Image copyright Donga Daily Image caption Turning up the dial on tensions - South Korea's broadcasts irritate the North's rulers

South Korea has responded to North Korea's recent claim to have tested a hydrogen bomb by switching back on its gigantic loudspeakers pointed across the border.

North Korea has in turn switched on its own giant speakers.

As the world continues to investigate whether the North's bomb claim is true and how it should respond, what are the two Koreas shouting at each other?

What's coming out of the speakers?

For the South, their purpose is propaganda - persuading North Korean soldiers to doubt their own regime or even defect.

The propaganda programming, running on and off since the Korean War, has become more subtle in recent years.

Image copyright AP Image caption Girl group Apink - and their song Just Let Us Love - has been used before in South Korea's audio bombardment

It includes weather reports - making it a useful thing for Northern soldiers to listen to - news from both Koreas and abroad which won't otherwise be heard over the border, dramas, favourable discussion of democracy, capitalism and life in South Korea, and less favourable comments on corruption and mismanagement in the North.

The speakers also blast music in the form of Korea's much-loved K-pop, which is banned in the North. Songs from Korean girl band Apink, singer IU and boy band Big Bang - including their megahit Bang Bang Bang - are on the propagandists' playlists.

The North's broadcasts are harder to hear - possibly the result of poor speakers - and carry its characteristically strident condemnations of Seoul and its allies.

They may not be as powerful, but it is thought they do help cancel out the sound of the South's speakers to some extent.

How many broadcasts a day?

A South Korean military spokesperson said there were two to six hours of broadcasts daily, day and night, at irregular hours.

Image copyright Getty Images Image caption Not your usual roadies: the huge speaker stacks are assembled by army engineers

While the exact distance the sound travels will depend on topography, weather conditions and so on, the South Korean military claim the broadcasts can be heard as much as 10 km (6.2 miles) across the border in the day, and up to 24 km (15 miles) across at night.

That would easily reach North Korean troops, and would be audible by any civilians in the area.

In August, when the South briefly turned its speakers back on after an 11-year break, the military said there were 11 loudspeaker sites. But it has not confirmed if that is still the case. Their exact location along the border is also not officially disclosed.

One South Korean government official said the North appears to have expanded its own speaker operations, from two sites to "several".

"In fact, the anti-South loudspeaker broadcasts appear to be coming from every location where we are broadcasting," the unnamed official told the Yonhap news agency.

Image copyright AP Image caption If the speakers' range is as far as claimed, they might be heard in Kaesong, the jointly-operated industrial zone in the North

Why does North Korea hate them so much?

Pyongyang says it considers them an act of war and has threatened to blow up the speakers. Apart from the regime's usual sensitivity to insults and threats, its anger could be because they might be working.

The pop-cultural "Korean Wave" has not just broken on distant shores - North Koreans too are fans of movies and dramas smuggled across the border, says Kim Yong Hun, president of Daily NK, an online newspaper reporting on North Korea, with a network of sources inside the country.

Image copyright Getty Images Image caption The South's speakers are more powerful than the North's

"Its popularity trickles down to ordinary residents and is especially favoured by younger generations. Soldiers are not exempt from the obsession; songs and cultural programming transmitted by their brethren in the South holds massive power to influence how young soldiers view the North Korean system."

"Prolonged listening of these broadcasts day and night typically has a gradated and ultimately transformative effect," Kim Yong Hun says. "The North Korean government's enraged response is proof positive of the threat these broadcasts pose to its grip on power."

How long will it continue?

It is impossible to know.

In 2004, the broadcasts were stopped as part of a North-South deal. Seoul threatened to restart theirs in 2010 - going as far as reinstalling them along the border, before settling for radio broadcasts instead.

Image copyright EPA Image caption It was just a month ago that both sides met to discuss improving relations

They finally did restart on 10 August 2015 - after a border landmine maimed two South Korean soldiers - only to end just weeks later, as part of another deal with the North to dial back tensions.

Some see the broadcasts as unnecessarily provocative. British Foreign Secretary Philip Hammond, on a recent visit to Japan, said that resuming the broadcasts was "simply rising to the bait".

But their defenders argue that the North is annoyed by them precisely because they do work, or at least that they are a useful bargaining chip to use in negotiations.

How else do messages cross the border?

Image copyright AP Image caption South Korea considers the balloon launches to be part of freedom of expression

The South also has a radio programme, called "The Voice of Freedom", which is transmitted into the North by radio. Dedicated listeners in Seoul can even tune in on FM107.3. Like the loudspeaker broadcasts, they are also sometimes halted. The North also attempts to jam the signal.

Other organisations, such as Unification Media Group, also broadcast radio into the North, though typically in a more neutral way than the military's efforts.

Still other groups, mostly made up of defectors, drop leaflets, DVDs, USB sticks and other material across the border, using balloons. Nearby residents say this could encourage the North to open fire, and while the government does not like the campaigns, it says it will not stop them.