Determining the mathematical relationship in log graph please?

Question

so lets say i have a log graph where the Y axis is measured in dB , while the X axis in Frequency.

Y axis = from 0 to 90 dB X axis = from 0 to million Hz

If 55 dB has 15KHz.

Q1) How can i know what frequency does the 50 dB has ?

Q2)How can i know what frequency does the 90 dB has ?

please show me the method / the math behind it because it's coming in the exam

I think that in able to answer we need two points. For example, you tell us that 15dB is 15kHz, and that xdB is for ykHz, then we can tell you how many kHz is 50dB — Claudio Avi Chami, Commented Apr 16, 2016 at 20:43
Or maybe your description is just confusing because it is not clear what '55dB has 15kHz' means. — Claudio Avi Chami, Commented Apr 16, 2016 at 20:49
yes 55 dB is at 15 KHz , what is the frequency of 50 dB in this bode plot ? — malken, Commented Apr 16, 2016 at 21:13
and if 95 dB is at 150 Hz , how do we know what is the frequency for 90dB ? — malken, Commented Apr 16, 2016 at 21:24

ya man · Accepted Answer · 2016-04-16 23:50:57Z

Your question is a bit confusing. The way I see it, you want to calculate the value in question rather than reading it from the plot.

I guess that there is no transfer function given. So you'll have to determine it first. This is basically done in three steps:

This video tutorial covers the whole process.

Now with the transfer function you'll be able to calculate any gain at a given frequency or vice versa.

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