Recorded vibrations of gray whale skulls to study how vibrations in the skull are amplified in the bony hearing complex to facilitate low frequency hearing

Mysticete whales have a bony ear complex (tympanoperiotic complex, TPC) that supports highly sensitive hearing at low frequencies. Components of the mysticete TPC include the tympanic bulla, which is suspended from the skull by two thin, flexible bones (pedicles), and the periotic bone, which encases the cochlea and is embedded in the skull. Between the bulla and the periotic are the ossicular chain (middle ear bones) that excite the cochlear fluid and hence the sensory organs of hearing. We conducted experiments to measure the vibrational dynamics between the tympanic bullae and the base of the skull in both a juvenile and an adult gray whale. For the juvenile, assessments were performed on three versions of the skull: a plastic replica produced by 3D printing from CT scans of the original specimen, the original skull after much of the soft tissue had been removed by dissection, and the denuded skull after hydrogen peroxide was used to erode soft tissues within the cavities of the skull. We excited vibrations in the juvenile skull underwater by projecting sound in a test pool, ranging from 170 - 1000 Hz. Additionally, we measured in-air vibrations of the plastic skull, the denuded skull, and the adult skull using a mechanical shaker to drive vibrations anteroposteriorly (rostrum-to-tail) from 150 - 1000 Hz. This dataset includes recordings from 7 uniaxial accelerometers during each of the experiments. Frequency response functions (vibration velocity amplitude vs. frequency) consistently revealed amplification of vibrations at the tympanic bullae compared to the base of the skull, demonstrating a mechanism by which low-frequency sound is transferred from the environment into the inner ear. 

Methods
We conducted experiments to measure the vibrational dynamics between the tympanic bullae and the base of the skull in both a juvenile (specimen: LACM 97758) and an adult (specimen: LACM 84202) gray whale. Measurements were made by accelerometers glued with specified locations and orientations on the skull while the skull was being driven at a specific frequency, either in air or underwater. The dataset includes amplitudes of acceleration measured by 7 accelerometers positioned on the gray whale skull. Accelerometer positions are described below (begins with 'Sensor labels') and are shown in the file 'sensor_positions.jpg' (a: annotated positions on plastic skull, b: sensors on natural skull, c: sensors on denuded skull, d: sensors on adult skull. 'Plastic skull' refers to a 3D-printed replica of the juvenile skull, 'natural skull' refers to the juvenile head after most of the soft tissue was removed, 'denuded skull' refers to the skull after the rest of the soft tissue was removed by bathing the natural skull in a 12% hydrogen peroxide solution, and 'adult skull' refers to the adult skull which has no soft tissue remaining.

In-water experiments were conducted on the natural, denuded, and plastic skulls at the Naval Information Warfare Center (NIWC) Pacific's Transducer Evaluation Center (TRANSDEC) facility in San Diego, CA. Experiments were conducted using a GeoSpectrum Technologies M72-110 C-Bass transducer. Each experiment was run as a frequency sweep in the range of 170-1000 Hz, stepping through the specified frequency range at 2 Hz intervals. Each frequency was projected for 500 milliseconds at one-second intervals. Accelerometer data were recorded during each step for 700 ms, allowing the amplitude of acceleration to be calculated using a standard FFT method. This dataset includes the amplitudes of acceleration from each sensor as well as acceleration divided by incident pressure. Details of each run are described in the file 'run_descriptions_TRANSDEC.csv.'

In-air experiments were conducted on the plastic and denuded skulls in the whale acoustics lab (ALab) at UC San Diego and on the adult skull at the marine mammal collections warehouse of the Los Angeles County Museum of Natural History (LACM). Experiments in air used an Unholtz-Dickie model 5PM shaker to drive anteroposterior (rostrum-to-tail) motions in the skull at specified frequencies, and measured acceleration at the same points used in the water experiments. Employing LabView as the signal generator, the shaker vibrated for 1 s, stepping from ~150 to 1000 (or 3000) Hz in 1 (or 2) Hz intervals. At each frequency step, accelerometer data were recorded for 1 s, allowing the amplitude of acceleration to be calculated using a standard FFT method. Details of each run are described in the file 'run_descriptions_ALab_LACM.csv.' This dataset includes the amplitudes of acceleration from each sensor.