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During the Philippine Sea experiment in May 2009, Deep Sound, a free-falling instrument platform, descended to a depth of 5.1 km and then returned to the surface. Two vertically aligned hydrophones monitored the ambient noise continuously throughout the descent and ascent. A heavy rainstorm passed over the area during the deployment, the noise from which was recorded over a frequency band from 5 Hz to 40 kHz. Eight kilometers from the deployment site, a rain gauge on board the R/V Kilo Moana provided estimates of the rainfall rate. The power spectral density of the rain noise shows two peaks around 5 and 30 kHz, elevated by as much as 20 dB above the background level, even at depths as great as 5 km. Periods of high noise intensity in the acoustic data correlate well with the rainfall rates recovered from the rain gauge. The vertical coherence function of the rain noise has well-defined zeros between 1 and 20 kHz, which are characteristic of a localized source on the sea surface. A curve-fitting procedure yields the vertical directional density function of the noise, which is sharply peaked, accurately tracking the storm as it passed over the sensor station. [ABSTRACT FROM AUTHOR]

In 2009, as part of PhilSea09, the instrument platform known as Deep Sound was deployed in the Philippine Sea, descending under gravity to a depth of 6000 m, where it released a drop weight, allowing buoyancy to return it to the surface. On the descent and ascent, at a speed of 0.6 m/s, Deep Sound continuously recorded broadband ambient noise on two vertically aligned hydrophones separated by 0.5 m. For frequencies between 1 and 10 kHz, essentially all the noise was found to be downward traveling, exhibiting a depth-independent directional density function having the simple form cos θ, where θ ≤ 90° is the polar angle measured from the zenith. The spatial coherence and cross-spectral density of the noise show no change in character in the vicinity of the critical depth, consistent with a local, wind-driven surface-source distribution. The coherence function accurately matches that predicted by a simple model of deep-water, wind-generated noise, provided that the theoretical coherence is evaluated using the local sound speed. A straightforward inversion procedure is introduced for recovering the sound speed profile from the cross-correlation function of the noise, returning sound speeds with a root-mean-square error relative to an independently measured profile of 8.2 m/s. [ABSTRACT FROM AUTHOR]

Ambient noise in the deep ocean is traditionally monitored using bottom-mounted or surface-suspended hydrophone arrays, An alternative approach has recently been developed in which an autonomous, untethered instrument platform free falls under gravity from the surface to a preassigned depth, where a drop weight is released, allowing the system to return to the surface under buoyancy. Referred to as Deep Sound, the instrument records acoustic, environmental, and system data continuously during the descent and ascent. The central component of Deep Sound is a Vitrovex glass sphere, formed of two hemispheres, which houses data acquisition and storage electronics, along with a microprocessor for system control. A suite of sensors on Deep Sound continuously monitor the ambient noise, temperature, salinity, pressure, and system orientation throughout the round trip from the surface to the bottom. In particular, several hydrophones return ambient noise time series, each with a bandwidth of 30 kHz, from which the noise spectral level, along with the vertical and horizontal coherence, are computed as functions of depth. After system recovery, the raw data are downloaded and the internal lithium ion batteries are recharged via throughputs in the sphere, which eliminates the need to separate the hemispheres between deployments. In May 2009, Deep Sound descended to a depth of 6 km in the Philippine Sea and successfully returned to the surface, bringing with it a unique data set on the broadband ambient noise within and below the deep sound channel. The next deep deployment is planned for November 2009, when Deep Sound will descend almost 11km, to the bottom of the Challenger Deep at the southern end of the Mariana Trench. If successful, it will return with continuous acoustic and environmental recordings taken from the sea surface to the bottom of the deepest ocean on Earth. [ABSTRACT FROM AUTHOR]