I've recorded the output hiss from a small battery powered radio operating inside a metal tin. From related posts, I can say that the radio is effectively shielded from receiving any FM transmission, and the resultant hiss is thermal noise in the early stages of the receiver. I believe that the radio can output frequencies up to 50 kHz via it's earphone socket. Can this be correct? My findings suggest that it can. When not constrained by handling FM audio, the input to the early receiver stages is thermal noise which (unconstrained) has an almost infinite bandwidth. The only constraint operating on it then comes from the radio's internal electronics. These will have spare capacity well above the audio range so as not to clip FM audio. I've attached screen grabs from Audacity. This is the recorded hiss, and visually compared to a basic 10 kHz sine wave you can deduce how the hiss might reach 50 kHz, albeit at lower amplitudes. I've also uploaded a spectrum analysis, and a spectrogram. These suggest Pink noise. And finally, I've uploaded a zipped FLAC file of 5 seconds of the hiss should anyone have a real spectrum analyser. This would help greatly Is there any reason to think that a pocket radio can't have electronics capable of handling frequencies up to 50 kHz..?