|WikiProject Physics / Acoustics||(Rated Start-class, Low-importance)|
This article claims that the minimum acoustic velocity occurs in the SOFAR channel because "the cumulative effect of water pressure, temperature, and salinity causes the water at this depth TO BE LESS DENSE than that of other parts of the water column". This cannot be correct.
The DENSITY is most certainly not at a minimum here. At constant pressure, acoustic speed in water DECREASES with depth because the density INCREASES steadily as the temperature drops. Indeed, the densest and heaviest water (at a temperature a few degrees above freezing) sinks to the bottom of the ocean, which is why ice floats and life exists on earth --otherwise, ice would sink and the oceans would be frozen solid.
On the other hand, the speed of sound INCREASES with depth because the bulk modulus (compressive stiffness) of the water grows in tandem with the pressure. Near the surface of the ocean, the change in speed is controlled mostly by the drop in temperature (increase in density of water and decrease in acoustic speed), but below the SOFAR channel, the speed increases again under the control of the high pressure (high bulk modulus), and thus the acoustic velocity increases with depth once more. The salinity plays a lesser role, because it does not change that much with depth at any given location. Near the poles, the water is very cold, so the SOFAR channel is controlled by pressure alone, and the minimum is at the surface.
Other factors that can also affect the acoustic speed of sound in water are the amplitude (intensity of the sound) and its frequency (especially for ultrasonic waves).
By the way,
- Yes, there are some problems here. The text says "Near Bermuda, the lowest density water occurs at a depth of around 1000 metres." which cant be right. Fig 10:29, 22 March 2007 (UTC)
The term SOFAR is way out of date and referred to a WWII downed plane location system. DSC is the preferred term.Adresia 19:52, 23 June 2007 (UTC)
- Agree I was going to propose a name change based on the entry in the Navy DOD dict. cited in the article, unless someone has a more authoritative ref. Dhaluza 20:58, 23 June 2007 (UTC)
Linearity of pressure increase with depth
The wave guide depth is defined by a local maximum of d(rho)/dz ie. the depth gradient of in situ density, positive downwards. This is within the pycnocline. Deep at western boundaries, and shallow at eastern. Long range sound propagation studies go back to WW126.96.36.199.205 (talk) 11:42, 13 January 2014 (UTC)