In a current MPSIMS threadsombody quotes a news story:
What kind of speakers are they using? And how do they work?
I can’t imagine anything that would produce a sound loud enough to be heard 10 frickin’ miles away, and still have the words be comprehensible.
I can imagine, maybe, a few things that can be heard at that distance: a huge explosion. Or masssive industrial noise of some sort.And I know that in WW II, the US Army used a deception tactic of playing recorded “traffic noise” at night–the rumblings of tanks and heavy trucks, designed to confuse the Germans into thinking that there were large numbers of vehicles nearby.But that was just general rumbling, nothing specific, and it was at pretty close range, (broadcast only at night-- because the enemy was so close that, in daylight, they would see the ruse.)
And I’ve heard a few sonic booms.
But when it comes to music and spoken words being amplified…well, the only vaguely similiar experience I know of is a rock concert in a stadium. That’s loud enough to damage your hearing…but not if you’re 10 miles away.
What are the S Koreans blasting at the North? And does that kind of audio technolgy have any other practical use?
There are pictures of the speakers in these articles:
Looks like just large arrays of loudspeakers.
I suspect that the speakers are set up in a phased array, and that the sound is only intelligible along a fairly narrow beam.
It’s always amazed me how far sound can travel on a calm, quiet, and still night. Using the inverse relationship of sound pressure to distance, it is close to being linear and not exponential. So the sound pressure waves dissipate linearly with distance, so the sound you hear at 10 miles is about 1/10 of the sound you hear at 1 mile.
So what if I stood directly in front of one of those speakers? Would my eardrums be destroyed?
Yes, by the incoming NK artillery barrage.
No, because if the speakers are a phased array, the low power speakers will add up at some point to have more loudness than an individual speaker. There’s like magic audio stuff going on to get them in phase, but hey, not an audio girl.
It would be a Disaster Area.
I’m reminded of the Wall Of Sound that was featured in The Grateful Dead Movie.
They must be using some of those loudspeakers used on pickup trucks in Thailand that drive around touting political parties at election times.
YaKi - Mo Truck and tune.
These pictures give a good idea of the system. Essentially a Lego brick system of horn loudspeakers that can be built into various size arrays. From the look of it they are constant voltage systems (probably 100v) with a transformer, so you simply bolt together a huge number of them, and use amplifiers rated for that nominal voltage and able to deliver the needed power. They are probably very efficient, and horrendously loud. Big compression driver on the back.
You will get pretty good directionality down to mid-frequencies from one of these. Assume an 8x8 array is about 4m wide, means you will be getting useful gain right down to say 100Hz. Spoken voice propaganda (300-3000Hz) will easily get you quite significant gain. Also where the ear is most sensitive.
They will however sound pretty horrid at a distance. Air is not linear, and the effect over distance is to cause the waveform to sharpen - adding higher harmonics. Plus there will be difficult to control reflections off the landscape that will generally mess up the sound. But clearly audible over 10kms away? Very likely.
I used to live roughly 10 miles from Gateshead International Arena, an athletics stadium which was often used for large-scale outdoor gigs by big-name bands in the 80s and 90s. It didn’t happen often, but if the conditions were right we could sometimes hear a muffled bass thump from concerts happening there if we happened to be outside. Not much in the way of clarity, though.
I remember Tina Turner being especially audible.
Yeah, you need to listen from inside a large concrete bunker. Preferably 37 miles away.
Got those in Japan too: Uyoku Dantai, extremely loud sound trucks operated by far right-wing Japanese nationalists. They’re loud enough to drown out polite conversation from a couple of blocks away.
Back of the envelope. (Easily contains mistakes) A typical rating for something like one of these horns would be say 50W continuous power handing, and efficiency of 110db SPL/watt at one metre. Since these are in a phased array, it is more than 110db SPL, as the multiple units will additively interfere, and get you another 12db. Also it doesn’t matter if you are quite a bit back from them, you will still be in the near field, and so the SPL/power will remain at about 122db SPL/watt until you are getting close to, say, 10m away. So with 50w per speaker, you would be sitting inside a sound field of about 135db SPL. That won’t send you deaf instantly (150db SPL does that) but you are seriously over the pain threshold (120db SPL), and any time at such levels will start to cause permanent damage. The permissible time for exposure is about one quarter of a second.
You might expect that the sound field right in front would be much louder - given the insane power involved. But you are saved because the speakers are spread over a large area in an array, and this means the areal power density is lower close in than would be the case if you somehow built a speaker that delivered the energy from just one unit.
