# A Feasibility Study for a Hand-Held Acoustic Imaging Camera
## 1 Introduction  
Acoustic imaging is an emerging methodology that aims to create spatial maps of sound sources analogous to conventional optical cameras. It digitally reconstructs acoustic fields based on the analysis of sound waves captured by microphone arrays and advanced signal processing algorithms [1, 2]. Well-known and widespread acoustic imaging applications include sonar and ultrasound [3]. Potential applications include pinpointing mechanical faults in machines [4], monitoring transport noise pollution [5], locating sniper fire in combat zones [6], validating room acoustics models [7], and many others spanning industrial inspection, public health, security, and virtual reality domains. While optical cameras form images along physical sight lines, acoustic cameras sample sound arriving from diverse directions and computationally focus on particular points in space to create visualizations of sound intensity and origin. This allows passive localization and separation of multiple simultaneous sources based on spatial diversity. The core signal processing operation is known as beamforming, which applies carefully engineered delays and filters to the microphone signals to isolate particular propagation directions [3]. However, performance is subject to physical constraints and trade-offs inherent to the microphone array design [2, 8]. In particular, existing real-time acoustic imaging systems utilize large multi-microphone apertures to achieve sufficient angular resolution and sensitivity [9].  
This leads to bulky configurations unsuitable for portable applications with limited size, weight, and power budgets (see for instance [[https://www.fir.com/browse/industrial/acoustic-imaging-cameras/](https://www.fir.com/browse/industrial/acoustic-imaging-cameras/)]([https://www.fir.com/browse/industrial/acoustic-imaging-cameras/](https://www.fir.com/browse/industrial/acoustic-imaging-cameras/)), accessed on 14 June 2023). There is strong motivation to miniaturize such cameras for more accessible and more extensive deployment. This paper investigates methodologies to reduce the form factor of real-time acoustic imaging systems by an order of magnitude while minimizing losses in spatial filtering fidelity. We specifically consider the case study of the _Dual Cam_ (Figure 1), an acoustic camera prototype developed at the Italian Institute of Technology [10]. It combines a 0.5 \(\times\) 0.5 m\({}^{2}\), 128\(-\)element microphone array with an embedded system for real-time beamforming and visualization over wideband [500, 6400] Hz. While high-performing, the large stationary apparatus restricts usage scenarios. Our approach is to co-optimize the array configuration and beamforming filters through simulations to retain directional acoustic sensing capability on dramatically smaller footprints. We quantitatively demonstrate that a 32\(-\)microphone array over a 0.21 \(\times\) 0.21 m\({}^{2}\) aperture optimized for the acoustic frequencies of interest can provide better directivity than the 128\(-\)microphone, 0.50 m aperture _Dual Cam_ array from 2 kHz to 6.4 kHz. This supports reducing system size by up to 4\(\times\) with tolerable imaging trade-offs. Ongoing efforts are focused on constructing miniature microphone arrays guided by these simulations to develop portable acoustic cameras that interface with tablets/laptops and smartphones for easy deployment. Enabling compact, real-time acoustic imaging could expand applications in machine health monitoring where vibration analysis indicates developing faults before catastrophic failure [11], urban noise pollution mapping to improve public health interventions [12], and augmented/virtual reality scene analysis for realistic audio rendering [13; 14]. The methodologies and insights presented provide an array of signal processing starting points for researchers and engineers aiming to transform acoustic imaging capabilities from the lab to the field.