft noise source characterisation, industrial noise source ranking, and architectural acoustic fault detection. The technique uses the delay in arrival times of a sound wave across the array to determine the source direction, and creates an acoustic map showing the relative contribution of different source regions.

The fundamental principle — delay-and-sum beamforming — calculates, for each candidate source position on a reference plane, the time delays that would result from sound arriving from that position at each microphone, applies compensating delays to the measured signals, and sums the time-aligned signals. If the candidate position matches the actual source position, the signals add coherently and a peak appears in the acoustic map. At non-source positions, the signals are incoherent and partially cancel.

Array Design Principles

• Aperture — the overall array diameter determines the angular resolution: larger aperture = finer spatial resolution at a given frequency
• Element spacing — the spacing between microphones determines the upper frequency limit before spatial aliasing: smaller spacing = higher frequency coverage
• Element count — more microphones improve the sidelobe suppression and dynamic range of the acoustic map
• Spiral and random array layouts — reduce periodic sidelobe patterns compared to uniform spacing

Beyond Delay-and-Sum: Advanced Algorithms

• CLEAN-SC (Source Coherence deconvolution) — removes the PSF smearing effect; improves spatial resolution for distributed sources
• DAMAS (Deconvolution Approach for Mapping of Acoustic Sources) — iterative deconvolution; highest fidelity source maps for well-separated sources
• Frequency-domain beamforming — enables efficient analysis across full frequency range simultaneously

Multi-channel simultaneous recording is essential for array beamforming — all channels must be sampled at exactly the same instant, as inter-channel timing errors directly corrupt the delay-and-sum calculation. PLACID PQ1601 (16-channel) and PQ801 (8-channel) DAQ systems with simultaneous sampling are used in array measurement applications.

Calibration Requirements for Array Measurement

For quantitative acoustic imaging — where the acoustic map represents calibrated sound pressure levels at each position rather than just relative source contributions — each microphone in the array must have a known, calibrated sensitivity. Sensitivity variation across the array elements introduces systematic errors in the source level estimate. PLACID array microphone sets are supplied with individual ISO/IEC 17025 accredited calibration certificates for each microphone position. The calibrated sensitivity values are stored in a configuration file that the analysis software applies automatically when calculating source SPL from the array data.