Vaquita: beleaguered porpoise of the Gulf of California, México

The vaquita (Phocoena sinus), an endemic porpoise of the Gulf of California, México, was first described scientifically in 1958, from three skulls. It is considered a sister taxon of an ancestor of the Southern Hemisphere Burmeister’s porpoise (P. spinipinnis) and spectacled porpoise (P. dioptrica), a case of antitropical distribution and speciation. Vaquita in modern times seem to have existed largely in waters 10 to 30 m deep of the very northern Gulf of California, and may have already existed in relatively low numbers by the 1950s and 1960s. The external appearance of the vaquita was not described until the late 1970s, and not until the 1980s and 1990s did additional information about ecology and biology emerge. Those studies and more recent shipboard and aerial visual line transect surveys, as well as stationary and boat-towed acoustic arrays, mapped occurrence patterns and approximate numbers in greater detail than before. The first credible estimates of abundance appeared in the 1990s, with numbers in the mid-hundreds and declining. While several reasons for the decline were originally postulated, mortality due to entanglement in nets has been established as the only known cause of decline, especially due to bycatch in large-mesh gillnets set for the endangered croaker fish totoaba (Totoaba macdonaldi). This fish is prized in China for human consumption of its swim bladder, generally ground up for purported therapeutic purposes. An extensive, lucrative fishery for totoaba, now illegal for many decades, has existed since at least the 1920s, and has recently increased. Although there have been laudable attempts to stem or halt totoaba fishing, these have largely been unsuccessful, and as of this writing the vaquita is on the brink of extinction. However, rapid concentrated action against illegal fishing with gillnets may yet save the species, and hope (with attendant action) must be kept alive. This overview is followed by an appendix of a previously unpublished popular essay by K.S. Norris describing when, where, and how he first discovered the species, and subsequent early work relative to this newly-described porpoise.

Partially activated reconfigurable arrays to guide acoustic waves

Recent studies have exemplified the potential for curved origami-inspired acoustic arrays to focus waves. Yet, reconfigurable structures that adopt curvatures are often difficult to translate to practice due to mechanical deformation of the facets that inhibit straightforward folding. In addition, not all tessellations that curve upon folding are also flat-foldable, which is a key advantage of portability inherent to many origami-inspired structures. This research introduces a new concept of partially activated reconfigurable acoustic arrays as a means to mitigate these drawbacks. Here, tessellations are studied where a subset of the facet surfaces are considered to radiate acoustic waves. The analytical results reveal focusing behaviors in such arrays that are otherwise not manifest for the array when fully activated. The focused waves are more intense in amplitude and space for partially activated arrays than fully activated counterparts. These trends are verified by experiment and are also found to be applicable to multiple reconfigurable array geometries. The results encourage broader study of the design space accessible in reconfigurable arrays to capitalize on all of the functionality afforded by origami-inspired wave guiding structures.

Binaural Heterophasic Superdirective Beamforming

The superdirective beamformer, while attractive for processing broadband acoustic signals, often suffers from the problem of white noise amplification. So, its application requires well-designed acoustic arrays with sensors of extremely low self-noise level, which is difficult if not impossible to attain. In this paper, a new binaural superdirective beamformer is proposed, which is divided into two sub-beamformers. Based on studies and facts in psychoacoustics, these two filters are designed in such a way that they are orthogonal to each other to make the white noise components in the binaural beamforming outputs incoherent while maximizing the output interaural coherence of the diffuse noise, which is important for the brain to localize the sound source of interest. As a result, the signal of interest in the binaural superdirective beamformer’s outputs is in phase but the white noise components in the outputs are random phase, so the human auditory system can better separate the acoustic signal of interest from white noise by listening to the outputs of the proposed approach. Experimental results show that the derived binaural superdirective beamformer is superior to its conventional monaural counterpart.

Reconfigurable Acoustic Arrays With Deployable Structure Based on a Hoberman–Miura System Synthesis

Curved surfaces are often used to radiate and focus acoustic waves. Yet, when tessellated into reconfigurable surfaces for sake of deployability needs, origami-inspired acoustic arrays may be challenging to hold into curved shape and may not retain flat foldability. On the other hand, deployable mechanisms such as the Hoberman ring are as low-dimensional as many origami tessellations and may maintain curved shape with ease due to ideal rigid bar compositions. This research explores an interface between a Hoberman ring and Miura-ori tessellation that maintain kinematic and geometric compatibility for sake of maintaining curved shapes for sound focusing. The Miura-ori facets are considered to vibrate like baffled pistons and generate acoustic waves that radiate from the ring structure. An analytical model is built to reveal the near field acoustic behavior of acoustic arrays resulting from a Hoberman–Miura system synthesis. Acoustic wave focusing capability is scrutinized and validated through proof-of-principle experiments. Studies reveal wave focusing phenomena distinct to this manifestation of the acoustic array and uncover design and operational influences on wave focusing effectiveness. The results encourage exploration of new interfaces between reconfigurable mechanisms and origami devices where low-dimensional shape change is desired.