The question is quite relevant and interesting. You submerge a flat inner tube of your bicycle under water to find a small hole. You won’t be able to do that with the ISS.
That goes on sound. Escaping gas through a leak will vibrate and thus make noise. You can detect that sound with a detector.
In 2004, a drop in air pressure was also detected on board the ISS, which indicates a leak. It took two astronauts three weeks to locate it using a portable ultrasound probe to detect the high-frequency “scream” of escaping air.
If this had not worked, the alternative would have been to seal the station module by module until the leak was isolated, with all the effort and loss of mission time involved.
The crew was lucky. A larger hole would have drastically diminished the chances of finding it before the astronauts were faced with the choice of either closing off a station section or retreating to the Soyuz capsule that was kept on board as a lifeboat. With this in mind, NASA is looking for an alternative.
The problem is that the ISS is a very noisy place full of fans, pumps, gasses and liquids flowing, buzzing electronics, and all the other noises associated with people living in a series of cans. The usual procedure is for a drop of pressure to sound an alarm and the astronauts to look for the cause, which could be a leaky hull or defective equipment. Even with an ultrasonic listening device, detecting a leak is a slow process made more difficult by the fact that equipment or structural components can cover the leak. Moreover, it doesn’t help that the station is leaking, no matter how well sealed it is.
Eric Madaras, an aerospace technologist at NASA’s Langley Research Center, is the lead investigator of the Ultrasonic Background Noise Test (UBNT) and is leading a team working on a new way to quickly detect air leaks.
The UBNT approach is not just to hunt for a leak, but to address all other high-frequency noises in the ISS as part of the development of an automated leak detection system. This includes the installation of 14 distributed impact detection systems (DIDS) units inside the pressure hulls of the Destiny and Tranquility modules on the station. Each DIDS has four pressure-sensitive transducers, which act like the pickup coils on an electric guitar. This allows the system to listen for the leak through the metal of the hull itself, and the signals to the various units can help triangulate the leak.
But the smart thing is that Madaras wants to routinely identify and characterize background noises so that it is possible to filter them out if a leak occurs. “One way to look at that is to think about a cocktail party,” he says. “It is sometimes very difficult to hear people, even near you, because of all the background noise. That’s the same phenomenon that this has to do with. How do you get that cleared up so that you can hear the specific type of sound – the signal -? what you are looking for? “
The aim of the system would be to give astronauts more time to find and fix leaks before so much air is lost that the only alternatives are to leave the station or shut down a module. “The idea of giving them more time, trying to help them and getting that part done so they can get to the leak, and now they have the tools to fix it,” Madaras says. “That would be a good deal for me.” (source)