Fin whales are among the largest predators on earth, yet little is known about their foraging behavior at depth. These whales obtain their prey by lunge-feeding, an extraordinary biomechanical event where large amounts of water and prey are engulfed and filtered. This process entails a high energetic cost that effectively decreases dive duration and increases post-dive recovery time. To examine the body mechanics of fin whales during foraging dives we attached high-resolution digital tags, equipped with a hydrophone, a depth gauge and a dual-axis accelerometer, to the backs of surfacing fin whales in the Southern California Bight. Body pitch and roll were estimated by changes in static gravitational acceleration detected by orthogonal axes of the accelerometer, while higher frequency, smaller amplitude oscillations in the accelerometer signals were interpreted as bouts of active fluking. Instantaneous velocity of the whale was determined from the magnitude of turbulent flow noise measured by the hydrophone and confirmed by kinematic analysis. Fin whales employed gliding gaits during descent, executed a series of lunges at depth and ascended to the surface by steady fluking. Our examination of body kinematics at depth reveals variable lunge-feeding behavior in the context of distinct kinematic modes, which exhibit temporal coordination of rotational torques with translational accelerations. Maximum swimming speeds during lunges match previous estimates of the flow-induced pressure needed to completely expand the buccal cavity during feeding.

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Kinematics of foraging dives and lunge-feeding in fin whales

The external ear protects the eardrum from mechanical damage by providing a narrow and extended entrance to the auditory system. It also transforms the sound field modifying and augmenting the directionality associated with head diffraction and adding substantial acoustic gain at the higher frequencies. These properties are intimately connected with the spatial perception of sound and are of considerable importance in auditory measurements and instrument design. To understand these properties it is useful to divide the acoustic antenna system into its functional components: the head, the torso and the pinna flange (Fig. 1) acting as diffracting bodies, the concha and the earcanal serving as acoustic resonators, and the eardrum providing an acoustic termination. But these components must always be seen as parts of an integrated system for it is the sound pressure transformation from the free field to the eardrum as a function of frequency, direction, and perhaps distance which is most directly linked with monaural and binaural localization and with the overall sensitivity of the hearing system.

The External Ear

A hydrophone-equipped tag recorded exposure to noise, as well as physiological and behavioral sounds, on free-ranging northern elephant seals (Mirounga angustirostris). The compact acoustic probe (CAP) consisted of the hydrophone, a thermistor, and a pressure transducer in a 36 cm long, 10 cm diameter cylindrical hydrodynamic housing capable of withstanding 2000 m depth. The enclosed logging electronics included a programmable “TattleTale 7” data acquisition engine and a 340 Mb hard disk. A custom low-power operating system supported multi-channel interrupt-driven sampling at 5 kHz. The complete tag weighed 0.9 kg in water and displaced 2.1 l. During five deployments on juveniles translocated from and returning to Ano Nuevo, California, CAP tags measured dive pattern, ambient and vessel noise exposure, oceanographic ranging (RAFOS) and thermometry (ATOC) beacons, acoustic signatures of swim stroke, surface respiration, and cardiac function, and possible vocalizations.

A programmable acoustic recording tag and first results from free-ranging northern elephant seals

Acoustic particle velocity is commonly inferred from measurement of pressure or pressure gradient; however, in water, direct measurement is simple. A moving‐coil sensor embedded in a neutrally buoyant package produces a voltage directly proportional to the particle velocity in the surrounding fluid for frequencies above the mass‐spring resonance. Leslie et al. [J. Acoust. Soc. Am. 28, 711–715 (1956)] built such a sensor by mounting a moving‐coil element inside a hollow brass sphere. The sensor described in this paper is identical in principle but is considerably easier to fabricate. Useful from tens of hertz to several kilohertz, this sensor consists of a glass‐ microballoon‐and‐epoxy composite cast around a small, commercial geophone. The sensor is inexpensive, rugged, and has good immunity to interference. In conjunction with a pressure hydrophone, acoustic intensity can be measured without the errors associated with subtraction of nearly equal signals (as in the two‐hydrophone method).

A simple neutrally buoyant sensor for direct measurement of particle velocity and intensity in water

The theoretical directivity of a single combined acoustic receiver, a device that can measure many quantities of an acoustic field at a collocated point, is presented here. The formulation is developed using a Taylor series expansion of acoustic pressure about the origin of a Cartesian coordinate system. For example, the quantities measured by a second-order combined receiver, denoted a dyadic sensor, are acoustic pressure, the three orthogonal components of acoustic particle velocity, and the nine spatial gradients of the velocity vector. The power series expansion, which can be of any order, is cast into an expression that defines the directivity of a single receiving element. It is shown that a single highly directional dyadic sensor can have a directivity index of up to 9.5 dB. However, there is a price to pay with highly directive sensors; these sensors can be significantly more sensitive to nonacoustic noise sources.

Highly directional acoustic receivers.

A circular flow tank facility has been developed which features extremely low acoustic and vibration ambient conditions. This facility is described, operating procedures associated with its use are presented, and limitations in the measurement procedures are mentioned. The flow facility has been used to measure the noise output of pressure and pressure gradient hydrophones during actual flow conditions and results of these tests are presented and discussed.

Measurements of low‐velocity flow noise on pressure and pressure gradient hydrophones

A decade ago, two of the authors presented a paper reporting on diffraction and interaural delay of a progressive wave caused by the human head [J. Acoust. Soc. Am. 36, 1993(A) (1964)]. Six live subjects were used in those experiments. The publication was delayed pending measurements on artificial heads then being planned. In this paper, two series of data obtained with the aid of the Acoustical Manikin [J. Acoust. Soc. Am. 42, 204–207 (1967)] are compared with the earlier experiments: (a) Data based on phase measurements; (b) Data obtained using a bandwidth‐limited delta pulse. All three methods provide useful information about interaural diffraction and delay, albeit the third method appears to be the most reliable.

Measurements of diffraction and interaural delay of a progressive sound wave caused by the human head. II

External ear replica for acoustical testing noting sensing element, ear canal and eardrum impedance

External‐Ear Replica for Acoustical Testing

A gradient hydrophone calibrator consists of a precisely made water‐filled tank, with ports to receive the hydrophones to be calibrated and the reference pressure hydrophones, which is set into axial oscillation with suitable drivers to develop a standing wave of sound in the water. An improved calibrator is described, which, by using a very rigid tank, covers a range of 3–2500 Hz. Measurements are simplified through use of electrical feedback, which maintains a constant pressure‐gradient versus frequency function. The applications and limitations of this system to the measurement of sensitivity, frequency response, and phase response of pressure‐gradient and pressure hydrophones, and for obtaining the polar patterns of gradient hydrophones, are described.

Louis A. Abbagnaro

Papers

Measurements of low‐velocity flow noise on pressure and pressure gradient hydrophones

Measurements of diffraction and interaural delay of a progressive sound wave caused by the human head. II

External‐Ear Replica for Acoustical Testing

Wide‐Range Calibration System for Pressure Gradient Hydrophones

The Ghent Microphone System for SQ- Quadraphonic Recording and Broadcasting