People who know how to do maths - how can I do log values in a spreadsheet?

MikeHunt79 · 5 Mar 2010 at 17:20

I need to use decibels which is a log scale.

Say I have a voltage (10v) and I reduce it by 6dB, it should become 5v.

But, I need to know what the voltage will be if I reduce it by 2dB for example...

This isn't schoolwork, I'm trying to build a stepped attenuator!

Rich_L · 5 Mar 2010 at 17:23

=log(number,base) I believe

MikeHunt79 · 5 Mar 2010 at 17:35

Rich_L said:
=log(number,base) I believe

Just tried that and it works great, thanks.

I did tried to google 'speadsheet log' but got loads of unrelated rubbish...

Ricochet J · 5 Mar 2010 at 17:45

Just to confuse you Excel also does natural log!! I.e. ln

p4radox · 5 Mar 2010 at 17:50

If "dB" is your change in dB, V2 is your new voltage and V1 is your original voltage:

dB = 20 x log (V2/V1)

Rearranged:

V2 = V1 x 10 ^ (dB/20)

This should give you your new voltage. If you try the above with a dB value of "-6" and a V1 of 10V it gives you a V2 of 5V, which is correct

.

/edit: oh, it sounds like you just needed the Excel syntax. Oh well.

paradigm · 5 Mar 2010 at 18:10

MikeHunt79 said:
I did tried to google 'speadsheet log' but got loads of unrelated rubbish

Should have tried "spreadsheet logarithmic".

Spreadsheet log could mean anything.

Westyfield2 · 5 Mar 2010 at 18:31

Ricochet J said:
Just to confuse you Excel also does natural log!! I.e. ln

Tbh by the end on A-Level ln was all that I used for any logs.

p4radox · 5 Mar 2010 at 19:02

Westyfield2 said:
Tbh by the end on A-Level ln was all that I used for any logs.

Unfortunately the decibel scale uses base 10 for some reason.

MikeHunt79 · 5 Mar 2010 at 21:37

p4radox said:
If "dB" is your change in dB, V2 is your new voltage and V1 is your original voltage:

dB = 20 x log (V2/V1)

Rearranged:

V2 = V1 x 10 ^ (dB/20)

This should give you your new voltage. If you try the above with a dB value of "-6" and a V1 of 10V it gives you a V2 of 5V, which is correct.

/edit: oh, it sounds like you just needed the Excel syntax. Oh well.

Well, I was originally struggling to just to be able to put log into the spreadsheet, then to figure out how to determine the voltage, so this info is ideal, thanks.