A 400 V, 10 KVA transformer at 50 Hz, is operated at the frequency

A 400 V, 10 KVA transformer at 50 Hz, is operated at the frequency of 40 Hz, then the humming ___________

A. increases

B. decreases

C. remains same

D. increases to very high

This question was addressed to me in an online quiz.

I’d like to ask this question from Three Phase Transformers topic in division Transformers of Electrical Machines

Right option is A. increases

The explanation: If the frequency is reduced, the core flux density increases, so the noise also will increase.