These simple equations comes up as an error

[nilamo]

x = "float(DB)/10"
y = "float(x)+11"
z = "float(y)/100"

Those are strings. Strings are what they are, they aren't equations. If you want to do math, stop putting things in a string.

ie:

>>> DB = 4
>>> x = "float(DB)/10"
>>> y = "float(x)+11"
>>> z = "float(y)/100"
>>> x
'float(DB)/10'
>>> y
'float(x)+11'
>>> z
'float(y)/100'
>>>
>>> x = float(DB)/10
>>> x
0.4
>>> y = x+11
>>> y
11.4
>>> z = y/100
>>> z
0.114