Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity

Differential Rotation in Weakly-Magnetized Neutron Star Atmospheres

The atmospheres of weakly-magnetized neutron stars expand hydrostatically and rotate differentially during thermonuclear X-ray bursts. Differential rotation is probably related to the frequency drifts of millisecond burst oscillations exhibited by about a dozen nuclear-powered X-ray pulsars. Here, we analyze the linear stability of this differential rotation with respect to local, axisymmetric, multi-diffusive MHD perturbations, at various heights in the neutron star atmosphere. Unstable magneto-rotational modes are identified from within to well above the burning layers. Properties of the fastest growing modes depend sensitively on the local magnetic field geometry. Linear estimates suggest that momentum transport due to magneto-rotational instabilities can affect atmospheric rotation profiles on time-scales relevant to burst oscillations. This transport would likely strengthen the coherence of burst oscillations and contribute to their drifts.Comment: 11 pages, 6 figures, 2 tables, accepted for publication in MNRA

A Thoracic Mechanism of Mild Traumatic Brain Injury Due to Blast Pressure Waves

The mechanisms by which blast pressure waves cause mild to moderate traumatic brain injury (mTBI) are an open question. Possibilities include acceleration of the head, direct passage of the blast wave via the cranium, and propagation of the blast wave to the brain via a thoracic mechanism. The hypothesis that the blast pressure wave reaches the brain via a thoracic mechanism is considered in light of ballistic and blast pressure wave research. Ballistic pressure waves, caused by penetrating ballistic projectiles or ballistic impacts to body armor, can only reach the brain via an internal mechanism and have been shown to cause cerebral effects. Similar effects have been documented when a blast pressure wave has been applied to the whole body or focused on the thorax in animal models. While vagotomy reduces apnea and bradycardia due to ballistic or blast pressure waves, it does not eliminate neural damage in the brain, suggesting that the pressure wave directly affects the brain cells via a thoracic mechanism. An experiment is proposed which isolates the thoracic mechanism from cranial mechanisms of mTBI due to blast wave exposure. Results have implications for evaluating risk of mTBI due to blast exposure and for developing effective protection

Noise attenuation and communication enhancement characteristics of the USCG boat crew communication system

The US Coast Guard is prototyping a new small boat communication system which consists of the MSA Sordin Supreme Pro headset in combination with a wireless communications system. The MSA Sordin headset receives wireless communications input independently from an amplitude-sensitive sound transmission feature which amplifies ambient noise in certain frequency ranges. The purpose of this study was to determine the extent to which the communication system improves speech intelligibility in noise and to measure the noise reducing capabilities of the headset with the amplification feature activated and with it turned off. Overall noise reduction was calculated based on four different noise spectrums to compare actual noise reduction to the manufacturer advertised Noise Reduction Rating (NRR). Additionally, clamp force was measured to determine its relationship with noise reduction. This study found that the communication system, consisting of wireless communications and the activated headset amplification feature, drastically improved verbal communications when compared to the case of the headset donned but wireless communication disconnected and headset amplification off (88% vs 44% intelligibility score in 90 dBA background noise; 82% vs negligible intelligibility in 100 dBA background noise). The MSA Sordin headset amplification feature had a profound effect on the hearing protector\u27s noise reducing capability. When it was activated, noise reduction was dramatically lower in all frequencies above 315 Hz. This resulted in lower overall noise reduction when this feature was on. Mean overall noise reduction values ranged from 11.2 to 27 dBA with the amplification feature turned off and 6.7 to 0.2 dBA with the amplification feature activated. The difference was least in low frequency dominant noise (11.2 vs 6.7 dBA) and greatest in high frequency dominant noise (27 vs 0.2 dBA). The low frequency dominant spectrum used in this study was recorded onboard an operational USCG 47\u27 Motor Life Boat, the system\u27s intended operating environment. In this intended environment, the calculated overall noise reduction was less than the manufacturer advertised rating of 18 dB (study values; 11.2 dBA without amplification, 6.7 dBA with amplification). A weak positive correlation was found between clamp force and noise reduction but the association was not statistically significant, meaning that clamp force was not the reason for the noise reduction performance of the MSA Sordin headset

Estimation of Outer-Middle Ear Transmission using DPOAEs and Fractional-Order Modeling of Human Middle Ear

Our ability to hear depends primarily on sound waves traveling through the outer and middle ear toward the inner ear. Hence, the characteristics of the outer and middle ear affect sound transmission to/from the inner ear. The role of the middle and outer ear in sound transmission is particularly important for otoacoustic emissions (OAEs), which are sound signals generated in a healthy cochlea, and recorded by a sensitive microphone placed in the ear canal. OAEs are used to evaluate the health and function of the cochlea; however, they are also affected by outer and middle ear characteristics. To better assess cochlear health using OAEs, it is critical to quantify the impact of the outer and middle ear on sound transmission. The reported research introduces a noninvasive approach to estimate outer-middle ear transmission using distortion product otoacoustic emissions (DPOAEs). In addition, the role of the outer and middle ear on sound transmission was investigated by developing a physical/mathematical model, which employed fractional-order lumped elements to include the viscoelastic characteristics of biological tissues. Impedance estimations from wideband refectance measurements were used for parameter fitting of the model. The model was validated comparing its estimates of the outer-middle ear sound transmission with those given by DPOAEs. The outer-middle ear transmission by the model was defined as the sum of forward and reverse outer-middle ear transmissions. To estimate the reverse transmission by the model, the probe-microphone impedance was calculated through estimating the Thevenin-equivalent circuit of the probe-microphone. The Thevenin-equivalent circuit was calculated using measurements in a number of test cavities. Such modeling enhances our understanding of the roles of different parts of the outer and middle ear and how they work together to determine their function. In addition, the model would be potentially helpful in diagnosing pathologies of cochlear or middle ear origin