math.log(x) returns unexpected results in Python

Question

I am attempting to do some basic dB calculations using Python. If I use either Excel or a scientific calculator:

20*log(0.1) = -20

in Python:

20*log(0.1) = -46.0517018599

to simplify further, Excel and Scientific calc:

log(0.1) = -1

Python:

log(0.1) = -2.30258509299

I start my script with

import math

log = math.log

Can someone explain why this is and how I can fix it?

Sidenote: you can from math import log and save yourself a line. — jonrsharpe
– jonrsharpe, Commented Feb 2, 2016 at 21:04
It always helps to read the docs: docs.python.org/2/library/math.html. This is important especially when using a logarithm function since log() can mean either base 10 or base e depending on the programmer. — Code-Apprentice
– Code-Apprentice, Commented Feb 2, 2016 at 22:14

zondo · Accepted Answer · 2016-02-02 22:15:01Z

11

math.log is actually ln (with a base of e). To get the expected results, use math.log(0.1, 10) or math.log10(0.1).

edited Feb 2, 2016 at 22:15

answered Feb 2, 2016 at 21:03

zondo

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3 Comments

Code-Apprentice Over a year ago

Alternatively, you can use math.log10()

zondo Over a year ago

@Code-Apprentice That is true. I have updated my answer.

Davros Over a year ago

Thank you for this answer, it certainly solved my issue and now its been layed out makes me feel a bit stupid as i was actually close to this answer from reading the documentation but simply didn't implement it correctly!! however I now simply start my code with import math log = math.log10 instead of doing it in the equation as x, log(x) is derived from a simple division using variables causing additional complication and required me to learn/use the float function. so now looks somthing like this >>> rxcalc_dst = pntsrc_spl + 20 * log(float(pntsrc_dst) / float(pntrx_dst)) Thanks again!

bibi · Accepted Answer · 2016-02-02 21:45:15Z

1

If you want the base 10 logarithm, you have to use math.log10(0.1)

Documentation is HERE

Fortran, C, C++, Java, tcl and many others they all use log as base-n logarithm (natural logarithm)

edited Feb 2, 2016 at 21:45

answered Feb 2, 2016 at 21:26

bibi

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