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Low-frequency radio noise is the electromagnetic background radiation which is compared here to the luminosity of 39 sprites recorded with a low-light video camera. It is found that the sprite luminosities coincide with ∼10-30 ms long sudden enhancements of the electromagnetic background radiation ∼6-8 μV mHz(∼6-9 dB) with a relative maximum near ∼125 kHz as measured with a wideband (∼1-400 kHz) digital radio receiver. The sprites cluster in 10 groups of 2-5 consecutive sprites which are paralleled by up to ∼1 s long slowly varying enhancements of the electromagnetic background radiation ∼4-5 μV mHz (∼2-4 dB). The observed electric field strengths place an upper bound on the low-frequency radiation from the electron multiplication associated with the exponential growth and branching sprite streamers predicted by Qin et al. [2012a]. This upper bound corresponds to a maximum of ∼300-5000 sprite streamers at ∼40 km height above thunderclouds. Some part of the observed electromagnetic background radiation might result from the superposition of low-frequency radiation emanating from the quick succession of numerous horizontal lightning strokes and/or stepped leaders inside thunderclouds which would constitute a fundamentally novel quasi-static discharge process inside thunderclouds radiating slowly varying low frequency radio noise.