Problem in flyback converter

When reading document, I found that below:

"Generally, when the frequency of operation of known power supplies of this type (flyback converter) drop to frequencies within the audio frequency range,such as within 20Hz to 20kHz, undesirable audio noise is generated by the transformer of the power supply."

"When the load is light, the lower current limit result in low flux density through the transformer, so unacceptable audio noise is not generated even the swithing regulator operate at lower frequencies within the audible frequency."

"In another word, the switching regulator will not generate undesired audible noise because the flux density is limited to be below a sufficiently low threshold value."


my proble is

1. Why the transformer does not generate audio noise when the operating frequency within the audible frequency range? Is there some relationship between the noise and the flux density ?
2. What the "threshold value" means mentioned above ?


thanks

These vibrations (noise) caused by switching current will always be generated. I would rather call this "threshold of your hearing" as at very low load switching current / flux will also be very low and the generated noise will be below level (threshold) of your hearing .. or if you prefer - flux density threshold ..

hi, IanP
I think it is just the answer. Thank you !