"I would just like to have that hard evidence ... photograph evidence [before saying] that this is the final resting place of MH370," Houston said.

Finding the sound again is crucial to narrowing the search area so a small submarine can be deployed to chart a potential debris field on the seafloor. If the autonomous sub were used now with the sparse data collected so far, covering all the potential places from which the pings might have come would take many days.

"The better Ocean Shield can define the area, the easier it will be for the autonomous underwater vehicle to subsequently search for aircraft wreckage," Houston said.

The locator beacons on the black boxes have a battery life of only about a month. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task.

Tuesday marks one month since the plane vanished during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board — setting off an international search that started off the southern coast of Vietnam and then shifted to the southern Indian Ocean as information from radar and satellite data was refined.

Family members of passengers on board the flight held a candlelight vigil in Beijing on Tuesday to mark how long it has been since contact with the plane was lost. About two-thirds of people on board were Chinese.

"We've been waiting and holding on here for already 31 days," said Steve Wang, one of the relatives.

"Don't cry anymore. Don't hurt anymore. Don't despair. Don't feel lost," he counseled others who gathered for the vigil.

The length of the search and lack of any information about why the plane went so far off course has transfixed the world.

Al Jazeera and wire services