The AF447 report lists the effective range of the ULBs on the two recorders as 2 to 3 km. That's slant range, so if the water is deep, the hydrophones need to be lowered substantially, and of course good bathymetry (i.e., and underwater terrain map) helps a lot.
Main page for AF447 data:
FLIGHT AF 447
Sea Search Operations (31 MB):
"Both of the flight recorders of the aeroplane were equipped with a ULB designed to locate it when immersed. The duration of ULB transmission is at least thirty days from immersion(10). . . . As a rule, acoustic searches should always be preferred during the transmission time of the beacons. They are more effective than searches using sonar, magnetometers or video cameras.
The maximum range(11) of these beacons is of the order of 2,000 to 3,000 m. However, in the search area the average depth was 3,000 m. It was therefore necessary to bring the hydrophones(12) closer to the source of transmission, by towing specialized equipment nearer to the seabed.
(10)The regulatory minimum is 30 days, but the ULB manufacturer stated that the transmission duration was in reality of the order of forty days. This figure is determined by the capacity of the internal battery.
(11)It may be necessary to take into account the propagation of acoustic waves in a liquid medium, which depends on numerous interconnected parameters, such as the salinity and temperature of the water. When an acoustic wave is propagated in the sea, it is subject to refractions, which generate multiple trajectories. It can also happen that the acoustic waves are deflected in such a way that there is an area of shadow that is never reached by these waves.
(12)Undersea microphone. "
"The TPL20 and TPL40 systems are deep-towed devices belonging to the family of the “Towed Pinger Locators” manufactured by Phoenix International for the US Navy. The United States government made both the equipment and the associated operators freely available to the French government (17 people distributed on the two ships). In June 2009 the TPLs were the only systems capable of carrying out passive acoustic searches over large areas at significant depths.
The two TPLs are towed devices each equipped with an omni-directional hydrophone which can operate down to depths of six thousand metres with towing speeds ranging from 1.5 to 5 knots. They can be installed on all types of appropriate vessels capable of carrying a load weighing around 25 tonnes. A mapping software application uses GPS positioning information to follow the ship’s movements and the position of the towed device. The latter is equipped with a pressure sensor that permanently transmits the immersed device’s approximate depth of submersion. Management of the deployed cable length and ship towing speed is used to place the acoustic sensor at the required average submersion depth For example, an average submersion depth of 2,300 m for the TPL is achieved by deploying approximately 6,000 m of cable at a towing speed of 3 knots."
Much more interesting information in the report, such as description of an acoustic repeater that downconverts the 37.5 kHz signal to 12 kHz to be compatible with other sensors onboard a ship.
On using the French nuclear-powered submarine Emeraude:
"The Emeraude is equipped with numerous acoustic sensors including a sonar interceptor which was used during the search operations. This equipment was not originally designed to detect and localise ULB type acoustic beacons, but thanks to the optimisation of its settings and the use of additional computer software from 30 June 2009, its signal detection capabilities were enhanced."
"Deployment of the nuclear-powered submarine improved the search system and provided an interesting mobile device in terms of the surface areas covered (its average speed in the zone , between 6 and 10 kt, was higher than that of the other resources deployed). However its use proved difficult, given the safety constraints resulting from its integration in the search operations.
The presence of the nuclear-powered submarine meant that a vast safety zone had to be put in place around its patrol area in order to avoid any collision between the various pieces of towed equipment and the submarine. This permanent preoccupation with safety required delicate management of the undersea zones."
Interim report on search for ULBs:
"The first search phase aimed at detecting and locating the acoustic signals transmitted by the Underwater Locator Beacon (ULB) fitted on each flight recorder(8). As a priority, the aeroplane’s planned flight path as well as the greatest possible area inside the 40 NM circle was swept by two Towed Pinger Locators (TPL)(9).
No signal from either of the beacons was detected by the sensors deployed in the area despite TPL passing by, on two occasions, not far from the debris field, on 22 and 23 June 2009.
(8)There were two beacons on the A-330, one attached to the cockpit voice recorder (CVR) and the other to the flight data recorder (FDR).
(9)The two US Navy TPL’s are the only two towed hydrophones in the world able to operate to a depth of up to 6,000 metres."
Report on tests of similar ULBs:
"Masked areas are present on the beacon’s vertical axis as well as the horizontal axis, leaving the possibility of significant of "shadow" zones. . . .
- It should be remembered that each time the level falls 6 dB(micro)
Pa, the signal’s theoretical range falls by half. For beacons attached in this way on a flight recorder this can be very noticeably reduced, depending on orientation, in relation to the theoretical range of a single beacon."