This is the electrical hum heard on 81-717 metro trains, found commonly in Eastern European subway systems. (Moscow, Saint-Petersburg, Kyiv, Budapest, Prague, Warsaw, and Sofia metros, to name a few).
This is produced by the BPSN-5U2 converter (BPSN stands for auxiliary power supply block), a set of two static inverters that convert 750V (or 825V) DC from the third rail to 80V DC for the onboard control systems in the cabin, and 220V AC for the interior lights. One of the converters produces a 300Hz tone, the other produces a 200Hz tone. On later trains, the 200Hz tone is inaudible due to the replacement of the secondary inverter.
Later 81-717 trains have had these power supplies replaced with quieter BPSN-5U2M or BBE units, or by completely silent BPN-115 and DIP power supplies.
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Wave (.wav)
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16 bit
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Stereo
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