SBIR-STTR Award

Both acoustic and electromagnetic waves exert forces of radiation on targets in their path of propagation. The radiation force (pressure) results from the change in wave momentum due to scattering by the target, averaged over one period of oscillation. Modulated radiation pressure (MRP) utilizes the acoustic radiation force by using a double-sided suppressed carrier modulated (DSB-SCM) incident signal, which when averaged over one period of the carrier frequency, results in a signal that oscillates at the modulation frequency. Target detection and classification can be facilitated by using the high carrier frequency to create a narrow beam with surgical accuracy, which can be used to probe the target with the ability to shake it at the modulation frequency. A frequency scan can reveal target resonances, while a physical scan along the length of the target can reveal the mode shapes corresponding to those resonances, which can be used to estimate target size and its content. The purpose of this effort is to investigate the use of MRP in detection, classification and identification of underwater targets. In the last five years we have been investigating the use of MRP with scaled targets using funding from ONR. We plan to build on this experience to design and fabricate a prototype transducer that will eventually be modified for use with real-size targets.

Benefit:
Modulated radiation pressure of focused beams can provide the means to interrogate a target in ways that cannot be afforded by conventional sonar. It can extract clues about target shape, type, dimensions and content as a consequence of the focusing. Except for insignificant parametrically generated low-frequency sound, the target response at the modulation frequency depends on the focal location in such a way that the neighboring clutter is primarily ensonified by the radiation of the target and not the incident signal. Thus, the presence of clutter is mitigated since the resulting radiation from it is weaker than if it had been directly ensonified by the incident signal as would be the case in conventional sonar. The ability to apply this method in air as well as water, makes its applications that much wider. For example, it can be used in non-destructive testing to assess damage in mechanical parts (such as hydraulic turbines/propellers, aircraft parts, machine parts and so on) by a frequency scan, which can determine the type and extent of damage by evaluating the shift in the objects resonant frequencies and its Q-factor. This technology can also benefit the medical ultrasound community where this method has wide applications.

Keywords:
Scattering, Scattering, acoustic lens, modulated radiation pressure, radiation force, Target Resonances

In the past few years, we have experimentally demonstrated that modulated radiation pressure (MRP) can be used to extract classification clues such as resonant frequencies, mode shapes and sizes for targets of interest. These experiments were carried out at Washington State University (WSU) between (2015-2020) in water tanks involving scaled targets. Using a 15-cm acoustic lens, MRP was applied to a variety of metallic targets, including circular plates, solid aluminum cylinders, open-ended cylindrical shells, and even an aluminum replica of a UXO, and their resonant frequencies and mode shapes were measured. Modulated radiation pressure exploits the phenomenon of the acoustic radiation force, which is produced due to the rate of change of momentum that an incoming wave experiences when it is scattered by a target. The acoustic radiation force has no obvious use for a regular incident signal, but if the incident field is a double-sided suppressed carrier modulated (DSB-SCM) signal, the way in which a physical system responds to such an excitation, results in a signal that oscillates at twice the frequency of the modulation oscillator. This creates a process by which a target can be interrogated by the surgical accuracy of a narrow beam, produced by the high carrier frequency, while being shaken at a single low modulation frequency. A modulation frequency sweep (frequency scan) can reveal target resonances, while a physical scan along its length can reveal its mode shapes corresponding to those resonances, which can be used to estimate target size and content. The above experiments were conducted in highly controlled laboratory environments at short standoff distances due to the short (15 cm) focal lengths of the available acoustic lenses. Prior to the start of this STTR, an acoustic lens with a focal length of one meter was designed and fabricated at WSU. During Phase I of this STTR, this lens was used to conduct MRP experiments involving a variety of targets including a solid aluminum cylinder and an open-ended, thin aluminum cylindrical shell. It was demonstrated that MRP could excite these targets at a standoff distance of one meter and for the case of the cylindrical shell it was also demonstrated that it could extract its mode shape by a physical scan along its length. The focus of the Phase II work is 1) to demonstrate that MRP can be used in a realistic ocean environment and 2) that it is capable of extracting classification clues for real-size targets at longer standoff distances of up to three meters.

Benefit:
So far, modulated radiation pressure (MRP) has been applied to scaled targets in water tanks at short standoff distances of one meter. If successful, the proposed work will enable target interrogation at a standoff distance of three meters in a realistic ocean environment, where target parameters such as resonant frequencies, vibrational modes, size, and content can be extracted without physical contact.

Keywords:
Scattering, Acoustic Radiation Force, modulated radiation pressure, resonant frequencies