Sorry for my english. My question:

I´m analyzing a three parallel axis gearbox, four helicoid gears: E1-slow drive wheel, E2-intermediate pinion, driven by E1, E3- intermediate wheel, joined to E2, E4-fast pinion, driven by E3.
I have the following information: z1=?; z2=24
teeth, z3=78 teeth, z4=34 teeth, n4=1500 cpm.
The gearmesh frequencies I´ve calculated was f34=51000 cpm and
f12=15700 cpm, aprox. However I didn´t find this f12 frec. in the espectrums, and I find a 11530 cpm peak., with a lot of sidebands.
¿Could you tell me what this means?.